222 



Observations of the Magnetic 



[July 



mate accuracy, it would be worth while to reduce the times of vibra- 

 tion under these circumstances to the times of describing an infinitely 

 small arc, as has been done by Hansteen, and on account of buoyancy, 

 to a vacuum ; but since such is not the case, the result will be obtained 

 to all useful accuracy by supposing the correction common to each set 

 of observations, by which the reductions, which are rather operose, 

 are avoided : the reduction to a temperature of 60° is effected by ap- 

 plying the correction, 0,00017 t. (where t represents the time of per- 

 forming 100 vibrations) ; — a formula which is derived from experi- 

 ment. The two needles used in the following observations are distin- 

 guished from one another by a sign x on one °f them. This needle in 

 London at a temperature of 60° performed 100 vibrations in 442,76 

 seconds of mean time, whereas the other needle performed 100 vibra- 

 tions under the same circumstances in 461,96 seconds; the former 

 needle is further distinguished from the latter from its having been 

 long in use in England, and as having exhibited a remarkable degree 

 of steadiness in its magnetism during the late magnetical experiments 

 instituted in Ireland under the auspices of the British Association ; 

 added to which, these needles are calculated to excite a more than 

 ordinary degree of interest from the circumstance of their having 

 been employed by Sir John Ross in the perilous North Polar Expedi- 

 tion, from which he has lately so fortunately returned. The observa- 

 tions at Madras are as follows. 



No. 3, Private mark x 



1837 Arc. Ther. Vib. Mean Time. Inter v. Mean Int. 



h. m. s. s. s. 



April 30th 20* 0 88.0 1 0 43 49.4 ) o no 9 } o m k 7 



1 ]2 45 101 48 51.3 \ ( ^ UL5 ' 



8 15 201 53 53.1 301.5 i > « . , 



4 0 — 301 58 54.1 301.0 3 cor. for temp. 1.43 



300.14 



Another set. 20 0 88.0 1 1 1 49.0 > m 2 ^ 3^ 



]2 4o 101 b 51.2) r 



8 15 201 11 52.6 301.4 C f t j 43 



4 15 87.8 301 16 53.8 301.2 ) cor, 101 temp. 



300.17 



May 3d 20 0 87.0 1 0 45 37-7 ) o 09 * ) 301 97 



12 45 101 50 40.2^ 0i -° ( ^ l ' J/ 



8 30 201 55 42.1 301.9 C f t j 38 



5 0 _ 301 1 0 43.6 301.5 ) cor. tor temp. 



300.59 



