Chapter XIV: Introduction: 203 (4)

In 1954, several antibiotics were donated to the Division including a mold of _Penicillium notatum_ prepared and presented to the Smithsonian Institution by Sir Alexander Fleming (1881-1955), the discoverer of penicillium (1929), and a few Petri dishes used by botanist Benjamin M. Daggar who, while working for Lederle Laboratories, developed Aureomycin (chlortetracycline) in 1948. The Forest D. Dodrill--G.M.R. mechanical heart (1952), the first machine reported to be used successfully for the complete bypass of one side of the human heart during a surgical operation,[17] was presented to the Smithsonian Institution.

The following year, 1955, the Division acquired one of the earliest Einthoven string galvanometers (named after the Dutch physiologist Willem Einthoven, 1860-1927) made in the United States in 1914 by Charles F. Hindle for an electrocardiograph. Also added to the Division's collections was the electrocardiograph used by Dr. Frank E. Wilson of the United States, a pioneer educator in this field. Two temporary exhibits on allergy and surgical dressings were installed in the gallery. In the same year, Curator Griffenhagen published _Early American Pharmacies_, a catalog on 28 pharmacy restorations in this country.

In 1956, among many publications of interest in the fields of medical and pharmaceutical history, was Curator Griffenhagen's _Pharmacy Museum_, with a foreword by Laurence V. Coleman, who termed it a useful catalog and "a good reflection of the history of the museum movement at large." A third x-ray tube of Wilhelm Konrad Roentgen (1845-1922) was added to the collection in 1957 as well as a complete set of hospital-ward fixtures of about 1900 from the Massachusetts General Hospital, rare patent medicines, 18th-century microscopes, and a 13th-century mortar and pestle made in Persia.

In 1957, Mr. Griffenhagen published a series of illustrated articles in the _Journal of the American Pharmaceutical Association, Practical Pharmacy Edition_, which were later reprinted by the Association in a booklet entitled, _Tools of the Apothecary_. In it, he described several pharmaceutical specimens in the collection and their place in history.

Division of Medical Sciences (1957 to Present)

The U.S. National Museum was reorganized on July 1, 1957, into two units, the Natural History Museum and the Museum of History and Technology. At the same time, and in view of the widening scope of the Division, its more scientifically based planning, and the constantly increasing collection with equal emphasis on all branches of the healing arts, the Division's title was changed to the Division of Medical Sciences--the title it still bears in 1964. With the reorganization, the Department of Engineering and Industries, under which the Division fell administratively, was renamed the Department of Science and Technology of the Museum of History and Technology. It was also the first time since its establishment in 1881 that the Division had two curators, for on July 1, 1957, Dr. John B. Blake joined the staff.

As a result of these changes, the Division was subdivided into a Section of Pharmaceutical History and Health and a Section of Medical and Dental History. The former was planned to encompass the collections of _materia medica_, pharmaceutical equipment, and all material related to the history of pharmacy, toxicology, pharmacology, and biochemistry, as well as the Hall of Health which was opened November 2, 1957, and which emphasizes man's progressing knowledge of his body and the functions of its major organs.[18] The latter Section was planned to include all that belongs to the development of surgery, medicine, dentistry, and nursing, especially in relation to hospitals.

In October 1957, the Division acquired a collection of rare, ceramic, drug jars which included two, 13th-century, North Syrian and Persian, albarello-shaped, majolica jars; a 15th-century, Hispano-Moresque drug container; and a 16th-century, Italian faience, dragon-spout ewer. During the following two years, Curator Griffenhagen periodically toured museums and medical and pharmaceutical institutions in this country, South America, and Europe gathering specimens and information for the Division and for publication, respectively. However, on June 27, 1959, he resigned his curatorship to join the staff of the American Pharmaceutical Association in Washington, D.C. Dr. Blake became the curator in charge of the Division and Mr. Griffenhagen was succeeded on September 24, 1959, by the author of this paper as associate curator in charge of the Section of Pharmaceutical History and Health.

Dr. Blake, as curator of the Section of Medical and Dental History, acquired a large number of valuable and varied specimens for the Division's collections. They included optometric refracting instruments, an early 1920's General Electric, portable, x-ray machine, the Charles A. Lindbergh and Alexis Carrel pump (designed in 1935 to perfuse life-sustaining fluids to the organs of the body), the Sewell heart pump (1950) to control delivery of air pressure and suction to the pumping mechanism, and a large and valuable collection of dental equipment formerly at the universities of Pennsylvania and Illinois. Dr. Blake wrote the explanatory material and supervised the design and production of the majority of exhibits in the renovated hall of medical and dental history. He also contributed several scholarly articles and a book (see bibliography) on the history of the healing arts and public health in particular. He resigned on September 2, 1961, to join the staff of the National Library of Medicine as chief of the History of Medicine Division, and was succeeded by the author as curator of the Division. From the summer of 1962 to April 1964, the Division benefited from the expert advice of Dr. Alfred R. Henderson as consultant in the preparation and designing of the surgical and medical exhibits of the Museum of History and Technology.

During the period from 1961 to May 1964, the Division's collections expanded greatly through its medical, dental, and pharmaceutical acquisitions. Specimens of antiques acquired from 1961 through 1963 numbered up to 1,539 and included gifts from leading institutions and individual philanthropists. The scope of these gifts and acquisitions ranges from electronic resuscitators, microscopes, x-ray equipment, and spectacles, to patent medicines, amulets, apothecary tools, dental instruments, and office material of practitioners.

In the last decade, the interest in the national endeavor for promoting research and scholarship in the history of medicine has increased greatly. It was most appropriate, therefore, for the Smithsonian Institution to play host on May 2 for two sessions of the 37th annual meeting of the American Association for the History of Medicine held in the Washington, D.C., area from April 30 through May 2, 1964. In welcoming the members to the morning session in the auditorium of the new Museum of History and Technology, Frank A. Taylor, director of the United States National Museum, expressed the feeling that the meeting of the Association was, in a sense, a dedication of the new auditorium and an opportunity for the Smithsonian to reaffirm its deep interest and commitment in fostering research and furthering the appreciation of scholarly endeavor in the history of the healing arts.

