821 



instruments. Tables are appended to the paper, showing the monthly, 



quarterly and yearly mean temperatures, with those of groups of 

 years, and other tables exhibiting the de|PB,rture of every individual 

 result from the mean of all. 



The author concludes by stating, that hitherto the mean tempera- 

 ture at Somerset House has been estimated a great deal too high. 

 He does not here enter into the investigation as to whether the tem- 

 perature as now determined is too high for the geographical position 

 and elevation of Somerset House, but proposes to do so, in a paper 

 he is preparing with the view of connecting the Somerset House 

 with the Greenwich series, and of bringing up all the results to the 

 present time. He hopes also, at some future time, to present results 

 from the barometrical observations arranged in a similar manner. 



May 10, 1849. 



The EARL OF ROSSE, President, in the Chair. 



The following communication was read : — " Remarks on M. De la 

 Rive's Theory for the Physical Explanation of the Causes which 

 produce the Diurnal Variation of the Magnetic Declination," ia 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 Annales de Chimie et de Physique for March last contains a 

 letter from M. De la 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 facts" 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 w^hich 

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



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