Chapter V: Front Matter (5)

The whole heavens, north and south alike, have been divided into zones, and the chief observatories on the earth’s surface are busy night after night in taking photographs of that part intrusted to them. The whole heavens are thus being made to write their autobiography, and the total gain to the astronomy of the future of this most priceless record can perhaps be scarcely grasped as yet, although the advantage of being able at any point of future time to see on a photographic plate what the heavens are telling now is sufficiently obvious.

Catalogues of the stars have, of course, led to other minor catalogues of various classes of stars, binary, variable, and the like. In the later years catalogues of stars according to their spectra have enriched science.

The first extensive catalogue of stellar spectra was published by Vogel. It dealt with four thousand and fifty-one stars, and appeared in 1883; it has since been followed by the Draper catalogue, based upon photographs of the spectra, which contains a much larger number. With regard to nebulæ, Herschel published his third catalogue in 1802. The last catalogue of this nature is by Dreyer (1888), and contains seven thousand eight hundred and forty of these objects. In the time of Tycho they could be counted on the fingers of one hand.

INVESTIGATIONS OF SOME IMPORTANT ASTRONOMICAL CONSTANTS

The nineteenth century was fruitful in the determination of many numerical values which are all important in enabling us to determine the distance and masses of the heavenly bodies, thereby giving us a firm grasp not only of the dimensions of our own system, but of those scattered in the celestial spaces.

To take the distances first. We must begin with the exact measure of the earth; for this we must measure the exact length of an arc of meridian or of parallel—that is, a stretch of the earth’s surface lying north and south or east and west, between places of which the latitudes are accurately known in the former case, and the longitude in the latter. In either case we can determine the number of miles which go to a degree. Beginning at the opening of the nineteenth century with an arc of meridian of two degrees measured by Gauss, from Göttingen to Altona, the arcs of meridian grew longer as the century grew older, till, at the close, the measurement of an arc of meridian from the Cape to Cairo, embracing something like sixty-eight degrees of latitude, was mooted.

The measurements of arcs of parallel have been developed by the rapid extension of telegraphic communications, which now permit the longitude of the terminal stations to be determined with the greatest accuracy.

Thanks to this work, we now have the size of our planet to a few miles. The polar diameter is 41,709,790 feet, but the equator is not a circle: the equatorial diameter from longitude 8 degrees 15 minutes west to longitude 188 degrees 15 minutes west is 41,853,258 feet; that at right angles to it is 41,850,210 feet—that is, some thousand yards shorter. The earth, then, is shaped like an orange slightly squeezed.

Knowing the earth’s diameter, we can obtain the sun’s distance by several methods, the old one by observing transits of Venus, one of which Cook went out to observe in 1768, and two of which recurred in 1874 and 1882; new ones by observations of Mars or one of the minor planets at a favorable opposition, and by determining the velocity of light.

The recent discovery of a minor planet, Eros, which in one part of its orbit is nearer the earth than Mars, has recently revived interest in this method, and a combined attack is in contemplation.

It has been long known that light has a finite velocity, but we had to wait till the 60’s before Fizeau and Foucault showed us how to determine its exact value. The methods introduced by them have been recently applied by Cornu, Newcomb, and Michelson, and the resulting value is slightly less than three hundred thousand metres per second. Combining this with the constant of aberration, the distance of the sun can be determined.

It is wonderful how these vastly different methods agree in the resulting mean distance. At the beginning of the century it stood roughly at ninety-five million miles; this has been reduced to ninety-three million nine hundred and sixty-five thousand miles. The extreme difference between the old and new values of the solar parallax, two-fifths of a second of arc, is represented by the apparent breadth of a human hair viewed at a distance of about one hundred and twenty-five feet.

