SOLAR-DIURNAL VARIATION OF THE DECLINATION. 507 



The station, St. Helena, which first placed in a clear and 

 unmistakable point of view the peculiar features of the 

 semi-annual inequality, and the at least approximate 

 absence of any other diurnal variation, is in 15 56' south 

 latitude, and has a magnetic dip of 22. The distance, 

 therefore, is considerable, both from the terrestrial 

 equator and from the line of no dip ; and in respect to 

 the sun's vertically, he is in the zenith of St. Helena in 

 the first week of November and the first week of 

 February, whereas the equinoxes are the epochs when 

 the opposite phases of the semi-annual inequality pass 

 into each other, and all diurnal variation disappears. 

 On the other hand, St. Helena is situated in the close 

 vicinity .of the present position of the line of least force, 

 which, in consequence of the great distance apart (in 

 that quarter of the globe) of the two foci of maximum 

 force in the southern hemisphere, has a large southerly 

 inflection, and passes between St. Helena and the 

 Cape of Grood Hope, but somewhat nearer to St. Helena. 

 The Cape of (rood Hope furnishes still stronger evi- 

 dence to the same effect: the phenomena are similar, 

 indeed nearly identical with those at St. Helena, mark- 

 ing the Cape distinctly as a magnetically-equatorial 

 station, whilst its geographical latitude is 33 56 ', and 

 its dip 53 (very distant therefore from the line of no 

 dip), and the sun is not vertical in any part of the year. 

 A very instructive contrast is presented when we com- 

 pare the diurnal variation at Algiers and at the Cape of 

 Good Hope ; the two stations are at opposite points of 

 the same continent, Algiers in 36 north, and the Cape in 

 34 south latitude, and are therefore nearly equidistant 

 from the terrestrial equator : and as the dip is 57 north 



