tached one end of a suspension skein, that passes over a glass pulley three feet above the magnet. The diameter of the pulley is fourteen millimetres; it should, however, equal the distance between the ends of the suspension skein—ten millimetres. The present pulley is to be changed for one of that diameter.
The pulley is turned into such a position that the pull of the suspension skein brings the magnet approximately at right angles to the magnetic meridian. The magnet is in equilibrium under the action of three forces: gravity, the pull of the threads, and the horizontal component of the earth's magnetism. The first two forces being constant, the equilibrium is not destroyed save by a variation in the intensity of the third force. The changes in the direction of that force are never sufficiently great to appreciably alter the position of this magnet. The diameter of the pulley being greater than it should be, increases the leverage of the pull of the threads, and so lessens the ratio of the variation of the horizontal force to the sum of the opposing forces. The delicacy of the instrument is thus slightly impaired.
On each side of the point of suspension of the magnet is a place for a small weight. By weighting the magnet its angular position is slightly changed. A comparison of the effect thus produced with the changes due to variations in the horizontal force gives us a measure of that force. It is, in truth, weighing the magnetism.
Observe what takes place when the instrument is heated. Neither the glass pulley nor the glass rod would be sensibly affected. The magnet, however, would lose some of its power, and consequently be less strongly pulled by the horizontal force, which we wish to measure. To counterbalance this loss of magnetic power, the effect of one of the opposing forces must be diminished by an equal amount. This is effected by the zinc tubes, whose expansion brings the ends of the suspension skein nearer together, and thus lessens the pull of that skein.
I now come to the most delicate of all the instruments—the balance magnetometer. Attached rigidly to the axis of this instrument, and at right angles to the same, is an axis, resting, through the interposition of agate knife-edges, upon an agate plate. By changing the position of small brass balls that screw upon vertical and horizontal arms of this axis, the centre of gravity of the instrument may be accurately adjusted to any desired position. None of these balls weigh over fifty grains, and the distance between two successive threads of the screw upon which they work is only the hundredth part of an inch; yet, if one of those for shifting the centre of gravity horizontally be turned through so much as the twentieth part of a revolution, thus advancing it the two-thousandth of an inch, the instrument will be so tilted as never to right itself. This extreme delicacy is attained by bringing the centre of gravity of the instrument close up under the axis of suspension. To prevent unnecessary wear of the agate