Knowing the distance of the sun, the way is open to us to determine, by a method suggested by Galileo, the distances of those stars which occupy a different position among their fellows, as seen from opposite points in the earth’s orbit round the sun, points one hundred and eighty-six million miles apart. We now know the distances of many such stars, Bessel having determined the first in 1838. The nearest star to us, so far as we know, is Centauri, the light of which takes four and a half years to reach us. Not many years ago Pritchard applied photography to this branch of inquiry; we may, therefore, expect a still more rapid progress in the future.

With regard to masses. We naturally must first know that of the earth; having its size, if we can determine its density, the rest follows.

The problem of determining the mean density of the earth occupied the minds of many workers during the nineteenth century. Newton (about 1728) pointed out how it could be deduced by observing the deviation from the vertical of a plumb-line suspended near a large mass of matter—a mountain, the volume and density of which could be previously determined. This method, which is very laborious and requires the greatest skill and most delicate instruments, has been employed several times, by Bouguer and Condamine, in 1738, at Chimborazo; Maskelyne, in 1774, at Schehallien in Scotland; and James, at Arthur’s Seat, near Edinburgh.

At the beginning of the century another method was introduced by Cavendish. This consists in measuring the attraction of two large spheres of known size and mass, such as two balls of lead on two very small and light spheres, by means of a torsion balance constructed by Mitchell for this purpose.

The most recent determination by this method, and one which is considered to give us perhaps the most accurate value, is that which is due to the skill and ingenuity of Professor Boys. His improvement consisted in constructing a most delicate torsion balance; the attracted spheres consisted of small gold balls suspended by a quartz fibre carrying a mirror to indicate the amount of twist. The whole instrument was quite small, and could easily be protected from air currents and changes of temperature, while the use of the quartz fibres reduced to a minimum one of the greatest difficulties of the Cavendish experiment. The value of the mean density of the earth is now considered to be 5.6, which means that if we have a globe of water exactly the same size as our own earth, the real earth would weigh just 5.6 times this globe of water. The earth’s weight, in tons, does not convey much idea, but that it is six thousand trillions may interest the curious. This determination has enabled the masses of the sun, moon, planets and satellites, and many sidereal systems to be accurately known in relation to the mass of the earth.

SOME ACHIEVEMENTS OF MATHEMATICAL ANALYSIS

Uranus, a planet unknown to the ancients, was discovered by its movement among the stars by William Herschel in 1781. It was not until 1846 that another major planet was added to the solar system, and this discovery was one of the sensations of the century.

The story of the independent discovery of Neptune by Adams and Le Verrier, who were both driven to the conclusion that certain apparent regularities in the motion of Uranus were due to the attraction of another body travelling on an orbit outside it, has been often told. The subsequent discovery of the external body not far from the place at which their mathematical analysis had led them to believe it would be seen, will forever be regarded as a fine triumph of the human intellect.

But the results of the inquiries which now concern us are generally of not so sensational a character, although they lie at the root of our knowledge of celestial motions. They more often take the shape of tables and discussions relating to the movements of the bodies which make up our solar system.

Gauss may be said to have led the way during the nineteenth century by his _Theoria molus corporum coelestium solem ambientium_. This was a worthy sequel to the _Méchanique Céleste_, in which work, towards the end of the eighteenth century, Laplace had enshrined all that was known on the planetary results of gravitation.

In later years Le Verrier and Newcomb have been among the chief workers on whom the mantle of such distinguished predecessors has fallen. From them the planet and satellite tables now in use have been derived.

But the motion of our own satellite, the moon, has had fascinations for other analysts besides those we have named.

The problem, indeed, of the moon’s motion is one of the most difficult, and has taxed the ingenuity of astronomers from an early date. Even at the present day it is impossible to predict the exact position of the moon at any one moment owing to inequalities and perturbations, the exact varying values of which are not known.

The two most important theories of the motion of the moon completed towards the middle of the century were due to Hansen and Delaunay. The former’s appeared in 1838, the lunar tables being published later (1857), while the latter’s was published in 1860.

