162 METEOKOLOGY [TERRESTRIAL MAGNETISM. possible. To keep the state of the thread constant the glass shade should be rendered air-tight, and should contain some substance for absorbing moisture, such as chloride of calcium. It is clear that if the state of the thread remains the same, and if the position of the magnetic axis of the magnet does not change, this instrument should record faithfully the various changes of declination. The Horizontal Force Magneto meter is exhibited in fig. 25. Here the magnet * has been twisted round into a position at right angles to the magnetic meridian. It is sus pended by means of two very fine steel wires some little distance apart, and thus the instrument is often called the bifilar magneto meter. These wires have the plane passing through their lower ex tremities differing very consider ably from that of their upper. If the magnet should suddenly lose its magnetism the whole arrange ment would be twisted round until the two planes coincided. This difference of plane gives rise to a couple tending to twist the magnet round in one direction while the horizontal magnetic force of the earth constitutes an equal and opposite ^J^J^s^gZ couple, the two couples ^Ss^S^ keeping the magnet in ;* J^** equilibrium. The couple :g*l^ 1^- depending upon the bi- j^ss filar arrangement may . for the present be re garded as constant, that depending on the hori zontal force of the earth FIG. 24. Declinometer. as variable. If the latter increase or diminish, the magnet will be slightly twisted round in one direction 26), the magnet is or the other. In the Vertical Force Magnetometer (fi balanced by means of a knife-edge resting on an agate plane. By means of two screws working hori zontally and vertically the centre of gravity may be thrown to either side of the point of suspension, or it may be raised or lowered and the sensibility of the magnet when balanced thereby increased or diminished. These screws are so arranged that there is a preponder ance of weight towards the south side of the magnet. This is neutralized partly by the magnetic force tending to pull the north end down and partly by a slip of brass standing out horizontally towards the north side. Let us suppose the system to be in equilibrium at a certain temperature ; if the tem perature rise (since brass expands more than steel), the leverage of the weight at the north side will increase more than that of the weight at the south. There will thus be a slight preponderance towards the north, and this may be arranged so as to neutralize to a great extent the de crease in the magnetic moment which an in crease of temperature produces. 21. Magnctograplis. ^IG. 25. Horizontal Force Magnetometer. The arrangement by means of which these instruments are con verted into self-recording magnetographs is very simple. In fig. 23 we see a gas flame burning behind a vertical slit and placed end wise in order to render its light more intense. The light from this illuminated slit passes through a lens, and being reflected from the mirror of the declination magnet throws an image of the 1 All the magnets are of the same size. slit upon some sensitized paper in the central box. To speak more properly, two images are thrown, one reflected from the upper and movable half and the other from the fixed half of the mirror. The sensitive paper is wrapped round a horizontal cylinder (fig. 27), and the two images are therefore thrown upon different parts of FIG. 26. Vertical Force Magnetometer. this cylinder. But before reaching the cylinder these two images are_by means of a hemicylindrical lens (shown in fig. 27) crushed up into two dots of light. The cylinder moves round regularly by clock-work once in twenty-four hours, and hence the course on the moving paper of the dot of light which comes from the fixed half- mirror will be a straight line, while that of the dot from the moving half-mirror will be a curved line depending on the motions of the magnet. When the paper is developed these lines appear black. The arrangement for the horizontal force instrument is precisely similar to that for the declinometer ; for the vertical force it is somewhat different, the illuminated slit being horizontal and not vertical, while the mirror oscillates on a horizontal axis and not on a vertical one ; the hemicylindrical lens too and the cylinder are vertical and not horizontal. It was found necessary to put the plane of motion of the vertical force magnet 15 out of the magnetic meridian for the following reason. The axes of the telescopes are respectively 30 inclined to the tubes which go from the magneto- FIG. 27. Magnetograph. meters to the central box, and hence had the vertical force magnet swung in the magnetic meridian it would have been necessary to place the mirror inclined at the angle of 15 to the axis of motion of the magnet. This was tried, but it was found that in this posi tion of the mirror the correction for temperature was so excessive that the instrument became a thermometer and not a magnetometer. The mirror was therefore put in a plane passing through the axis of motion of the needle, the needle being made to move in a plane inclined 15 to the magnetic meridian. 22. Scale Coefficients of Differential Instruments. It is necessary to know the value of one division of the scale in the magnetometer or of one inch difference in the ordinate of the curve impressed on the photographic paper in the magnetograph. In the declination instrument it is only necessary to obtain the angular deviation cor responding to one division, and this may be done at once by a series of measurements. In the horizontal and vertical force instruments we wish to obtain the value of one division in parts of force. There is more than one method by which this can be accomplished, but that of John Allan Broun is probably the simplest, and it is, we believe, the one adopted at most of the various observatories .pos sessing self-recording instruments. It is given in the British Association Reports, 1859. 23. Temperature Coefficients of Differential Force Instruments. Broun has devoted a great deal of attention to the subject of these coefficients, and has come to the conclusion that the best
and most unobjectionable method of determining them is to