

Magnetic Variations at different Places, 



201 



or whether the irregularities are wholly accidental, and that the 

 magnetic impulse in these motions is everywhere simultaneous, we can- 

 iiot say. The data are not sufficient to permit of the formation of 

 ;any definite conclusion. 



But whatever may be the explanation of the irregularities of 

 deviation, it would seem that over a certain portion of the earth's 

 surface the magnetic action in these initial impulses is practically 

 simultaneous. The values of mean deviation for Greenwich, Paw- 

 lowsk, Mauritius, and Bombay are O'l m., 0'2m., +0'5m., and 

 O'l m., respectively, an agreement which is certainly striking, and 

 which can hardly be explained on any supposition but that of 

 simultaneous action. For if we divide the series for these places 

 (Table I) into two groups, say of 8 and 9 values each respectively, 

 and take their means, we have as follows : 



In the first set the Greenwich and Bombay values agree, so also in 

 the second set (within 0*1 m.). Greenwich and Bombay are thus in 

 close accord, and in a remarkable degree, the agreement amounting 

 almost to a confirmation of the adopted difference of longitude. The 

 extreme difference between the four valuesat the different stations is 

 seen to be, in the first set l'8m., and in the second set 0'7m. And 

 the differences between the separate values at each station are 

 severally 0'4 m., 1'2 m., TO m., and 0'3 m. Now the latter differences, 

 which may be taken to indicate the amount of accidental irregularity 

 in the values, bear a considerable proportion to the differences 

 between the values at the separate stations (1'8 m. and 0*7 m.), so that 

 these differences are probably wholly accidental. Again, the mean of 

 the first four values is +O1 m., and of the last four OO m., showing 

 on the whole no real difference^ All this indicates that at Greenwich, 

 Pawlowsk, Mauritius, and Bombay the magnetic impulse would appear 

 to be really simultaneous, or at any rate that the variations between 

 the observed deviations at these places afford no trustworthy evi- 

 dence against such conclusion. 



We come thus to an interesting result. The magnetic impulse 

 over that portion of the earth's surface covered by the places last 

 mentioned would evidently appear to be, on the whole, simultaneous ; 

 .although whether, in like manner, simultaneous over the whole earth 