Hansen’s theory had for its chief object the formation of tables; to avoid the inconvenience of using in his calculations series which slowly converge, he inserted numerical values throughout. In Hansen’s solution the problem is one actually presented by nature, allowance being made for every known cause of disturbance. There is one disadvantage, namely, that should observations demand a change in any of the constants used, there is no means of making any correction in the results.

Delaunay’s theory surmounted this difficulty, but at the expense of still greater inconvenience for making an ephemeris. The slow convergence of certain series involved an immense amount of labor to give sufficiently approximate results.

More recently, as the century was closing, Dr. Brown took up the subject and made a fresh attempt to calculate the motion of our satellite. It may be stated that he adopts all Delaunay’s modifications of the problem and works them out algebraically; but there are many technical differences which it would be out of place to mention here.

Enough has been stated to show that there is not likely to be any breach of continuity in the treatment of this most important problem.

Another attack on the moon, and, incidentally, its motion, has recently been made by another analyst, Professor George Darwin; grappling with all the consequences of tidal friction, he has been able to present to us the past and future history of our satellite. Beginning as a part of the material congeries from which subsequently some fifty million years ago both earth and moon, as separate bodies, were formed, it has ever since been extending its orbit, and so retreating farther away from its centre of motion, while the period of the earth’s rotation has been increasing at the same time, from a possible period of some three hours when the moon was born, to one of one thousand four hundred hours when the day and month will be equal, something like one hundred and fifty million years being required for the process.

STELLAR EVOLUTION

It was only in the 80’s, after thousands of observations of the spectra of stars, nebulæ, and comets had been secured, that the full meaning of the revelations of the spectroscope began to dawn upon the world.

Before the introduction of spectrum analysis all stars were supposed to be suns, and the only difference recognized among them was one of brilliancy and the variation of brilliancy in the case of some of them.

It ultimately came out that great classes might be recognized by the differences of their spectra, which were ultimately traced to differences in their chemistry and in their temperature, as determined by the extension of the spectra in the ultra-violet, the whiter stars being hotter than the red ones, as a white-hot poker is hotter than a red-hot poker.

Next there was evidence to show that a large proportion of the stars were not stars at all like the sun, but swarms of meteorites; and in this way the mysterious new stars which appear from time to time in the heavens, and a large number of variable stars, were explained as arising from collisions among such swarms.

The inquiry which dealt with the spectroscopic results, having thus introduced the ideas of meteor swarms and collisions to explain many stellar phenomena, went further and showed that the various chemical changes observed in passing from star to star might also be explained by supposing the whole stellar constitution to arise from cool meteoritic swarms represented by nebulæ, the changes up to a certain point being explained by a rise of temperature due to condensation towards a centre. Here the new view was opposed to that of Laplace, advanced during the last century, that the stars were produced by condensation and cooling; but Kelvin had shown, before the new view was enunciated, that Laplace’s view was contrary to thermodynamics, a branch of science which had developed since Laplace published his famous _Exposition du Système du Monde_.

After all the meteorites in the parent swarm had been condensed into the central gaseous mass, that mass had to cool. So that we had in the heavens not only stars more or less meteoritic in structure, of _rising_ temperature, but stars chiefly gaseous, of falling temperature. It was obvious that representatives of both these classes of stars might have nearly the same mean effective temperature, and therefore more or less the same spectrum. A minute inquiry entirely justified these conclusions.

So far had the detailed chemistry of the stars been carried in the latter years of the century that the question of stellar evolution gave rise to that of inorganic evolution generally, the sequence in the phenomena of which can only be studied in the stars, for laboratory work without stint has shown that in them we have celestial furnaces, the heat of which transcends that of our most powerful electric sparks. In this way astronomy is paying the debt she owes to chemistry.

THE SUN AND HIS SYSTEM

Although the outer confines of space have, as we have seen, been compelled to bring their tribute of new knowledge by means of the penetrating power possessed by modern telescopes, and the cameras and spectroscopes attached to them, the study of the _near_ has by no means been neglected, and for the reason that in astronomy especially we must content ourselves in the case of the more distant bodies by surmising what happens in them from the facts gathered in the region where alone detailed observations are possible.

