394 MAGNETIC OBSERVATORY AT 



This instrument is designed to measure and record the variations 

 in the vertical component of the earth's magnetic force, to which 

 end a magnet is balanced horizontally in a plane at right angles to 

 the magnetic meridian, in which position its balance is affected by the 

 vertical part of the force only, and variations in the latter will pro- 

 duce corresponding proportional changes of direction in the position 

 of the magnet. In the annexed diagram, fig. 7, is a top view, fig. 8, 

 a side elevation, and fig. 9, an end view of the instrument, such parts 

 being omitted in each as would confuse the representation. In fig. 7 

 we see the balance beam, with speculum attached on the right, and 

 magnet on the left hand, the latter surrounded by the copper damper, 

 which is further shown in fig. 9, but omitted in fig. 8. The beam 

 rests with agate knife edges on agate planes, and can be lifted off or 

 let down upon the latter by means of a vertical sliding frame, (in 

 fig. 8, and shown endways on the right hand,) which can be moved 

 by an eccentric, operated by means of a key from the outside of the 

 surrounding case. Whenever the instrument is handled for the 

 purpose of adjustment or otherwise, it is lifted off the agates, and 

 afterwards slowly lowered. There are attached to the beam two 

 balls on screw stems, one for adjusting the balance, the other for 

 adjusting the height of the centre of gravity, and thereby the value 

 of the scale divisions. The small thermometer attached to the 

 balance frames parallel with the magnet is for the purpose of com- 

 pensating the effect of temperature. The whole instrument rests on 

 a black marble slab, and is covered by a mahogany case, blackened 

 on the inside, with glazed apertures for the speculum and plane 

 mirror. 



In order to determine the scale value, according to Lloyd's method, 

 we observe T h the time of horizontal vibration of the magnet in its 

 frame with attachments; also T v the time of vertical vibration of 

 the same on its knife edges. Then if F designates the vertical 

 force, I the dip, D the distance from the mirror to the scale or 

 paper, and x the space on the paper corresponding to a certain 

 change of force d F, as, for instance, the one-thousandth part, we 

 have — 



dF T h 2 u _ x 



— = ^ 2 X cot. I X ^ 



The compensation for temperature is effected by means of a ther- 

 mometer, whose bulb and stem are so adjusted to the temperature 

 and scale coefficients that the translation of a portion of the mercury 

 towards the north end of the magnet by an increase of temperature 

 will exactly counterbalance the loss of force in the magnet. This 

 mode of compensation, also due to Mr. Brooke, enables us to compen- 

 sate for the effect of the second as well as the first power of the change 

 of temperature; for the statical moment of the mercury displaced from 

 the bulb by any given elevation of temperature, as t, above 32° 

 Fahr., may be represented by the same formula which expresses the 

 temperature coefficients, namely, c t -}- e ft. 



For let iv be the weight of mercury contained in one degree of the 



