Chapter XII

148.--=The Prismatic Compass=, shown Fig. 35, was invented by Charles August Schmalcalder in 1812. It is the most convenient portable instrument for reading magnetic bearings. Angles may be taken with great rapidity within about 15′ of arc by holding the instrument in the hand, or perhaps within 5′ if the instrument is of 4 to 6 inches diameter and placed on a stand. It is a most valuable instrument for filling in close details, such as may occur among buildings, trees, etc., after the principal points have been laid down from observations taken with the theodolite. The principles of the reflection of a prism were discussed, art. 55, Fig. 3, p. 29.

149.--_Prismatic Compasses_ are made from 2½ to 6 inches in diameter. The compass needle is sometimes made to carry a card dial for the 2½-inch size; for larger sizes the ring is now made uniformly of aluminium. The reading of the compass ring is effected by means of a glass prism, Fig. 36, _P_, which is cut to 45° upon one face and 90° for the two others, one 90° face being worked convex, so as to give magnifying power simultaneously with reflection of the ring at right angles, so that the reading of the compass appears to stand erect before the user of the instrument, and to be considerably magnified. As the reading is made on the side of the ring nearest the observer, the figures on the ring are engraved right to left. The prism is placed in a box, with a vertical sight slit _SS_ over it, which cuts a line with the centre of the top of the prism. The box with its prism moves upwards or downwards in a sliding fitting _SL_ by means of a _thumb nail_ stud, which adjusts the prism until it is in exact focus with the divisions on the ring. The back of the prism-box has a hinge _H_, so that this box may be closed down to the level of the compass-box to render it portable when out of use. On the opposite side of the compass-box to that upon which the prism is placed a long vertical window _SV_ is attached, having a central hair placed so as to cut a direct line from the slit _SS_ in the prism-box across the axis of the needle. This window-piece is jointed to turn down upon the face of the compass-box and simultaneously to lift the compass needle off its centre by a part of it pressing the outer end of the lifting lever _L_. To prevent too great a continuity of the oscillation of the compass needle and the ring, through unsteadiness of the hand in holding it, a pin is placed at _S_, through the compass-box under the window, which carries a light spring _B_ that just touches the ring lightly when the pin is pressed in, and thereby brings the compass ring to rest, or fixes it for reading with some degree of certainty. An open ring under the prism-box is sometimes used for placing a piece of ribbon through it, to attach it to some part of the person to save dropping the compass accidentally when it is used in the hand. When the instrument is out of use a metal cover is provided to protect the glass. The instrument is uniformly carried in a leather case with strap to pass over the shoulder. As these instruments are often carried by military surveyors, they are better made of a stiff aluminium alloy, which makes the instrument less than half its ordinary weight.

150.--=Additional Parts= commonly provided with the prismatic compass are a mirror and sunshades, shown only in section Fig. 36. The mirror _M_ is carried in a frame attached with a sliding piece to the window, upon which it can be placed either upwards or downwards. It is jointed with a hinge so as to be set at any angle. By reflection from the mirror, bearings in azimuth are taken much above or below the horizontal plane. Sun-glasses are also provided in front of the prism, which are used for taking the sun's place either with or without the mirror, a single sun-glass being also used very comfortably for working towards the sun at all times. The sun-glasses, which are simply small, dark-coloured glass circles in frames, are not shown in the engraving.

151.--=To Prepare to take Observations with the Prismatic Compass.= After the window and prism are opened out, the prism is adjusted to read the divided ring sharply when the compass is about level, by raising or lowering the prism _P_ by pressure of the thumb and forefinger of the right hand upon the stud placed upon the prism slide fitting, shown below _SL_, until the divisions appear clear.

152.--=In Using the Prismatic Compass=, the compass-box is held with the thumb of the right hand under the prism at _SL_ and the forefinger upon the stud _S_. The object which it is desired to observe is sighted through the slit _SS_, cutting the left-hand side of the hair in the window _SV_, while the division which comes opposite the reading point at its edge by the reflection from the prism is noted. The ring when free oscillates for a time, but is easily brought to rest for reading by gently pressing the pin _S_ upon which the forefinger is placed.

153.--Where objects are observed for taking their bearings above the horizontal plane, the length of the window will be sufficient to take in a vertical angle of 20° to 30°; but for such altitudes it is necessary to take very great care that the compass is held level, to get magnetic angles even approximately true. Below the horizon, angles can be obtained with somewhat greater certainty by means of reflections from the mirror. Altogether, except for taking nearly horizontal angles, or for very close work in filling in after the theodolite, it is much better to have the prismatic compass mounted upon a tripod stand. With a stand, where the angle in azimuth is much above or below the horizontal plane, it is better to have a small glass level, described further on, art. 181, to place across the compass when setting it up. If the compass ring is very carefully balanced across 90° to 270° two bright wire points may be placed inside the compass-box, level with the compass ring, which will answer for the cross levelling.

154.--=Stands.=--The author has made a very simple and inexpensive tripod stand for the prismatic compass, the head of which consists of a ball and socket only, clamped by a large milled-head screw. An axis through the ball permits horizontal adjustment, shown in section, Fig. 37.

