Royal Society* 467 



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

 priateness, as he considers, 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 theory 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 theory, 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 ; 

 whilst the second extract describes what he believes to be the facts 

 of the phenomenon at St. Helena. 



Extract 1. 



" La limite qui s6pare les regions occupies par chacun de ces 

 deux grands courants n'est pas I'equateur proprement dit, car elle 

 doit etre variable : elle est, d'apres la theorie que je developpe, celui 

 des paralleles compris entre les tropiques, qui a le soleil a son zenith ; 

 elle change par consequent chaque jour." 



Extract 2. 



" A St. Hel^ne, la variation diurne a lieu k I'ouest tant que le 

 soleil est au sud de I'ile, a Test d6s que le soleil est au nord. En 

 eiFet, dans le premier cas, ainsi que j'ai remarque plus haut, St. 

 Helene doit faire partie de la region dans laquelle les courants elec- 

 triques vont sur la surface de la terre du pole boreal aux regions 

 6quatoriales ; et, dans le second cas, de la region dans laquelle ces 

 courants vont du pole austral vers I'equateur." 



Whoever will be at the pains to refer to the paper printed in 

 the Philosophical Transactions, describing the phenomena of St. 

 Helena, or to the volume containing the details of the observations 



2H2 