A New Dimension For the Healing Arts

"One day the United States will have a National Museum of science, engineering, and industry, as most large nations have." This was the prediction made in 1946 by the director of the U.S. National Museum, Mr. Frank A. Taylor, then curator of the Division of Engineering.[19] It was in 1963, that the new $36,000,000 building of the Museum of History and Technology was completed, and opened to the public in 1964. The offices of the Division of Medical Sciences as well as the reference and study collections were moved to the fifth floor of the new building. The exhibits, however, will be displayed in the gallery at the southwest corner of the first floor. These exhibits, it is hoped, will show a new dimension and an unprecedented approach in displaying the development of the healing arts throughout the ages and the instruments and equipment associated with health professions. They also present the expanding objectives and plans of the Division's growth as an integral part of the Smithsonian Institution. Conveniently, the exhibits form four, closely connected halls in one large gallery which will be open to the public in the summers of 1965 to 1966.

1. THE HALL OF HEALTH displays models and graphic and historical exhibit materials to demonstrate the function of the various healthy organs of the human body. The main topics emphasized are: embryology and childbirth; tooth structure; the heart and blood circulation; respiration; the endocrine glands; kidneys and the urinary-excretory system; the brain and the nervous system; the ear; and vision and the use of eyeglasses.

The most appreciated exhibit of all in this Hall is the "transparent woman" figure which rotates, automatically, every 15 minutes with a recorded message describing the function of each major organ of the body at the same time that the organ is electronically lighted, so that the viewer can see its place in the body.

2. THE HALL OF MEDICINE AND DENTISTRY will depict the history of these two sciences with exhibits of the equipment used through the centuries. In the medical field, early trephining and other surgical instruments will be displayed along with a diorama of an 1805 surgical operation performed by Dr. Philip Syng Physick in the amphitheater of the Pennsylvania Hospital. Diagnostic instruments such as stethoscopes, endoscopes, speculums, and blood-pressure measuring devices will be exhibited with a series of microscopes illustrating the development of these instruments. Exhibits of original galvanometers and other apparatus will trace the development of cardiography. The early use of anesthesia will be shown by apparatus of William Morton and Crawford W. Long, American pioneers in this field. The development of the devices of modern medicine and surgery will be shown by exhibits of the iron lung and x-ray tubes, including a tube used by W. K. Roentgen. Medicine chests and surgical kits of different periods will graphically summarize the state of medical science in the period each represents.

Exhibits on the development of dentistry and dental surgery will display examples of tooth-filling and extracting tools, drilling apparatus from the early hand and foot engines to the first ultrasonic cutting instrument (1954), and the original contra-angle, hydraulic and air-turbine handpiece model[20] which revolutionized the field of instrumentation for dental surgery (with speeds of 200,000 to 400,000 rpm). This hydraulic turbine of Dr. Robert J. Nelson and associates of the National Bureau of Standards set the design pattern for the remarkable and successful high-speed, air-turbine handpiece developed by Paul H. Tanner and Oscar P. Nagel of the U.S. Naval Dental School in 1956. Also underway is the reconstruction of the offices of famous dentists such as G. V. Black and the father of American orthodontia, Edward H. Angle, using their original equipment and instruments. In addition, an exhibit is planned to include x-ray tubes and the electric dental engine, the first to be operated in a human mouth by the pioneer dentist on dental skiagraphy, Charles E. Kells (1856-1928).[21]

3. THE HALL OF PHARMACEUTICAL HISTORY will feature exhibits on the reconstruction of two pharmacy shops: an 18th-century apothecary shop, originally from Germany, with a very elegant collection of drug jars, decorated medicinal bottles, balances, mortars and pestles, and other tools and documents pertaining to the apothecary art, and a late 19th-century American drugstore with shelves filled with patent medicines and drug containers of various sizes and shapes. The window will also feature symbols of pharmacy and beautiful show globes. Displays will show the development of antibiotics and the early tools used in the manufacture of the so-called "miracle drugs," including a mold from Sir Alexander Fleming, the discoverer of penicillin. In addition, a platform will be reconstructed to display a variety of pharmaceutical apparatus used in the preparation and manufacture of drugs, such as tablet and capsule machines and drug mills and percolators. Recently, with the assistance of Professor Glenn Sonnedecker, the Division acquired a fine collection of pharmaceutical equipment and devices from the School of Pharmacy of the University of Wisconsin.

Since the Division houses the largest collection of _materia medica_ in the country, a representative cross section of crude drugs will be displayed in alphabetical order as well as a display illustrating the role of cinchona and antimalarial drugs in the fight against disease. An exhibit will portray the "origin of drugs" from the three natural kingdoms, animal, vegetable, and mineral, together with synthetic drugs including the manufacture of vitamins.

Plans are being made for an elaborate exhibit of weights and balances used in many countries throughout the centuries, their impact on accuracy of dosage and weighing of drugs, and their use in the apothecary art.

The Division will also display pictorial and printed materials, as well as artifacts from all periods and all countries. These collections are intended to help in presenting a more complete picture of the story of the medical sciences for educational purposes and research, and to increase man's knowledge in fighting disease and promoting health.

Thus, from a few hundred specimens of crude drugs in the Section of Materia Medica of 83 years ago, there has developed a Museum Division today which embraces the evolution of the health professions through the ages. This Division now has the largest collection in the Western Hemisphere of historical objects which are related to the healing arts. The reference collections are available to the researcher and scholar, and the exhibits are intended for pleasure and educational purposes in these fields. The plans for expansion have no limitation as we keep pace with man's progress in the medical sciences and continue to collect materials that contributed to the historical development in the fight against diseases and the attempts to secure better health for everyone.

BIBLIOGRAPHY

The _Annual report of the Board of Regents of the Smithsonian
Institution_ from 1872 to date and the _Proceedings of the
United States National Museum_ from 1881 to date were used
extensively as sources in this survey. In the latter, see in
particular, the year 1881, pp. 545-546; 1882, pp. 1-2; and 1884,
pp. 431-475.

ATKINSON, WILLIAM B. _The physicians and surgeons of the United States._ Philadelphia, 1878. [On Dr. Toner.]

BLAKE, JOHN B. Dental history and the Smithsonian Institution. _Journal of the American College of Dentists_ (1961), vol. 28, pp. 125-127.

---- _Public health in the town of Boston, 1630-1822._ Cambridge, Mass.: Harvard University Press, 1959.

[BRAISTED, WILLIAM C.] The biography of Dr. Beyer. Page 94 in _Dictionary of American medical biography_, by HOWARD A. KELLY and WALTER L. BURRAGE; NEW YORK: D. Appleton and Co., 1928.

CLARK, LEILA F. The library of the Smithsonian Institution. _Science_ (1946), vol. 104, p. 143.

