Unifilae Magnetometee. xxiii 



up to the 96th vibration. A moveable object-glass fits upon the object-glass of the 

 reading telescope, in order to bring the paper slip into view without altering the ad- 

 justment of the telescope. 



23. The following are the values of the constants used in the preceding formulae, 

 o for the declinometer thread, with the deflecting bar suspended, is obtained from 

 the value when the long scale magnet is suspended by multiplying the latter by the 

 ratio of the moments of the two magnets ; thus, moment of long scale magnet is to 

 moment of deflecting bar, as I'OOO is to 0*942, whence, from the values of * for 

 the long scale magnet (No. 10), we have for the deflecting bar and declinometer 

 thread, 



During 1845 and 1846, o = 0-001482. 



The rate of the Observatory clock was generally less then 2 seconds a-day. No 

 correction for rate was therefore employed in the determination of T. 



24. The following are the details for the determination of K. 



The deflecting bar is 1*25 feet (= a) in length ; 0'0719 feet (= b) in breadth ; 



6216*7 grains in weight (= W). The value of K had been previously determined 



a^ + 6^ 

 from the formula — — — x W. In January 1848, it was thought desirable to de- 



termine it by the more accurate formula given previously ; as it was doubtful whether 

 the declinometer thread could sustain the deflecting bar and an additional weight, 

 a stronger thread was substituted, for which * was determined. 



The deflecting bar was vibrated in a stirrup of silk without any appendages, 

 the results are : 



Jan. ll<i 1848. Observations made by chronometer. Dent, No. 1665, rate — 5^0. 



Semiarcs of vibration, arc - ^ a = 10^° arc -^ a' = 6°. Temperature of bar 38°-4 Fahr. 



Mean observed time of one vibration from 160 vibrations, To' = 15s-9037. 



Bifilar magnetometer observed every 5°* during vibration, mean during vibration corrected 

 for temperature = 553-9 so. div. 



Deflecting bar with scale and lens suspended in a stirrup of silk as during vibration, * = 000200. 

 Deflecting bar with scale and lens suspended in brass stirrup, O =0-00193. 

 The true value of one vibration, T^ = 15^-9014. 



The deflecting bar having been placed in a stirrup of silk fibres, of the thick- 

 ness of the suspension thread, it was vibrated with the vertical circle of the inclinometer 

 balanced horizontally upon it ; the results are : 



Internal radius of ring = 0*39541 feet. External radius of ring = 043779 feet. 



Weight of ring = 9628-6 grains. 



Semiarcs of vibration, arc - ^ a = 12°, arc " ^ a' = 7f°, temperature of bar = 38°0 Fahr. 



Rate of chronometer, Dent, No. 1665, s = — 5^-0. 



Mean observed time of one vibration from 120 vibrations, T/ = 27^-8006. 



