MAGNETIC SURVEY OF GREAT BRITAIN. 179 



can have no sensible effect upon the deduced values of the in- 

 tensity. 



The ratio of the intensity at any station to that at the base- 

 station being denoted by (f), we have (Fifth Report, p. 147,) 

 _ cos sin {Si ~ 6^ 

 ^ ~ cos 6, sin [h — 6)' 



Hence, supposing S and S, to vary by any small and equal amount, 

 A h, the corresponding variation of <^ will be expressed by the 

 formula 



^ = { cotan (S, ^ e;) - cotan {8-d)} AS. 



Now the quantity, A S, is very small, and (where the stations 

 are not widely separate) the coefficient by which it is multi- 

 plied is likewise small; for such, stations, then, the result- 

 ing value of —^ is inconsiderable. On substituting the nume- 

 rical values of S, S^, 6, 6i, for the extreme stations of the present 

 series, it will be seen that the correction does not affect the fourth 

 place of decimals. 



Values of the Intensity at the Base stations. — The following 

 is a summary of the comparisons of the intensity at London, 

 Dublin, and Limerick, as contained in Table LXXV. 



Intensity at Dublin, referred to London : 

 Aug. Sept. 1834 . . . Needle L 4 Int. = 1-0194 



Sept. Oct. Nov. 1835 

 April, May, 1836 . . 

 April, May, 1836 . . 

 June 1837, Oct. 1838 



— L 4 — = 1-0212 



— L 3 — = 1-0194 



— L 4 — = 1-0189 



— S 2 — = 1-0183 



Mean . . =1-0194 

 Intensity at Limerick, referred to London : 

 June, July, Aug. 1834 . Needle L 4 Int. = 1-0262 



Intensity at Limerick, referred to Dublin : 

 Aug. Sept. 1835 .. . — L 4 Int. = 1-0030 



July 1836 — S 2 — = 1-0062 



Mean . . = 1-0046 

 We have therefore (Fifth Report, p. 148), 

 a = 1-0194, h = 1-0262, c = 1-0046; 



c^ = — = 1-0067, 0,-0= -0021 ; 



A = 5, B = 1, C = 2. 