Thus what we can learn about the sun helps to explain what we discern much more dimly in the case of stars; a study of the moon’s face we are compelled to take as showing us the possibilities relating to the surface condition of other satellites so far removed from us that they only appear as points of light.

To begin, then, with the sun. Where a volume might be written, a few words must suffice. I have already stated that at the beginning of the nineteenth century the prevailing opinion was that it was a habitable globe. It was limited to the fiery ball we see. At the end of the century it is a body of the fiercest heat, and the ball we see is only a central portion of a huge and terribly interesting mechanism, the outer portions of which heave and throb every eleven years. Spots, prominences, corona, everything feels this throbbing.

Although the discovery of spots on the sun was among Galileo’s first achievements, it was reserved for the last half of the nineteenth century to demonstrate their almost perfect periodicity.

Thanks to the labors of Schwabe, Wolf, Carrington, and De la Rue, Stewart, and Loewy, we now know that every eleven years the spots wax and wane; Tacchini and Ricco, during the last thirty years, have proved that the prominences follow suit, and the fact that the corona also obeys the same law was established during the American eclipse of 1878.

The study of solar physics consists in watching and recording the thermal, chemical, and other changes which accompany this period. Some of these effects can be best studied during those times when the ball itself is covered by the moon in an eclipse. Then the outer portions of the sun are revealed in all their beauty and majesty, and all the world goes to see.

But it is the quiet daily work in the laboratory which has enabled us to study the sun’s place in relation to the other stars, and so to found a chemical classification of all the stars that shine.

From the sun we may pass to his system, and first consider the nearest body to us—the moon.

While some astronomers have been discussing the movements and evolution of our satellite, others have been engaged upon maps of its surface, upon questions dealing with a lunar atmosphere, or a study of the origin of the present conformations and of possible changes. The science of selenology may be said to have been founded by Schröter at the beginning of the century, but it required the application of photography in later years to put it on a firm basis. Maps of the moon have been prepared by Lohrmann, Beer and Mädler, and Schmidt, the latter showing the positions of more than thirty thousand craters.

Very erroneous notions are held by some as to what we may hope to do in the examination of the moon’s surface by a powerful telescope. A power of a thousand enables us to see it as if we were looking at York from London. It is recorded that Lassell once said that with his largest reflector in a “fit” of the finest definition he thought he might be able to detect whether a carpet as large as Lincoln’s Inn Fields was round or square. Under these circumstances, then, we may well understand that the question of changes on the surface has been raised from time to time never to be absolutely settled one way or the other. By many the existence of an atmosphere is denied, and this is a condition which would negative changes, anything like the geological changes brought about on the surface of the earth, but the idea is now held by many that there is still an atmosphere, though of great tenuity.

The last few years of the century were rendered memorable from the lunar point of view by the publication and minute study of a most admirable series of photographs of the moon obtained by the great equatorial Coudé of the Paris Observatory by Loewy and Puiseaux. One of the chief points aimed at has been to determine the sequence of the various events represented by the rills, craters, and walled plains, the mountain ranges and seas. This work is still in progress, the fourth part of the atlas being published in 1900; but enough has already appeared to indicate that the results of the inquiry when completed will be of the most important kind. The authors have already come to the conclusion that the lunar and terrestrial sea-bottoms much resemble each other, inasmuch as both have convex surfaces. The lunar seas began by sinking of vast regions; the formidable volcanic eruptions of which the moon has been the scene have taken place in times equivalent to those labelled “recent” in geological parlance. There is evidence that the axis of the moon has undergone great displacements, and four great periods of change have been made out. Finally they state that there is serious ground to believe that there is an atmosphere of some sort remaining.

