468 lioyal Society. 



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 

 different portions of the year takes place at the Cape as at St. He- 

 lena ; the two portions of the year in which the opposite phenomena 

 prevail, are also identical at the two stations ; and at both the 

 change in the direction of the deviation takes place when the sun 

 crosses the equinoctial line ; the deviation being to the west at both 

 stations when the sun is in the northern signs, and to the east when 

 he is in the southern signs. 



3. But in considering a theory which comes before us, claiming 

 the high distinction of affording a physical explanation of facts which 

 are known to us by well-assured observation, we ought not to con- 

 fine our view to those points only for which it professes to supply 

 the explanation : these are certainly tests as far as they go ; — and in 

 the present instance the conclusion from them is not favourable to 

 the theory proposed ; — but we should also notice its deficiencies ; or 

 those points wherein it neither furnishes, nor attempts to furnish, ex- 

 planations of circumstances which are certainly amongst the most 

 remarkable facts of the case. They may be possibly amongst the 

 most difficult to explain, but no physical theory can be regarded as 

 meeting the facts which does not at least attempt an explanation of 

 them. I may name as the most prominent in interest amongst these 

 the striking fact, that the Cape of Good Hope should be one of the 

 stations at which this remarkable peculiarity of a contrariety of move- 

 ment at different periods of the year takes place. 



