176 



REPORT — 1859. 



thus obtained both— and m X, either of the quantities m, X may now be found. 

 j\ 



The earth's horizontal force for any station being thus found, we have 

 only to divide it by the cosine of the observed dip at the station taken at 

 the same time, in order to find the total magnetic force. 



The following observations with the instrument used in the survey were 

 made at Kew as a base station. 



Table VII. 

 Total Magnetic Force obtained by the Survey Unifilar. 

 Date of observation, Total force. 



Aug. 5 10-291 



„ „ 10-297 



Oct. 14 10-292 



„ 15 10-301 



1858. 



June 21 10281 



„ „ 10291 



„ „ 10-300 



10-304 



Mean 10-295 



We may without error regard this mean result as the value in British 

 units of the total magnetic force given by the instrument used in the survey 

 at Kew Observatory, and corresponding to the epoch 1st January, 1858. 

 The following series of observations made by another equally reliable in- 

 strument, also at Kew, affords perhaps a still more reliable determi- 

 nation. 



Table VIII. 



Total Magnetic Force obtained by the Kew Unifilar. 



Date of observation, Total force, 



monthly means. 



1857. 



April 10 



May 10 



June 10 



July 10 



August 10 



September 10 



October 10 



1858. 



January 10 



February , 10 



March 10 



April 

 May. , 

 June 

 July. 



Augus 



10 

 10 

 10 

 10 

 10 



September 10 



October 10 



November 10 



3025 



3080 



300 



306 



287 



306 



315 



282 



299 



296 



291 



3075 



280 



296 



302 



2875 



317 



301 



Total force, January 1858, most probable value. . . . 10-299 