155.--=Hutchinson's Prismatic Compass=, Fig. 38, is now very generally used by military men. In this compass the metal cover is fixed on the top of the compass-box, and a glazed opening is placed in the cover, occupying about one-eighth of its area, near the prism. This opening gives sufficient light to the compass card to permit it to be easily read, and the loose cover is dispensed with; besides which, the cover being fixed, this, as well as the whole instrument, may be made much lighter, while retaining equal rigidity for wear. This compass is not fitted with shade and mirror arrangements as before described. Size, 2½ inches diameter, ¾ inch in thickness; weight, only 8½ oz. in brass; 3¼ oz. in aluminium.

156.--=Captain Burnier's Military Compass.=--This portable compass is more generally used on the Continent than other forms. It is generally combined with a clinometer, therefore the illustration is deferred, _seq._ with clinometers. The compass ring is set up vertical to the plane of the needle, and is read by an index point by means of a cylindrical lens. It has a pair of sights formed of a slit near the eye-piece, and a hair in the window as in the prismatic. When this instrument is held horizontally, at about a foot distance from the eye, the sight line and the index line read distinctly into the graduations of the ring. A lifter is provided to raise the compass off its centre, as with the prismatic compass, and a spring clutch to prevent continuity of oscillation. It is adapted to be set up on a plain rod stand, the socket fitting to which is held in the hand when it is used as a hand instrument.

157.--=Surveying with the Compass only.=--In modern practice very little surveying is performed with the compass, except for sketch or exploring maps and filling in details, wherein the prismatic compass is useful. The magnetic needle was formerly much used for surface work, and depended upon almost entirely for underground work; but this has been found practically in many cases unsafe, from the uncertainty of magnetic variations, local and other, in the district surveyed. Mining compasses, or _dials_, as they are termed, are now in modern practice made with means of taking angles with the compass, and independently of it. This subject will therefore be deferred to a future chapter on mining instruments.

158.--=In Plotting Military Sketch Surveys= from angles taken with the prismatic compass, the paper employed is ruled lightly all over with parallel lines an inch or less apart. The angles taken with the prismatic compass from 0° to 360° (northern zero) are set off with an ivory military protractor, which has lines to correspond with latitudinal lines drawn over its face at 90° to its base, so that the protractor may be placed transverse to any line drawn on the paper with its centre in any position. Particulars of this method are given in every detail in Major Jackson's _Course of Military Surveying_, and in my work on Drawing Instruments. The military protractor is shown Fig. 39.

159.--For making a sketch plan with the prismatic compass, a very convenient way is to use the tee-square, the upper edge of the blade of which represents magnetic east to west, the upper end of the board magnetic north and the lower end south, according to the reading of the compass. The bearings taken from any starting-point are set off on the plot by a semicircular protractor with its base resting along the tee-square. The northern angles are raised with the square at the left-hand side of the board and the southern with it at the right. The distances from the station for all bearings are measured and set off by scale.

160.--It is indifferent how many stations are taken by the prismatic compass. The measurements in any direction may continue all round an estate, and will be found fairly correct if carefully made, as the small personal errors in reading the prismatic, which may be _plus or minus_, tend to correct each other on the whole, and to tie up the lines.

161.--The rolling parallel rule may replace the tee-square, if it is thought desirable to place the plan in a direction other than that erect to magnetic north with the paper, or that it is inconvenient to use the tee-square. In this case a few parallel lines may be at first drawn correctly across the paper, at about equal distances, with a sharp pencil E. to W. for references to reset the parallel rule at any position desired.

162.--=Pocket Magnetic Compasses.=--The subject of compasses will scarcely be complete without mention of the small pocket compasses which are so useful and universal. Several well-known forms are shown in the next illustration. The square form shown first, Fig. 40, will be found the most useful for very rough sketching. The edges may be sighted for the direction of roads, etc., or the box may be placed against a wall for taking the magnetic direction of a building. In like manner also the compass-box may be laid on a drawing and lines drawn along by the edges of the box to the magnetic directions taken. This in most cases is sufficiently accurate for architectural work, in which the exact direction is not generally thought to be important. Fig. 41 is a French form of compass with step reading level with the upper surface of the needle. Fig. 42 is an old English form with enamelled dial, with lifter under the bow of the handle. Fig. 43 is the same make in a hunter case. In this the lifter rises upon the case being closed.

163.--The author has made a small pocket magnetic compass, which is represented in the illustration above. The needle is placed in a long box. It reads at its point into a single line when the needle is exactly parallel with the sides of the box. The lid turns up endwise. The needle is lifted by closing the box.

164.--In a form of compass similar to the above, the author has added a thin slide to the under side of the box, by means of which the magnetic variation may be adjusted, as shown Fig. 45. This slide moves out just the amount of magnetic variation, the stud _S_ being made concentric for this adjustment. If the slide box be made of ivory a few useful scales may be divided upon it. The compass slips into a light leather case, and is the most portable for its length of needle of any compass made. The edges of the box are used as directing lines, as above described for the square form. The illustrations show a compass made for Great Britain, but a similar instrument is also made universal. In this case the box is a little wider, with the centre of the slide in the middle, so that the magnetic variation can be set off west or east. A rider also on the needle enables it to be balanced in southern latitudes.

165.--=Barker's Luminous Compass=, with floating dial of mother-of-pearl, one-half of this being engraved with black figures and the other half painted black with the figures left white, permits magnetic direction to be observed in the dusk and by moonlight. These compasses, Fig. 46, are much used by travellers. Mr. Francis Barker has also designed a compass in which the needle carries a bar coated with luminous paint.

FOOTNOTE:

[2] See _Phil. Trans._ 1896, vol. 188, Map. 9.