Unifilar Magnetometer. xxiii 



north and south respectively, its centre being in the prolongation of the bifilar 

 magnet, b is the bifilar reading with the deflecting bar away. 



The deflecting bar was vibrated in the declinometer box after the observations 

 of deflection ; it was suspended in a stirrup of silk fibres of the same thickness as 

 the suspension thread, and a small thread of paper being gummed to the extremity 

 next the reading telescope, the times of transits of the paper at the wire of the read- 

 ing telescope towards right and left were observed for every 5 vibrations each way 

 up to the 96th vibration. (See Tables, pages 168 and 169.) 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 adjustment of the telescope. 



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

 $ 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 l'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, 



Aug. 11— Dec. 18, 1843, * = 0001556. During 1844, * = 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 (=6) in breadth ; 



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



a? + b 2 

 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 d 1848. Observations made by chronometer, Dent, No. 1665, rate — 5 S, 0. 



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



Mean observed time of one vibration from 160 vibrations, T ' = 15 s, 9037. 



Bifilar magnetometer observed every 5 m during vibration, mean during vibration corrected 

 for temperature = 5539 sc. div. 



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

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

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



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