COLEMAN, LAURENCE VAIL. _The museum in America: A critical study._ 3 vols. Baltimore: Waverly Press, 1939. [Printed for the American Association of Museums, Washington, D.C.] See vol. 1, pp. 3, 11-12, 32-33, 143-146, 222, 318; vol. 3, p. 471.

DAUKES, S. H. _The medical museum: modern developments, organization and technical methods based on a new system of visual teaching._ London: Wellcome Foundation Ltd., 1929.

DODRILL, FOREST D., and others. Pulmonary volvuloplasty under direct vision using the mechanical heart for a complete bypass of the right heart in a patient with congenital pulmonary stenosis. _Journal of Thoracic Surgery_ (1953), vol. 26, pp. 584-595.

---- Temporary mechanical substitution for the left ventricle in man. _Journal of the American Medical Association_ (1952), vol. 150, pp. 642-644.

DUNGLISON, ROBLEY. _A dictionary of medical science._ Rev. ed. Pp. 629-630. Philadelphia: Lea, 1874.

EDWARDS, J. J., and EDWARDS, M. J. _Medical museum technology._ London: Oxford University Press, 1959. [See in particular, pp. 33-62, 142-159.]

FERCHL, FRITZ. Die Moerser der Sammlung Jo Mayer--Wiesbaden; Libri rari et curiosi der Sammlung Dr. Jo Mayer--Wiesbaden; Bildnisse und Bilder der Sammlung Jo Mayer--Wiesbaden; Kuriositaeten und Antiquitaeten der Sammlung Jo Mayer--Wiesbaden; and Glaeser, Majoliken und Faensen der Sammlung Jo Mayer--Wiesbaden. _Pharmazeutische Zeitung_ (Berlin, 1930), vol. 75: January 4, no. 2, pp. 19-24; February 15, no. 14, pp. 219-223; March 8, no. 20, pp. 309-314; April 19, no. 32, pp. 487-489; and June 21, no. 50, pp. 735-740.

FLINT, JAMES M. Classification and arrangement of the materia medica collection. _Proceedings of the United States National Museum_ (1881), vol. 4, app. no. 6.

---- Classification of the materia medica collection of the United States National Museum, and catalogue of specimens. _Proceedings of the United States National Museum_ (1883), vol. 6, app. 19, pp. 431-475.

---- Directions for collecting information and objects illustrating the history of medicine. Part S of _Bulletin of the United States National Museum_ (1905). No. 39.

---- Memoranda for collectors of drugs for the materia medica section of the National Museum. _Proceedings of the United States National Museum_ (1881), vol. 4, app. 8.

FOLEY, MATTHEW O. Smithsonian Institution devotes much space to hospital exhibit. _Hospital Management_ (April 1929), pp. 271-287.

GALDSTON, IAGO. Research in the United States. _Ciba Symposia_ (June-July 1946), vol. 8, nos. 3 and 4, p. 366.

GARRISON, FIELDING H. _An introduction to the history of medicine._ 2d ed. p. 38. Philadelphia: Saunders, 1917.

GEBHARD, BRUNO. From medicine show to health museum. _Ciba Symposia_ (March 1947), vol. 8, no. 12, p. 579.

GOODE, GEORGE BROWN. _The Smithsonian Institution (1846-1896): The history of its first half century._ Pp. 325-329, 362-363. Washington, 1897.

GRIFFENHAGEN, GEORGE. _Pharmacy museums._ Madison, Wis.: American Institute of the History of Pharmacy, 1956.

---- and HUGHES, CALVIN H. The history of the mechanical heart. _Annual report of the Board of Regents of the Smithsonian Institution for the year ended June 30, 1955_ (Washington, 1956), pp. 339-356.

HAMARNEH, SAMI. At the Smithsonian ... exhibits on pharmaceutical dosage forms. _Journal of the American Pharmaceutical Association_ (1962), new ser., vol. 2, pp. 478-479.

----For the collector, facts and artifacts. _Pharmacy in History_ (1961), vol. 6, p. 48.

---- Historical and educational exhibits on dentistry at the Smithsonian Institution. _Journal of the American-Dental Association_ (July 1962), vol. 65, pp. 111-114.

---- New dental exhibits at the Smithsonian Institution. _Journal of the American Dental Association_ (May 1963), vol. 66, pp. 676-678.

HAYNES, WILLIAM. Out of alchemy into chemistry. _The Scientific Monthly_ (November 1952), vol. 75, p. 268.

HOLT, L. EMMETT. A sketch of the development of the Rockefeller Institute for Medical Research. _Science_ (July 6, 1906), new ser., vol. 24, no. 601, p. 1.

HOWELL, WILLIAM H. The American Physiological Society during its first twenty-five years. Pp. 21-22 [biography of Dr. Beyer] in _History of the American Physiological Society semicentennial, 1881-1937_; Baltimore, 1938.

[KLEIN, ALLEN.] He directs pharmacy exhibits at the Smithsonian Institution. _Modern Pharmacy_ (July 1941), vol. 25, pp. 20-21.

LAWALL, CHARLES H. Ancient pharmacy on display. _Pacific Drug Review_ (1933), vol. 45, p. 18.

---- _The curious lore of drugs and medicines._ Garden City, N.Y.: Garden City Publishing Co., Inc., 1927. [See p. 453 on Division of Medical Sciences' collection.]

LEWTON, FREDERICK L. A national pharmaceutical collection. _Journal of the American Pharmaceutical Association_ (1919), vol. 8, pp. 45-46.

---- The opportunity for developing historical pharmacy collections at the National Museum. _Journal of the American Pharmaceutical Association_ (1917), vol. 6, pp. 259-262.

LONG, ESMOND R. The Army Medical Museum. _Military Medicine_ (May 1963), vol. 128, pp. 367-369.

MONELL, S. H. "Dental Skiagraphy" (pp. 313-336 in _A system in x-ray methods and medical uses of light hot-air, vibration and high-frequency currents_ by Monell; New York: Pelton, 1902).

MURRAY, DAVID. _Museums, their history and their use._ Glasgow: MacLehose, 1904. [See vol. 1, pp. 13-77.]

NELSON, ROBERT J.; PELANDER, CARL E.; and KUMPULA, JOHN W. Hydraulic turbine, contra-angle handpiece. _Journal of the American Dental Association_ (September 1953), vol. 47, pp. 324-329.

_Official Catalogue of the Cotton States and International Exposition_: Atlanta, Georgia, September 18 to December 31, 1895. Atlanta: Claflin and Mellichamp, 1895. [See p. 204.]

PACKARD, FRANCES R. _History of medicine in the United States._ New York, 1931. [See vol. 1, pp. 5-6, 37-51, 168-176, 602-607 on Dr. Toner.]

