Force of the Earth's Magnetism. 



17 



from 40° to 50° higher ; and 3d, again when cooled to the 

 summer temperature ; a mean being taken between the 1st and 

 3d, to compare with the 2d. 



With these arrangements, needle No. 3 made the time of 70 

 vibrations as follows : — 



August, 1828. 

 In the ordinary temp., 19' 28".8 Th. 64°.5 

 In the cooled temp. .... 



In the cooled temp. .... 

 In the ordinary temp., 19' 28".0 Th. 64°.0 

 19' 28".4 Th. 64°.25 



19' 23".7 

 19' 22".8 



Th. 40° 

 Th. 47° 



July, 1829. 

 In the ordinary temp., 19' 26". 1 Th. 67° 

 In the heated temp. 

 In the ordinary temp., 19' 26".4 Th. 68° 



19' 23".25 Th. 43°.5 



19' 35".07 Th. 116° 



19' 26".25 Th. 67°5 19' 35".07 Th. 116° 

 Whence we have, by the first mode of experiment, an increase 

 of 5."15 in the time of vibration, caused by an increase of 

 20°. 75 in the temperature ; and by the second mode of expe- 

 riment, an increase of 8".82 in the time, caused by an increase 

 of 48°.5 in the temperature. Their sum, 13".97 -r 69°.25 = 

 0".2, the increase in the time of vibration for one degree of 

 Fahrenheit; and 0".2 -J- 19' 28" = .00017, the multiplier for 

 one degree. Then, if T be the time of vibration of this needle 

 at an observed temperature t, and if it be required to know the 



JULY— SEPT., 1829. C 