It may readily be understood that with each increase of optical power new satellites of the various planets have been discovered. Soon after the discovery of Neptune a satellite was noted by Lassell. In 1846 both he and the eagle-eyed observer Dawes independently discovered another satellite (Hyperion) of Saturn. Lassell was rewarded in the next year by the discovery of two more satellites of Uranus; but, strangest observation of all, in 1877 Hall discovered at Washington two satellites of Mars some six or seven miles only in diameter, one of them revolving round the planet in seven and one-half hours at a distance of less than four thousand miles. As the day on Mars is not far different in duration from our own, this tiny satellite must rise in the west and south three times a day!

Wonderful as this discovery was, it is certainly not less wonderful when we consider it in connection with a passage in _Gulliver’s Travels_, so true is it that truth is stranger than fiction. Swift, in his satirical reference to the inhabitants of Laputa, writes: “They have likewise discovered two lesser stars, or satellites, which revolve round Mars, whereof the innermost is distant from the centre of the primary planet exactly three of his diameters and the outermost five; the former revolves in the space of ten hours; and the latter in twenty-one and a half.”

The last discovery of this kind has been that of an inner satellite of Jupiter by Barnard in 1892.

The planets from Mercury to Saturn were known to the ancients. I have already referred to the discovery of Uranus by Herschel’s giant telescope, not long before the nineteenth century was born, and of Neptune, by analysis, towards the end of the first half of the century. With regard to what modern observations have done in regard to their physical appearance, the first place in general interest must be given to Saturn and Mars.

Saturn has always been regarded as the most interesting of the planetary family on account of its unique rings. Many subdivisions of the rings, and a dusky ring, first seen by Dawes and Bond, have been discovered during the last sixty years.

The meteoritic nature of the rings was suggested by Clerk Maxwell in 1857, and Keeler’s demonstration of the truth of this view by means of the spectroscope, a few years ago, was brilliant in conception and execution.

But during the last half of the century the interest centred in Mars has been gradually increasing. The drawings made during the opposition of 1862, when compared with those made by Beer and Mädler (1830–40), made it perfectly clear that in this planet we had to deal with one strangely like our own in many respects. There were obviously land and water surfaces; the snow at the poles melted in the summer-time; clouds were seen forming from time to time, and the changing tones of the water surfaces suggested fine and rough weather.

Afterwards came the revelation of the hawk-eyed Schiaparelli, beginning in the year 1877, and his wonderful map of the planet’s surface. The land surfaces, instead of being unbroken, were cut up, as an English farm is cut up by hedges; straight lines of different breadths and tints crossed the land surfaces in all directions, and at times some of them appeared double. Schiaparelli naturally concluded that they were rivers—water channels—and being an Italian he used the appropriate word _canali_. This, unfortunately, as it turned out, was translated _canals_. Now canals are dug, _ergo_ there were diggers. From this the demonstration, not of the habitability, but of the actual habitation, of Mars was a small step, and the best way of signalling to newly found kinsmen across some thirty millions of miles of space was discussed.

The world of science owes a debt of gratitude to Mr. Percival Lowell for having taken out to the pure air and low latitude of Arizona an eighteen-inch telescope for the sole purpose of accumulating facts tending to throw light upon this newly raised question. This he did in 1894. Schiaparelli has continued his magnificent observations through each opposition when the planet is most favorably situated for observation, and since 1896 Signor Cerulli, armed with a fifteen-inch Cooke, in the fine climate of Italy, has joined in the inquiry, so that facts are now being rapidly accumulated. It has been stated that markings similar to the strange so-called “canals” on Mars are to be seen on Mercury, Venus, and even on the satellites of Jupiter. Mr. Percival Lowell does not hesitate to proclaim himself in favor of their being due, _in Mars_, to an intelligent system of irrigation. Signor Cerulli claims that _wherever seen_ they are mere optical effects. We may be well content to leave to the twentieth century a general agreement on this interesting subject.