PICKARD, MADGE E. Government and science in the United States: Historical background. _Journal of the History of Medicine and Allied Sciences_ (1946), vol. 1, nos. 2 and 3, pp. 265-266, 289, 446-447, 478.

PURTLE, HELEN R. Notes on the Medical Museum of the Armed Forces Institute of Pathology. _Bulletin of the Medical Library Association_ (1956), vol. 44, no. 3, pp. 300-305.

RATHBUN, RICHARD. _A descriptive account of the building recently erected for the Departments of Natural History of the United States National Museum._ (U.S. National Museum Bulletin 80.) Washington, 1913. [See pp. 7-15.]

RHEES, WILLIAM J. _The Smithsonian Institution; documents relative to its origin and history, 1835-1899._ 2 vols. (Smithsonian Miscellaneous Collections: vol. 42, _1835-1881_; vol. 43, _1881-1899_.) Washington, 1901.

SHUFELDT, R. W. Suggestions for a national museum of medicine. _Medical Record_ (March 22, 1919), pp. 4-5. [Also reprinted, 1919, by William Wood and Co., New York.]

SIGERIST, HENRY E. _Primitive and archaic medicine._ (Vol. 1 of _A history of medicine_, by Sigerist.) New York: Oxford University Press, 1951. [See pp. 525-531.]

SILVER, EDWIN H. Description of the exhibit on conservation of vision placed in the United States Museum at Washington, D.C. _The Optical Journal and Review of Optometry_ (February 3, 1927), vol. 59, no. 5, pp. 39-40.

[SONNEDECKER, GLENN.] Apothecary shop nears completion. _Journal of the American Pharmaceutical Association, Practical Pharmacy Edition_ (1946), vol. 7, pp. 157.

---- Dr. Charles Whitebread, pharmacist and museum curator. _Journal of the American Pharmaceutical Association, Practical Pharmacy Edition_ (1946), vol. 7, p. 203.

---- Old apothecary shop. _Journal of the American Pharmaceutical Association, Practical Pharmacy Edition_ (1945), vol. 6, pp. 184-187.

---- Old apothecary shop opened. _Journal of the American Pharmaceutical Association, Practical Pharmacy Edition_ (1946), vol. 7, p. 427.

TAYLOR, FRANK A. A national museum of science, engineering and industry. _The Scientific Monthly_ (1946), vol. 63, pp. 359.

---- The background of the Smithsonian's Museum of Engineering and Industries. _Science_ (1946), vol. 104, no. 2693, pp. 130-132.

Toner Lectures:

1. J. J. WOODWARD. On the structure of cancerous tumors and the
mode in which adjacent parts are invaded. No. 266 in _Smithsonian
Miscellaneous Collections_, vol. 15; Washington, 1878. [Lecture
given on March 28, 1873.]

2. C. E. BROWN-SEQUARD. Dual character of the brain. No. 291 in
_Smithsonian Miscellaneous Collections_, vol. 15; Washington,
1878. [Lecture given on April 22, 1874.]

3. J. M. DA COSTA. On strain and over-action of the heart. No.
279 in _Smithsonian Miscellaneous Collections_, vol. 15;
Washington, 1878. [Lecture given on May 14, 1874.]

4. H. C. WOOD. A study of the nature and mechanism of fever. No.
282 in _Smithsonian Miscellaneous Collections_, vol. 15;
Washington, 1878. [Lecture given on January 20, 1875.]

5. WILLIAM W. KEEN. On the surgical complications and sequels of
the continued fevers. No. 300 in _Smithsonian Miscellaneous
Collections_, vol. 15; Washington, 1878. [Lecture given on
February 17, 1876.]

6. WILLIAM ADAMS. Subcutaneous surgery: Its principles, and its
recent extension in practice. No. 302 in _Smithsonian
Miscellaneous Collections_, vol. 15; Washington, 1878. [Lecture
given on September 13, 1876.]

7. EDWARD O. SHAKESPEARE. The nature of reparatory inflammation
in arteries after ligatures, acupressure, and torsion. No. 321 in
_Smithsonian Miscellaneous Collections_, vol. 16; Washington,
1880. [Lecture given on June 27, 1878.]

8. GEORGE E. WARING. Suggestions for the sanitary drainage of
Washington City. No. 349 in _Smithsonian Miscellaneous
Collections_, vol. 26; Washington, 1883. [Lecture given on May
26, 1880.]

9. CHARLES K. MILLS. Mental over-work and premature disease among
public and professional men. No. 594 in _Smithsonian
Miscellaneous Collections_, vol. 34; Washington, 1893. [Lecture
given on March 19, 1884.]

10. HARRISON ALLEN. A clinical study of the skull. No. 708 in
_Smithsonian Miscellaneous Collections_, vol. 34; Washington,
1893. [Lecture given on May 29, 1889.]

TRUE, WEBSTER P. _The Smithsonian Institution._ (Vol. 1 of the Smithsonian Scientific Series.) Washington, 1929.

URDANG, GEORGE, and NITARDY, F. W. _The Squibb ancient pharmacy._ New York, 1940. [Out of print, but remaining catalogs were given to the Division of Medicine to "be reserved for pharmaceutical educators, foreign dignitaries, pharmacists of national and international reputation, and pharmaceutical historians," according to a letter from Mr. Nitardy in 1945.]

WHITEBREAD, CHARLES. Animal pharmaceuticals of the past and present. _Journal of the American Pharmaceutical Association_ (1933), vol. 22, pp. 431-437.

---- An old apothecary shop of 1750. _National Capital Pharmacist_ (September 1946), vol. 8, pp. 11-13, 35.

---- Early American pharmaceutical inventions. _Journal of the American Pharmaceutical Association_ (1937), vol. 26, pp. 918-928.

---- _Handbook of the health exhibits of the United States National Museum._ Baltimore: Lord Baltimore Press [1924].

---- Health superstitions. _Journal of the American Pharmaceutical Association, Practical Pharmacy Edition_ (1942), vol. 3, pp. 268-274.

---- Medicine making as depicted by museum dioramas. _Journal of the American Pharmaceutical Association_ (January 1936), vol. 25, pp. 40-46.

---- Superstition, credulity and skepticism. _Journal of the American Pharmaceutical Association_ (1933), vol. 22, pp. 1140-1145.

---- The Indian medical exhibit of the Division of Medicine in the United States National Museum. Article 10 in vol. 67 of _Proceedings of the U.S. National Museum_; Washington, 1926.

