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TERRESTRIAL MAGNETISM, 



[Sect. ir. 



APP^ 



is deflected. Then if H dein 

 P the Earth's directTTe force. 



co-effi 





the m ean angle (in minutes of are) 

 90" — the mean angle (in minutes of arc)' 



2, The mr.:;- ■ ■, being at rest, must be made to vibrate in small arcs 

 by V ms of another magnet, and the time of 300 vibrations observed ia 

 the following manner ; — The arc of vibration being about 60 divisions 

 (or minutes) on either side of the central division, note the time when 

 the central division crosses the vertical wire of the tekscopoj passing 

 from left to right ; do the same when the central division passes from 

 left to right for the third, fifth, seventh, ninth, and eleventh times 

 corresponding to the completion of vibrations 4, 8, 12, 16, 20, aud 24. 

 This will gi%^e the approximate interval of time in which 20 vibrations 

 are performed ; and will indicate to the observer, without requiring him 

 to count tlie intermediate vibrations, when he should be prepared to ob- 

 serve the completion of the 60thj I20th, 180th, 240th, and 300th vibra- 

 tions. At the 300th vibration he should recommence taking- the time of 

 the 300th, 304th, 308th, 3I2th, 3ieth, 320th, and 324th vibrations. He 

 will then have seven partial results for the time in which 300 vibrations 

 are performed, viz. 



to 300 

 4 to 304 

 8 to 308 



12 te 312 



16 to 316 



20 to 320 



24 to 324 



of wlucli seven partial results he should take the mean. 



The temperature of the magnet should be observed by a thermometer 

 placed in the box with the magnet, before and after the experiments by 

 which its time of vibration is determined. 



The data required from the observer on each occasion are the follow- 

 ing:— The mean time of 300 vibrations, and the particulars from which 

 it is derived (or the partial results) ; the commencing and concluding 

 arcs of vibration : the temperature of the magnet ; the rate of the chro- 



by which the times are observed ; and the ratio of the torsion 



nomeier 



force to the Earth's directive force, or — • 



F 



If, then, T' is the observed time of a single vibration, and T the 

 corrected time, the corrections may be computed by the following 

 formula : 



T 



T 

 X 



I + 



TT 



2F 



a a 



Te 



8 



— ) 



in v^hich a and a^ are th- commencing and concluding arcs in parts of 

 radius, and x the rate of the chronometer. When the commencing arc 



^ 







friW 



It is 

 the ab: 



make 1 

 rinient 

 descrit 



such c 



gtation 



happen 

 The 



the cir 



of the 



magne 



momei 



withtl 



diamet 



denote 

 weight 



and if ' 

 of vibrj 

 the ma) 

 the fon 



e< 



Asth 



before t 

 ^^alue n 



stirrup , 



observe: 



If the 

 ofdefi 



the exf 



at the 1 



temper^ 



*^«^ req 

 <?ustonia: 



tempe, ^ 



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