Finally, in our survey of our own system, come comets and meteor swarms. One of the most fruitful discoveries of the century, that comets are meteor swarms, we owe to the genius of Schiaparelli, A. H. Newton, and other workers on those tiny celestial messengers which give rise to the phenomena of “falling” or “shooting” stars.

The magnificent displays of 1799, 1833, 1866, and, alas! that which failed to come in 1899, we now know must be associated with Tempel’s Comet. This is by no means the only case so far established; the connection will in the future be closer still when the orbits of the various swarms observed throughout the year shall be better known.

Comets which attract public attention by their brightness and grandeur of form are rather rare, and, in fact, only twenty-five of such have been seen since 1800. We have, however, with the great advance in instrumental equipment, been able to discover many which are scarcely visible to the naked eye, and this has swollen the number of comets very considerably. In the seventeenth century we find that only thirty-two were observed, while in the eighteenth this number was more than doubled (seventy-two). In the nineteenth century more than three hundred were placed on record, which is practically more than four times the number seen in the eighteenth.

The last great comet visible any considerable time was that discovered by Donati in 1858, and so carefully observed by Bond. It is unfortunate that since the importance, in so many directions, of spectroscopic observations of comets has been recognized they have been conspicuous by their absence.

THE CONNECTION BETWEEN SOLAR AND TERRESTRIAL WEATHER

Everybody agrees that all the energy utilized on this planet of ours, with the single exception of that supplied by the tides, comes from the sun. We are all familiar with the changes due to the earth’s daily rotation bringing us now on the side of our planet illuminated by the sun, then plunging us into darkness; that changes of season must necessarily follow from the earth’s yearly journey round the sun is universally recognized.

On the other hand, it is a modern idea that those solar phenomena which prove to us considerable changes of temperature in the sun itself, may, and indeed should, be echoed by changes on our planet, giving us thereby an eleven-year period to be considered, as well as a year and a day.

This response of the earth to solar changes was first observed in the continuous records of those instruments which register for us the earth’s magnetism at any one place. The magnetic effects were strongest when there were more spots, taking them as indicators of solar changes. Lamont first (without knowing it) made this out, at the beginning of the latter half of the century (1851), from the Göttingen observations of the daily range of the declination needle. Sabine the next year not only announced the same cycle in the violence of the “magnetic storms” observed at Toronto, but at once attributed them to solar influence, the two cycles running concurrently. It is now universally recognized that terrestrial magnetic effects, including auroræ, minutely echo the solar changes.

The eleven-year period is not one to be neglected.

Next comes the inquiry in relation to meteorology. Sir William Herschel, in the first year of the century, when there were practically neither sun-spot nor rainfall observations available, did not hesitate to attack the question whether the price of wheat was affected by the many-or-few-spot solar condition. He found the price to be high when the sun was spotless, and _vice versa_.

By 1872, however, we had both rainfall and sun-spot observations, and the cycle of the latter had been made out. Meldrum, the most distinguished meteorologist living at the time, and others, pronounced that the rainfall was greatest at sun-spot maximum, and, further, that the greatest number of cyclones occurred in the East and West Indies at such times.

This result with regard to rainfall was not generally accepted, but Chambers showed shortly afterwards an undoubted connection between the cycles of solar spots and barometric pressure in the Indian area.

By means of a study of the widened lines observed in sun spots an attempt has been recently made to study the temperature, history of the sun since about 1877, and the years of mean temperature and when the heat was in excess (+) and defect (-) made out, have been as follows:

Heat
condition mean + mean − mean + mean − mean
Years 1869 1876 1881 1886–87 1891–92
1870–75 1877–80 1882–86 1881–91 1892

Having these solar data, the next thing to do was to study the Indian rainfall during the southwest monsoon for the years 1877–1886, the object being to endeavor to ascertain if the + and − temperature pulses in the sun were echoed by + and − pulses of rainfall. The Indian rainfall was taken first because in the tropics the phenomena are known to be the simplest. It was found that in many parts of India the + and − conditions of solar temperature were accompanied by + and − pulses, producing pressure changes and heavy rains in the Indian Ocean and the surrounding land. These occurred generally in the first year following the mean condition, that is, in 1877–78 and 1882–83.