---- The magic, psychic, ancient Egyptian, Greek, and Roman medical collections of the Division of Medicine in the United States National Museum. Article 15 in vol. 65 of _Proceedings of the U.S. National Museum_; Washington, 1925.

---- The odd origin of medical discoveries. _Journal of the American Pharmaceutical Association, Practical Pharmacy Edition_ (1943), vol. 4, p. 321.

---- The United States National Museum pharmaceutical collection, its aims, problems, and accomplishments. _Journal of the American Pharmaceutical Association_ (1930), vol. 19, pp. 1125-1126.

WINTERS, S. R. Magic medicine. _Hygeia_ (July 1937), vol. 15, pp. 630-633.

* * * * *

FOOTNOTES

[1] _Annual Report of the Board of Regents of the Smithsonian
Institution for the Year 1882_ [hereinafter referred to as the
_Smithsonian Annual Report_], pp. 101-103; and introductory
"advertisement" to the lectures published by the Smithsonian
Institution in its Miscellaneous Collections (see bibliography).

[2] Dr. J. J. Woodward's lecture explained the progress of medical
knowledge of morbid growth and cancerous tumors from 1865 to 1872.
It cautioned that uncertain methods of diagnosis at that time
allowed charlatans and uneducated practitioners to report cures of
cancer in instances where nonmalignant growths were "removed by
their caustic pastes and plasters."

[3] The two longest intervals were in preparing the last two lectures:
the ninth in 1884, and the tenth, 1889. Both came after the
establishment in 1881 of the Section of Materia Medica in the U.S.
National Museum, to display the development and progress of the
health professions.

[4] _Annual Report of the Secretary of the Navy for the year 1883_,
pp. 190, 614-615.

[5] For classifying chemical compounds, Dr. Flint relied on the work
of H. E. Roscoe and C. Schorlemmez, _A Treatise on Chemistry_, 2
vols. (New York: D. Appleton, 1878-1800.)

[6] _Annual Report of the Secretary of the Navy for the year 1882_,
vol. 2, part 2, pp. 100, 228, 656-657. Dr. Flint in his article
"Report on Pharmacopoeias of All Nations," ibid., pp. 655-680,
remarks that there were then 19 official pharmacopoeias in the
world, besides three semiofficial formularies in certain
localities in Italy. The pharmacopoeias collected represent
Austria, Belgium, France, Germany, Great Britain, Greece, Holland,
India, Mexico, Norway, Portugal, Spain, Sweden, Switzerland (two),
and the United States.

[7] The _Universal Formulary_, by R. Eglesfeld Griffith, first edited
in March 1850 (3rd ed. rev. and enlarged by John M. Maisch,
Philadelphia: Lea, 1874) should not be considered an international
drug standard. It was mainly concerned with compiling a great
number of formulas and recipes, methods of preparing and
administering official and other medicines, and tables on weights
and measures for utilization by the U.S. practitioners of the
time.

[8] Other elaborate arrangements were also made to improve and expand
the Section's activities and services, though some have never
materialized. For example, a herbarium was suggested from which
specimens could be obtained for display of the actual drug with
painted pictures of its plant next to it. Consideration was given
to displaying enlarged drawings to show the minute structure of
the specimen for better identification. In addition, an exhibition
of several 10-liter vessels of the most popular mineral waters was
planned. The amount of saline substances which analysis had shown
to be present in each vessel was to be listed in a table to be
attached to that vessel, or the same amount of minerals was to be
put in a small bottle beside it. This plan was carried out to the
best advantage at the Cotton States and International Exposition
held in 1895 in Atlanta, Georgia.

[9] HOLT, "A Sketch of the Development of the Rockefeller Institute
for Medical Research," p. 1. A similar comment was voiced by
GALDSTON, "Research in the United States," p. 366.

[10] _Journal of the American Pharmaceutical Association_ (1918), vol.
7, pp. 376-377, 466.

[11] Two decades later, Dr. Whitebread designed a panel showing
photographs of famous medical pioneers of all nationalities. See
his article, "The Odd Origin of Medical Discoveries," p. 321.

[12] GEBHARD, "From Medicine Show to Health Museum," p. 579. The
original plan for this Hall of Health was to feature exhibits on
public health for popular educational purposes, including an
illustrated exhibit on hospital care. See FOLEY, "Smithsonian
Institution Devotes Much Space to Hospital Exhibit," pp. 43-44.

[13] Lack of space notwithstanding, valuable accessions were added
about 1930, including a collection of early x-ray tubes and
personal memorabilia of Drs. William T. G. Morton (1819-1868),
Crawford W. Long (1815-1878), and William Gorgas (1854-1920).

[14] D. RILEY MOORE published a series of short reports under the
title "Committee on Osteopathic Exhibits in the U.S. National
Museum," in the _Journal of the American Osteopathic Association_
(1933-1946), vols. 33-46, regarding the exhibit on osteopathy.

[15] [KLEIN], "He Directs Pharmacy Exhibits at the Smithsonian
Institution," pp. 20-21.

[16] Several other journals reported the exhibition with
illustrations: _Drug Topics_ (July 8, 1946), vol. 90, no. 2, pp.
2, 79; _National Capital Pharmacist_ (September 1945), vol. 7, p.
11, and (September 1946), vol. 8, pp. 11-13; and _The Scientific
Monthly_ (November 1952), vol. 75, p. 268.

[17] DODRILL, and others, "Temporary Mechanical Substitution for the
Left Ventricle in Man," pp. 642-644, and "Pulmonary Volvuloplasty
under Direct Vision using the Mechanical Heart for a Complete
Bypass of the Right Heart in a Patient with Congenital Pulmonary
Stenosis," pp. 584-595.

[18] For the design, expert arrangement of the exhibits, and the
legends that accompany each exhibit in the Hall of Health, we are
indebted to Drs. Bruno Gebhard, Richards H. Shryock, Thomas G.
Hull, James Laster, Walle J. H. Nauta, Leslie W. Knott, Theodore
Wiprud, and other physicians, dentists, and scholars who have
offered their advice, assistance, and expert skills.

[19] TAYLOR, "A National Museum of Science, Engineering and Industry,"
p. 359.

[20] NELSON, PELANDER, and KUMPULA, "Hydraulic Turbine, Contra-angle
Handpiece," pp. 324-329.

[21] MONELL, "Dental Skiagraphy," pp. 313-336.

