823 



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

 of the phenomena at St. Helena. 



Extract 1. 



" La limite qui separe les regions occupi^es par chacun de ces 

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

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

 des paralleles compris entre les tropiques, qui a le soleil a son z6nith ; 

 elle change par consequent chaque jour." 



Extract 2. 



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

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

 efFet, 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 

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

 courants vont du pole austral vers Tequateur." 



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

 the Philosophical Transactions, describing the phenomena at St. 

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

 on the diurnal variation in each month during the five years in 

 which hourly observations were maintained day and night at that 

 observatory, will perceive, — on evidence which admits of no uncer- 

 tainty, — that the two portionsof the year in which the diurnal variation 

 is in contrary directions at that island, are not determined, as M. De 

 la Rive supposes, by the declination of the sun relatively to the la- 

 titude of the place, but by the declination of the sun relatively to the 

 equinoctial line. The sun is vertical at St. Helena, passing to the 

 south in the first week of November ; and again when passing to the 

 north in the first week of February : consequently the two portions 

 into which the year is thus divided, are respectively the one of three, 

 and the other of nine months' duration ; but the actual portions in 

 which the contrary diurnal movements of the magnets take place at 

 St. Helena are of equal duration, and consist of six months and six 

 months ; the dividing periods coinciding unequivocally, not with the 

 sun's verticality at St. Helena, but with the equinoxes. 



2. But if M. De la Rive's explanation be thus inconsistent in respect 

 to the dates of the transition periods of the phenomena at St. Helena, 

 it must be regarded as altogether at variance with, and opposed to, 

 the phenomena described in the same paper at the Cape of Good 

 Hope, where also they have been observed at the Magnetic Obser- 

 vatory at that station with an exactness which leaves no uncertainty 

 whatsoever as to the facts themselves. The Cape is not situated within 

 the tropics; its latitude is 33° 56' south; the sun is consequently 

 throughout the year well to the north of its zenith ; and therefore, 

 according to M. De la Rive's theory, the deviations should be in one 

 and the same direction throughout the year. But the fact is not so ; 

 for the same contrariety in the direction of the diurnal variation at 

 difi^erent portions of the year takes place at the Cape as at St. He- 



