822 



professes to explain, and others which appear to me to be altogether 



at variance with, and opposed to it. 



1. M. De la Rive's theory, in a few words, is as follows : — 



In consequence of the inequalities of temperature in the higher 

 and lower strata of the atmosphere, electric currents are generated, 

 which in the higher regions proceed from the equator to the poles, 

 and return at the surface of the earth from the poles to the equator ; 

 the return current causing in the northern hemisphere the north end 

 of the magnet to deviate in the one direction, and in the southern 

 hemisphere in the opposite direction ; the deviation being at any 

 given place greatest at the hour (about 1^-30 p.m.) when the differ- 

 ence of temperature in the upper and lower strata of the atmosphere 

 is greatest, and of course increasing until that hour, and subse- 

 quently diminishing. 



That the north end of the magnet does thus deviate in the fore- 

 noon towards the west in the northern hemisphere, and towards the 

 east in the southern hemisphere, and return in both cases in the 

 opposite directions in the afternoon, were facts knoMm before the 

 establishment of the magnetic observatories ; but M. De la Rive's 

 explanation of them appears to have been suggested, and its appro- 

 priateness, as he considers, is shown, by its affirmed accordance with 

 the remarkable peculiarity in the phenomena made known to us by 

 the observations at the Magnetic Observatory at St. Helena, and 

 communicated to the Royal Society in the paper referred to. This 

 peculiarity is briefly as follows : the deviation which constitutes the 

 principal part of the diurnal variation at St. Helena is not uniform 

 in its direction throughout the year ; in one part of the year it is to 

 the west, and in the other part of the year to the east ; and conse- 

 quently during certain months of the year the movement of the 

 magnet is in the contrary direction to that which prevails at the 

 same hours during the other months of the year. 



Now St. Helena is situated within the tropics, and M. De la Rive 

 infers from his theory that in all places so situated, the diurnal va- 

 riation should be in one direction when the sun's declination is north 

 of the latitude of the place, and in the contrary direction when the 

 sun's declination is south of the latitude of the place : and hence he 

 too hastily concludes that his theorv' accords with the characteristics 

 of the diurnal variation at St. Helena. When however the facts are 

 more closely examined, it is seen that they do by no means accord 

 with M. De la Rive's supposition. 



That it may be quite clear that I do not misapprehend either M. 

 De la Rive's theor}', or his supposition in regard to the facts at St. 

 Helena, I subjoin his own expressions, which convey his meaning, 

 as that gentleman's writings generally do, with most commendable 

 precision. 



The first extract defines the limit which, according to his theory, 

 should separate the electric currents proceeding respectively from 

 each of the poles to the equator ; and should consequently separate 

 the parts of the globe in which the diurnal variation is in the one 

 direction, from the parts in which it is in the opposite direction ; 