* * * * *

Paper 43 - Transcriber's Note

Page 277: "the basis of scientific, historical"
was "the bases of scientific, historical"

* * * * *

CONTRIBUTIONS FROM THE
MUSEUM OF HISTORY AND TECHNOLOGY:

PAPER 44

DEVELOPMENT OF GRAVITY PENDULUMS IN THE 19TH CENTURY

by

Victor F. Lenzen and Robert P. Multhauf

GALILEO, HUYGENS, AND NEWTON 304

FIGURE OF THE EARTH 306

EARLY TYPES OF PENDULUMS 309

KATER'S CONVERTIBLE AND INVARIABLE PENDULUMS 314

REPSOLD-BESSEL REVERSIBLE PENDULUM 320

PEIRCE AND DEFFORGES INVARIABLE, REVERSIBLE PENDULUMS 327

VON STERNECK AND MENDENHALL PENDULUMS 331

ABSOLUTE VALUE OF GRAVITY AT POTSDAM 338

APPLICATION OF GRAVITY SURVEYS 342

SUMMARY 346

_Victor F. Lenzen and Robert P. Multhauf_

DEVELOPMENT OF GRAVITY PENDULUMS IN THE 19th CENTURY

_The history of gravity pendulums dates back to the time of
Galileo. After the discovery of the variation of the force of
gravity over the surface of the earth, gravity measurement
became a major concern of physics and geodesy. This article
traces the history of the development of instruments for this
purpose._

THE AUTHORS: _Victor F. Lenzen is Professor of Physics,
Emeritus, at the University of California at Berkeley and Robert
P. Multhauf is Chairman of the Department of Science and
Technology in the Smithsonian Institution's Museum of History
and Technology._

The intensity of gravity, or the acceleration of a freely falling body, is an important physical quantity for the several physical sciences. The intensity of gravity determines the weight of a standard pound or kilogram as a standard or unit of force. In physical experiments, the force on a body may be measured by determining the weight of a known mass which serves to establish equilibrium against it. Thus, in the absolute determination of the ampere with a current balance, the force between two coils carrying current is balanced by the earth's gravitational force upon a body of determinable mass. The intensity of gravity enters into determinations of the size of the earth from the angular velocity of the moon, its distance from the earth, and Newton's inverse square law of gravitation and the laws of motion. Prediction of the motion of an artificial satellite requires an accurate knowledge of gravity for this astronomical problem.

The gravity field of the earth also provides data for a determination of the figure of the earth, or geoid, but for this problem of geodesy relative values of gravity are sufficient. If g is the intensity of gravity at some reference station, and [Delta]g is the difference between intensities at two stations, the values of gravity in geodetic calculations enter as ratios ([Delta]g)/g over the surface of the earth. Gravimetric investigations in conjunction with other forms of geophysical investigation, such as seismology, furnish data to test hypotheses concerning the internal structure of the earth.

Whether the intensity of gravity is sought in absolute or relative measure, the most widely used instrument for its determination since the creation of classical mechanics has been the pendulum. In recent decades, there have been invented gravity meters based upon the principle of the spring, and these instruments have made possible the rapid determination of relative values of gravity to a high degree of accuracy. The gravity meter, however, must be calibrated at stations where the absolute value of gravity has been determined by other means if absolute values are sought. For absolute determinations of gravity, the pendulum historically has been the principal instrument employed. Although alternative methods of determining absolute values of gravity are now in use, the pendulum retains its value for absolute determinations, and even retains it for relative determinations, as is exemplified by the Cambridge Pendulum Apparatus and that of the Dominion Observatory at Ottawa, Ontario.

The pendulums employed for absolute or relative determinations of gravity have been of two basic types. The first form of pendulum used as a physical instrument consisted of a weight suspended by a fiber, cord, or fine wire, the upper end of which was attached to a fixed support. Such a pendulum may be called a "simple" pendulum; the enclosure of the word simple by quotation marks is to indicate that such a pendulum is an approximation to a simple, or mathematical pendulum, a conceptual object which consists of a mass-point suspended by a weightless inextensible cord. If l is the length of the simple pendulum, the time of swing (half-period in the sense of physics) for vibrations of infinitely small amplitude, as derived from Newton's laws of motion and the hypothesis that weight is proportional to mass, is T = [pi][sqrt](l/g).

The second form of pendulum is the compound, or physical, pendulum. It consists of an extended solid body which vibrates about a fixed axis under the action of the weight of the body. A compound pendulum may be constituted to oscillate about one axis only, in which case it is nonreversible and applicable only for relative measurements. Or a compound pendulum may be constituted to oscillate about two axes, in which case it is reversible (or "convertible") and may be used to determine absolute values of gravity. Capt. Henry Kater, F.R.S., during the years 1817-1818 was the first to design, construct, and use a compound pendulum for the absolute determination of gravity. He constructed a convertible pendulum with two knife edges and with it determined the absolute value of gravity at the house of Henry Browne, F.R.S., in Portland Place, London. He then constructed a similar compound pendulum with only one knife edge, and swung it to determine relative values of gravity at a number of stations in the British Isles. The 19th century witnessed the development of the theory and practice of observations with pendulums for the determination of absolute and relative values of gravity.

Galileo, Huygens, and Newton

The pendulum has been both an objective and an instrument of physical investigation since the foundations of classical mechanics were fashioned in the 17th century.[1] It is tradition that the youthful Galileo discovered that the period of oscillation of a pendulum is constant by observations of the swings of the great lamp suspended from the ceiling in the cathedral of Pisa.[2] The lamp was only a rough approximation to a simple pendulum, but Galileo later performed more accurate experiments with a "simple" pendulum which consisted of a heavy ball suspended by a cord. In an experiment designed to confirm his laws of falling bodies, Galileo lifted the ball to the level of a given altitude and released it. The ball ascended to the same level on the other side of the vertical equilibrium position and thereby confirmed a prediction from the laws. Galileo also discovered that the period of vibration of a "simple" pendulum varies as the square root of its length, a result which is expressed by the formula for the time of swing of the ideal simple pendulum. He also used a pendulum to measure lapse of time, and he designed a pendulum clock. Galileo's experimental results are important historically, but have required correction in the light of subsequent measurements of greater precision.

