INTRODUCTION. XXV 



Calculation of the values of T and T'. 



T =4*.81325 ; S = . 002054 1",= 14*. 270536 ; ^=.004005 



log. T = 0.6824385 log. T'. 1.1525187 



' ' ' -0.0000007 i--. . . .9.9999799 



0.6824392 1.1543762 



~ ~ ' ^.3087424 



86400 I* ' ~ 86400 ~ 1 G 



H. . log ..... % 0008704 l+|hog ...... 0.0017359 



T 2 =23'.21383 = 1.3657488 T' 2 = 204V39878 . . 2.3104783 



Calculation of the value of K. 

 r =2 0.3000 feet ; r 1 = 0.2525 feet ; W = 1100 troy grains. 



log$r . . . .9.1760913. . .log 2 . . .8.3521826 = 0.022500 

 log |/. . . .9.1012314. . .log 2 . . .8.2024628 = 0.015939 



8.5847721 = r> + r' 2 = 0.038439 



log$ 9.6989700 



log W .... 3 0413927 



K'= 1.3251348 



T' 2 = 204. 39878 K' log 1.3251348 



T 2 = 23.21383 T 2 log 1.3657486 



T' 2 T 2 = 181. 18495 ar.co.log 7.7418717 



= 2. 7086638 = 0.4327551 



The value of K, obtained from a mean of several partial results, and used throughout the 

 computations, is K = 2. 66092. 



OBSERVATIONS TO DETERMINE THE TEMPERATURE CORRECTION. 



For these experiments two magnetometers are required: one for observing the total angle of 

 deflection and variations in its amount produced by changes of temperature; the other for 

 observing the changes of declination occurring during: the same intervals. The two instru- 

 ments should be observed simultaneously, and, if possible, observations should also be made 

 with a bifilar magnetometer at the same time. We had no bifilar magnetometer. 



May 14, 1854. In the east hall of the Smithsonian Institution, beneath a floor thirty feet 

 distant, supported on iron breams placed nearly north and south. At a like distance from the 

 spot chosen for experiments, but on the floor of the same room used, there were a few iron 

 gas-pipes. 



The unifilar being set up and carefully adjusted, a copper box, eight inches long by five 



inches in breadth and depth, was so placed to the east of it that the centre of the deflecting 



magnet, when on the supports within the box, would occupy a point in the same horizontal 



plane one foot and three-tenths from the suspended magnet. When the latter had come to 



IV A 



