1839] 



Terrestrial Magnetic Force in Southern India. 



249 



3d. — On account of temperature. — It being found that a magnetized 

 needle exhibits a greater degree of intensity in cold than in warm 

 weather, it is necessary to render the observations comparable— that they 

 should either all be made at the same tempt-rature, or that, if the tem- 

 perature vary, a correction proportionate thereto should be employed : 

 for this purpose I have lately made several observations with the three 

 needles employed by myself, at temperatures between 70° and 80° ; by 

 observing in a cool room of uniform temperature, and afterwards in one 

 with a low roof with a higher temperature, when the following rates of 

 alteration were observed : 



No. ob- 

 servati- 

 ons. 



Needle 

 Faht. 

 therm. 



No. 1 



Time of 

 100 vib. 



Needle 

 Faht. 

 therm. 



? No. 2 j 

 Time of j 

 100 vib. 1 



Needl 

 Faht. 

 therm. 



e No. 3 

 Time of 

 100 vib. 



from 3 



0 



t'-t 8.0 

 r-|-il,0 



I, i- 0,37 

 1,4- ,70 

 1.4- ,57 



o 



t' 4-14 5 



\t' 108 

 \t' 4- 12.3 



s 



1 , 4- 0,90 

 I„ 4- 0.60 

 I.. 4- 0,70 



i ° ! ' 



f-^ 8.0 r,,, -h0 65 

 1^4- 8.8 I,,, + 0,27 

 \t ^ 9.l|l,,, f-0,44 



where I, I,, represent the time of performing 100 vibrations at the 

 lower temperature {i') — If I r'^present the time of performing 100 vibra- 

 tions at the standard teuiperature I ; and I, that of performai-'g the same 

 at any other temperature t' then the correction 

 I — I =al {t—t') 

 I-I, 



« = I {t-n 



s s s 



In which I^ I,, \,,, were 282, 312, and 277 respectively; whence 

 a = ,00018 for needle No. 1. 



_ ,00019 No. 2. 



= ,00019 No. 3. 



This near and unusual agreement inter se, arising no doubt from my 

 not having employed artificial heat (as has been usual in observations of 

 this nature), has induced me to suppose that the remaining two needles 

 require the same correction, which consequently has been applied in the 

 reductions (Table III). This premised, we may now proceed to the 



Discussion of the Intensity Observations. 



In the examination of the intensity observations, it will be found con- 

 venient and necessary, to divide them into three several classes, and to 

 discuss the observations with each needle — cylinder rather — in each 

 class separately ; thus — 