Mersenne in 1644 made the first determination of the length of the seconds pendulum,[3] that is, the length of a simple pendulum that beats seconds (half-period in the sense of physics). Subsequently, he proposed the problem to determine the length of the simple pendulum equivalent in period to a given compound pendulum. This problem was solved by Huygens, who in his famous work _Horologium oscillatorium_ ... (1673) set forth the theory of the compound pendulum.[4]

Huygens derived a theorem which has provided the basis for the employment of the reversible compound pendulum for the absolute determination of the intensity of gravity. The theorem is that a given compound pendulum possesses conjugate points on opposite sides of the center of gravity; about these points, the periods of oscillation are the same. For each of these points as center of suspension the other point is the center of oscillation, and the distance between them is the length of the equivalent simple pendulum. Earlier, in 1657, Huygens independently had invented and patented the pendulum clock, which rapidly came into use for the measurement of time. Huygens also created the theory of centripetal force which made it possible to calculate the effect of the rotation of the earth upon the observed value of gravity.

The theory of the gravity field of the earth was founded upon the laws of motion and the law of gravitation by Isaac Newton in his famous _Principia_ (1687). It follows from the Newtonian theory of gravitation that the acceleration of gravity as determined on the surface of the earth is the resultant of two factors: the principal factor is the gravitational attraction of the earth upon bodies, and the subsidiary factor is the effect of the rotation of the earth. A body at rest on the surface of the earth requires some of the gravitational attraction for the centripetal acceleration of the body as it is carried in a circle with constant speed by the rotation of the earth about its axis. If the rotating earth is used as a frame of reference, the effect of the rotation is expressed as a centrifugal force which acts to diminish the observed intensity of gravity.

* * * * *

GLOSSARY OF GRAVITY TERMINOLOGY

ABSOLUTE GRAVITY: the value of the acceleration of gravity, also expressed by the length of the seconds pendulum.

RELATIVE GRAVITY: the value of the acceleration of gravity relative to the value at some standard point.

SIMPLE PENDULUM: see theoretical pendulum.

THEORETICAL PENDULUM: a heavy bob (point-mass) at the end of a weightless rod.

SECONDS PENDULUM: a theoretical or simple pendulum of such length that its time of swing (half-period) is one second. (This length is about one meter.)

GRAVITY PENDULUM: a precisely made pendulum used for the measurement of gravity.

COMPOUND PENDULUM: a pendulum in which the supporting rod is not weightless; in other words, any actual pendulum.

CONVERTIBLE PENDULUM: a compound pendulum having knife edges at different distances from the center of gravity. Huygens demonstrated (1673) that if such a pendulum were to swing with equal periods from either knife edge, the distance between those knife edges would be equal to the length of a theoretical or simple pendulum of the same period.

REVERSIBLE PENDULUM: a convertible pendulum which is also symmetrical in form.

INVARIABLE PENDULUM: a compound pendulum with only one knife edge, used for relative measurement of gravity.

* * * * *

From Newton's laws of motion and the hypothesis that weight is proportional to mass, the formula for the half-period of a simple pendulum is given by T = [pi][sqrt](l/g). If a simple pendulum beats seconds, 1 = [pi][sqrt]([lambda]/g), where [lambda] is the length of the seconds pendulum. From T = [pi][sqrt](l/g) and 1 = [pi][sqrt]([lambda]/g), it follows that [lambda] = l/T^{2}. Then g = [pi]^{2}[lambda]. Thus, the intensity of gravity can be expressed in terms of the length of the seconds pendulum, as well as by the acceleration of a freely falling body. During the 19th century, gravity usually was expressed in terms of the length of the seconds pendulum, but present practice is to express gravity in terms of g, for which the unit is the gal, or one centimeter per second per second.

Figure of the Earth

A principal contribution of the pendulum as a physical instrument has been the determination of the figure of the earth.[5] That the earth is spherical in form was accepted doctrine among the ancient Greeks. Pythagoras is said to have been the first to describe the earth as a sphere, and this view was adopted by Eudoxus and Aristotle.

The Alexandrian scientist Eratosthenes made the first estimate of the diameter and circumference of a supposedly spherical earth by an astronomical-geodetic method. He measured the angle between the directions of the rays of the sun at Alexandria and Syene (Aswan), Egypt, and estimated the distance between these places from the length of time required by a caravan of camels to travel between them. From the central angle corresponding to the arc on the surface, he calculated the radius and hence the circumference of the earth. A second measurement was undertaken by Posidonius, who measured the altitudes of stars at Alexandria and Rhodes and estimated the distance between them from the time required to sail from one place to the other.

With the decline of classical antiquity, the doctrine of the spherical shape of the earth was lost, and only one investigation, that by the Arabs under Calif Al-Mamun in A.D. 827, is recorded until the 16th century. In 1525, the French mathematician Fernel measured the length of a degree of latitude between Paris and Amiens by the revolutions of the wheels of his carriage, the circumference of which he had determined. In England, Norwood in 1635 measured the length of an arc between London and York with a chain. An important forward step in geodesy was the measurement of distance by triangulation, first by Tycho Brahe, in Denmark, and later, in 1615, by Willebrord Snell, in Holland.

Of historic importance, was the use of telescopes in the triangulation for the measurement of a degree of arc by the Abbe Jean Picard in 1669.[6] He had been commissioned by the newly established Academy of Sciences to measure an arc corresponding to an angle of 1 deg., 22', 55" of the meridian between Amiens and Malvoisine, near Paris. Picard proposed to the Academy the measurement of the meridian of Paris through all of France, and this project was supported by Colbert, who obtained the approval of the King. In 1684, Giovanni-Domenico Cassini and De la Hire commenced a trigonometrical measure of an arc south of Paris; subsequently, Jacques Cassini, the son of Giovanni-Domenico, added the arc to the north of Paris. The project was completed in 1718. The length of a degree of arc south of Paris was found to be greater than the length north of Paris. From the difference, 57,097 toises[7] minus 56,960 toises, it was concluded that the polar diameter of the earth is larger than the equatorial diameter, i.e., that the earth is a prolate spheroid (fig. 3).

Meanwhile, Richer in 1672 had been sent to Cayenne, French Guiana, to make astronomical observations and to measure the length of the seconds pendulum.[8] He took with him a pendulum clock which had been adjusted to keep accurate time in Paris. At Cayenne, however, Richer found that the clock was retarded by 2 minutes and 28 seconds per day (fig. 1). He also fitted up a "simple" pendulum to vibrate in seconds and measured the length of this seconds pendulum several times every week for 10 months. Upon his return to Paris, he found that the length of the "simple" pendulum which beat seconds at Cayenne was 1-1/4 Paris lines[9] shorter than the length of the seconds pendulum at Paris. Huygens explained the reduction in the length of the seconds pendulum--and, therefore, the lesser intensity of gravity at the equator with respect to the value at Paris--in terms of his theory of centripetal force as applied to the rotation of the earth and pendulum.[10]

A more complete theory was given by Newton in the _Principia_.[11] Newton showed that if the earth is assumed to be a homogeneous, mutually gravitating fluid globe, its rotation will result in a bulging at the equator. The earth will then have the form of an oblate spheroid, and the intensity of gravity as a form of universal gravitation will vary with position on the surface of the earth. Newton took into account gravitational attraction and centrifugal action, and he calculated the ratio of the axes of the spheroid to be 230:229. He calculated and prepared a table of the lengths of a degree of latitude and of the seconds pendulum for every 5 deg. of latitude from the equator to the pole. A discrepancy between his predicted length of the seconds pendulum at the equator and Richer's measured length was explained by Newton in terms of the expansion of the scale with higher temperatures near the equator.

