4 



i. 



Appendix.] 



TERRE8TEIAL MAGNETISM. 





hA 



i1 



1 



, ^ 



V 



\ #■- 



h^ 



for the purpose of elimiaatiBg thib eifect. If two distances are employed, 

 they should be to each other in the proportion of 1 to 1*3 nearly ; or for 

 magnets of the specified lexigth and Btrength, 1 foot r,ad 1*3 foot mixy 

 ;enera]ly be found conveuient and suitable distances, in which case the 

 lesser distance should be denoted by r, ai^d the greater by r,, the corre- 

 sponding deflections being ii and u.. 



Where two distances are employed, no alteration is required m the ex=- 



periments of vibration ; but in those of deflection it wili be found con- 

 venient to adopt the following routine : 



i. Observe the angle of deflection -with the Borth end of the deflecting 

 magnet altei'nately to the east and to the west (with the proper number 



arm 



least of the two distances. 



2. Go through the same process with the magnet still on the eastern 



arm 



3. Repeat No. 2 oa the western arm, and 



4. Eepeat No. 1 on the western arm, the order of the distances in Nos. 

 1 and 2 being Inverted in Nos. 3 and 4. 



If the distance required to produce a suflicient deflection should be less 

 than four tinies the length of the deflecting magnet, it may be dt/rable to 

 obtain three or more values of w (u, u,^ u,,^ &c,) at three or more distar/.s 

 (r, fj, ryy, &c.) ; in such case it is convenient for the calculation that the 

 distances should have a common difference ; as, for example, (in dedaials 

 of a foot) '9, 1 • 1, and 1*3: 1*1 being supposed to be the distance ordina- 

 rily employed when time and circumstances will not pei-mit more than a 

 single distance ; and the other two distances to be employed iu addition 

 when there may be full leisure for the purpose- 



Vibration Experiments, 



1. For these experiments che tripod stand carries a table top, Tvhich is 

 furnished with an azimuthal motion, and can be clamped to the tripod, 

 A telescope with a scale attached to it for measuring the arc of vibra- 

 tion is fixed to the table, as is also a rectangular box of wood, with a 

 glazed side, furnished with a tube for the suspension of the 3' 67 inch 

 (or deflecting) magnet. The talde must be levelled by means of the 

 foot-screws, and turned in azimuth until the mirror attached to the 

 stirrup in which the magnet is supported refleels in the telescope the 

 centre division of the scale attached to the telescope. The magnet rests 

 during the experiments of vibration in a stiri'up carrying a mirror, and 

 is suspended by a silk thread of which the ratio of the torsion force to 

 the magnetic directive force must be ascertained, by turning the index 

 of the torsion circle 90°, first in one direction and then in the other, and 

 taking the mean of the correspondiug angles through which the magnet 





MH*t 