The rainfalls at Mauritius, Cape Town, and Batavia were next collated to see if the pulses felt in India were traceable in other regions surrounding the Indian Ocean to the south and east. This was found to be the case.

A wider inquiry was followed, we are told, with equal success, so that we are justified in hoping that the question of the dependence of terrestrial upon solar weather has made a step in advance.

But just as the general public and practical men took little heed of the connection between sun spots and magnetism until experience taught them that telegraphic messages often could not “get through” when there were many sun spots, so the same public will not consider the connection in regard to meteorology unless the forecasting of droughts and famines be possible.

The recent work suggests that, if the recent advances in solar physics be considered, the inquiries regarding rainfall may be placed on a firmer basis than they could possibly have had in 1872, and that such forecastings may become possible.

What was looked for in 1872 was a change in the quantity of rain at maximum sun spots only, the idea being that there might be an effective change of solar temperature, either in excess or defect, at such times and that there would be a gradual and continuous variation from maximum to maximum.

We see that the rainfalls referred to above justify the conclusions derived from the recent work that two effects ought to be expected in a sun-spot cycle instead of one. There was excess of rainfall, not only near the sun-spot maximum, but near the minimum.

If the authors of this communication to which I refer are right, then droughts and famines occur in India because the rain pulses, which are associated with the solar-heat pulses, are of short duration. When they cease the quantity of rain which falls in the Indian area is not sufficient, without water storage, for the purposes of agriculture; they are followed, therefore, by droughts, and at times subsequently by famines. They divide the period 1877—89 as under:

{ 1877.
Rain from − pulse { 1878.
{ 1879 (part).

{ 1879 (part).
No rain pulse { 1880 (central year).
{ 1881 (part).

{ 1881 (part).
{ 1882.
Rain from + pulse { 1883.
{ 1884 (part).

{ 1884 (part).
{ 1885 }
No rain pulse { 1886 } (central year).
{ 1887 (part).

{ 1887 (part).
Rain from − pulse { 1888.
{ 1889.

Their statement is based on the fact that all the famines which have devastated India for the last seventy years have occurred at intervals of eleven years, or thereabouts, working backward and forward from the central years 1880 and 1885–86 in the above table, the middle years, that is, between the pulses.

Mr. Willcocks, in a paper read at the Meteorological Congress at Chicago, remarked that “famines in India are generally years of low flood in Egypt.”

It is now pointed out that the highest Niles follow the years of the + and − pulses, as does the highest rainfall in the Indian area.

Even if these results, which were communicated to the Royal Society of London five weeks before the end of the century, be confirmed, it may be pointed out that Sir William Herschel’s suggestion of 1801 will have required a whole century for its fulfilment, so slowly do those branches of science move which have not already led to some practical development.

NORMAN LOCKYER.

PHILOSOPHY

It is a natural illusion that makes us think of each century as exhibiting the continuous development of one tendency of mind through a series of stages whose differences are only of secondary importance, and, on the other hand, to regard the steps from one century to another as corresponding to some marked transition of thought, as if the world had been suddenly precipitated into a new sphere of existence. For some purposes a rough generalization of this kind, that breaks at stated intervals the continuity of time, may, perhaps, be convenient. When, however, we begin to look at things more closely, we discover that it is impossible thus to cut through the historical connection of events, as it were, “with a hatchet.” We discover, for example, that the characteristics of the eighteenth century were strongly marked only in one period of it; and that what we call the spirit of the nineteenth century was born some time before the year 1800, and has never quite prevailed over other tendencies. At the same time, there is an important difference indicated by these two loosely used names, and as it is always easier to define things by contrast, it may help us to make our subject more definite to consider what they mean.