Newton's theory that the earth is an oblate spheroid was confirmed by the measurements of Richer, but was rejected by the Paris Academy of Sciences, for it contradicted the results of the Cassinis, father and son, whose measurements of arcs to the south and north of Paris had led to the conclusion that the earth is a prolate spheroid. Thus, a controversy arose between the English scientists and the Paris Academy. The conflict was finally resolved by the results of expeditions sent by the Academy to Peru and Sweden. The first expedition, under Bouguer, La Condamine, and Godin in 1735, went to a region in Peru, and, with the help of the Spaniard Ullo, measured a meridian arc of about 3 deg. 7' near Quito, now in Ecuador.[12] The second expedition, with Maupertuis and Clairaut in 1736, went to Lapland within the Arctic Circle and measured an arc of about 1 deg. in length.[13] The northern arc of 1 deg. was found to be longer than the Peruvian arc of 1 deg., and thus it was confirmed that the earth is an oblate spheroid, that is, flattened at the poles, as predicted by the theory of Newton.

After 1735, the clock was used only to time the swing of a detached pendulum, by the method of "coincidences." In this method, invented by J. J. Mairan, the length of the detached pendulum is first accurately measured, and the clock is corrected by astronomical observation. The detached pendulum is then swung before the clock pendulum as shown here. The two pendulums swing more or less out of phase, coming into coincidence each time one has gained a vibration. By counting the number of coincidences over several hours, the period of the detached pendulum can be very accurately determined. The length and period of the detached pendulum are the data required for the calculation of the force of gravity.]

The period from Eratosthenes to Picard has been called the spherical era of geodesy; the period from Picard to the end of the 19th century has been called the ellipsoidal period. During the latter period the earth was conceived to be an ellipsoid, and the determination of its ellipticity, that is, the difference of equatorial radius and polar radius divided by the equatorial radius, became an important geodetic problem. A significant contribution to the solution of this problem was made by determinations of gravity by the pendulum.

An epoch-making work during the ellipsoidal era of geodesy was Clairaut's treatise, _Theorie de la figure de la terre_.[14] On the hypothesis that the earth is a spheroid of equilibrium, that is, such that a layer of water would spread all over it, and that the internal density varies so that layers of equal density are coaxial spheroids, Clairaut derived a historic theorem: If [gamma]_{E}, [gamma]_{P} are the values of gravity at the equator and pole, respectively, and c the centrifugal force at the equator divided by [gamma]_{E}, then the ellipticity [alpha] = (5/2)c - ([gamma]_{P} - [gamma]_{E})/[gamma]_{E}.

Laplace showed that the surfaces of equal density might have any nearly spherical form, and Stokes showed that it is unnecessary to assume any law of density as long as the external surface is a spheroid of equilibrium.[15] It follows from Clairaut's theorem that if the earth is an oblate spheroid, its ellipticity can be determined from relative values of gravity and the absolute value at the equator involved in c. Observations with nonreversible, invariable compound pendulums have contributed to the application of Clairaut's theorem in its original and contemporary extended form for the determination of the figure and gravity field of the earth.

Early Types of Pendulums

The pendulum employed in observations of gravity prior to the 19th century usually consisted of a small weight suspended by a filament (figs. 4-6). The pioneer experimenters with "simple" pendulums changed the length of the suspension until the pendulum beat seconds. Picard in 1669 determined the length of the seconds pendulum at Paris with a "simple" pendulum which consisted of a copper ball an inch in diameter suspended by a fiber of pite from jaws (pite was a preparation of the leaf of a species of aloe and was not affected appreciably by moisture).

A celebrated set of experiments with a "simple" pendulum was conducted by Bouguer[16] in 1737 in the Andes, as part of the expedition to measure the Peruvian arc. The bob of the pendulum was a double truncated cone, and the length was measured from the jaw suspension to the center of oscillation of the thread and bob. Bouguer allowed for change of length of his measuring rod with temperature and also for the buoyancy of the air. He determined the time of swing by an elementary form of the method of coincidences. The thread of the pendulum was swung in front of a scale and Bouguer observed how long it took the pendulum to lose a number of vibrations on the seconds clock. For this purpose, he noted the time when the beat of the clock was heard and, simultaneously, the thread moved past the center of the scale. A historic aspect of Bouguer's method was that he employed an "invariable" pendulum, that is, the length was maintained the same at the various stations of observation, a procedure that has been described as having been invented by Bouguer.

Since T = [pi][sqrt](l/g), it follows that (T_{1})^{2}/(T_{2})^{2} = g_{2}/g_{1}. Thus, if the absolute value of gravity is known at one station, the value at any other station can be determined from the ratio of the squares of times of swing of an invariable pendulum at the two stations. From the above equation, if T_{1} is the time of swing at a station where the intensity of gravity is g, and T_{2} is the time at a station where the intensity is g + [Delta]g, then [Delta]g/g = (T_{1})^{2}/(T_{2})^{2} - 1.

Bouguer's investigations with his invariable pendulum yielded methods for the determination of the internal structure of the earth. On the Peruvian expedition, he determined the length of the seconds pendulum at three stations, including one at Quito, at varying distances above sea level. If values of gravity at stations of different elevation are to be compared, they must be reduced to the same level, usually to sea level. Since gravity decreases with height above sea level in accordance with the law of gravitation, a free-air reduction must be applied to values of gravity determined above the level of the sea. Bouguer originated the additional reduction for the increase in gravity on a mountain or plateau caused by the attraction of the matter in a plate. From the relative values of gravity at elevated stations in Peru and at sea level, Bouguer calculated that the mean density of the earth was 4.7 times greater than that of the _cordilleras_.[17] For greater accuracy in the study of the internal structure of the earth, in the 19th century the Bouguer plate reduction came to be supplemented by corrections for irregularities of terrain and by different types of isostatic reduction.