1839] Terrestrial Magnetic Force in Southern India. 



247 



V r 



+ 81 50 2,0934 for one geographical mile. 



100 1 2,3900 



89 32 2,1731 — 



Shewing that a flexure takes place in the direction of the isoclinal lines, 

 as represented in fig. 3 — much at variance with the supposed uniformity 

 of their curvature, — and with reference to r, a determined variation from 

 the theory (tan. 6 = 2 tan. X) in which r cannot exceed 2 :— If we now 

 refer to page 2 13 for the mean values of B and X : we can, from the above 

 values of r and 0, compute more correct values than we have yet obtain- 

 ed of the latitude of the magnetic equator; thus 



jii Lat. of Mag. Equator. 



O / // O / ff 



80 6 47 4- 9 53 4 



79 27 23 10 3 35 



77 3 38 9 58 53 



These values are no doubt very near the truth ; the only doubt which 

 can exist, is, whether we ought not to have rejected the observations at 

 Kalehannary and Vadinatrum ? If we had done so, the above 2d and 3d 

 values would each be increased about two minutes. 



Intensity Observations. 



I have already mentioned that the intensity needles, employed for a 

 great portion of this work, were constructed by myself, immediately be- 

 fore quitting Madras, — when 1 had observed with them for about 30 or 40 

 days only. During this time, no perceptible alteration in the magnetic 

 intensity* of either of them having taken place, I determined to trust 

 to their invariability f — or rather to give the results, if on my return to 

 Madras it appeared that the needles had not varied ; — and the needles 

 constructed by Mr. Caldecott, had not been in use for a day previously to 

 my quitting Trivandrum. Under these circumstances, it will naturally 

 be expected that our intensity observations are of little worth ; particu- 

 larly since the observations have been made over a tract of country,where 

 from theory we know that an almost invariable intensity must prevail ; 



* The needles of this construction employed in England for measuring magnetic in- 

 tensity, have generally been found to part with their intensity, during the first two years 

 after being magnetized. 



+ The two needles to which this remark belongs, remained suspended in their boxes 

 during this time : had they been handled even, experience since shews that a difference 

 tyould probably hav« been obtained. 



