824 



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 the deficiencies of the 

 theory ; or those points wherein it neither furnishes, nor attempts to 

 furnish, explanations 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 diff'erent periods of the year, takes place. 



It is known that it does not occur at places situated in correspond- 

 ing latitudes north of the geographical equator ; at Algiers, for ex- 

 ample, — which is moreover nearly in the same geographical meridian 

 as the Cape, and where the magnetic inclination is nearly the same 

 towards the north as is the case at the Cape towards the south. It 

 may be quite correct perhaps to view the corresponding phenomena 

 at St. Helena and the Cape as those belonging to mc!p'we/ic«%- equa- 

 torial stations ; but they are certainly not those peculiar to or cha- 

 racteristic of geographicaUi/-eqna.toYidl stations, which would be the 

 condition in M. De la Rive's theory. There are thus two parts in 

 the problem which await a physical explanation ; on the one hand, 

 the cause is required of the contrariety of movement, as well as of 

 the times at which the different movements occur, the latter having 

 obviously a dependence on the sun's position whether in the northern 

 or the southern signs ; and on the other hand, the cause must be 

 shown why certain stations are thus affected and others not : a di- 

 stinction which obviously does not depend on situation in regard to 

 the geographical equator or to the tropical divisions of the globe. 



I have myself been led to infer that the peculiarity in question re- 

 sults from and is indicative of proximity to the line of least mag- 

 netic force, regarded as physically the separating line on the surface 

 of the globe between the northern and southern magnetic hemi- 

 spheres ; under this explanation the peculiarity would be strictly a 

 magnetically- equatorial one. 



It results from the present position of the four points of maximum 

 intensity at the surface of the earth, that the intermediate line of 

 least intensity departs considerably in the Southern Atlantic from the 

 middle or geographically-equatorial portion of the globe, and passes 



