460 Royal Society, 



May 10. — " Remarks on M. De la Rive's Theory for the Physical 

 Explanation of the Causes which produce the Diurnal Variation of 

 the Magnetic Declination," in a letter to S. Hunter Christie, Esq., 

 Sec. R.S., from Lieut.-Col. Sabine, For. Sec.R.S. Communicated by 

 S. Hunter Christie, Esq. 



My dear Sir, Woolwich, April 16, 1849. 



The Annates de Chimie et de Physique for March last contains a 

 letter from M. Dela Rive to M. Arago*, in which a theory is pro- 

 posed, professing to explain, on physical principles, the general 

 phenomena of the diurnal variation of the magnetic declination, and, 

 in particular, the phenomena observed at St. Helena and at the Cape 

 of Good Hope, described in a paper communicated by me to the 

 Royal Society in 1847, and which has been honoured with a place 

 in the Philosophical Transactions. 



Although I doubt not that the inadequacy of the theory proposed 

 by M. De la Rive for the solution of this interesting problem will be 

 at once recognised by those who have carefully studied the facts 

 which have become known to us by means of the exact methods of 

 investigation, adopted in the magnetic observatories of recent esta- 

 blishment ; yet there is danger that the names of De la Rive and 

 Arago, held in high and deserved estimation as authorities on such 

 subjects, attached to a theory, — which moreover claims reception on 

 the ground of its accordance with "well-ascertained fticts" and 

 " with principles of physics positively established," — may operate 

 prejudicially in checking the inquiries which may be in progress in 

 other quarters into the causes which really occasion the phenomena 

 in question ; I have thought it desirable therefore to point out, in a 

 very brief communication, some of the important particulars in which 

 M. De la Rive's theory fails to represent correctly the facts which it 

 professes to explain, and others which are 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 known before the 

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



* [A translation of this letter appeared in the Phil, Mag. for April 1849, 

 p. 386.— Ed.] 



