458 THE POPULAR SCIENCE MONTHLY. 



tacbed 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 sus- 

 pension 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 con- 

 stant, the equilibrium is not destroyed save by a variation in the in- 

 tensity 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 oppos- 

 ing 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, woidd lose some of its power, and consequently be 

 less strongly pulled by the horizontal force, which we wish to meas- 

 ure. 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 interposi- 

 tion 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 ac- 

 curately 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 horizontal- 

 ly 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 



