498 



these progressively augment in arc-value, until in the 

 middle latitudes of both magnetic hemispheres the 

 extent of the variation, or the difference between the 

 extreme easterly and westerly values in the course of 

 twenty-four hours amounts, on the annual mean, to 

 nine or ten minutes. The deflections, or departures 

 from the mean declination are greatest during the hours 

 of the day; a circumstance which does not require, 

 however, the actual presence of the sun above the 

 horizon, as it takes place also in the arctic circle during 

 the polar winter. The extreme deflections occur, in the 

 middle latitudes of both hemispheres and in all meri- 

 dians, at nearly similar hours of solar time ; but with 

 the characteristic distinction, that, (speaking always 

 of the north end of the magnet,) in the northern 

 hemisphere the extreme easterly deflection takes place 

 in the forenoon, and the extreme westerly in the 

 afternoon ; and conversely, in the southern hemi- 

 sphere, the extreme westerly deflection in the fore- 

 noon and the extreme easterly in the afternoon. The 

 hours of the extremes are, for the forenoon, from 7 

 to 10 A.M., and for the afternoon from 1 to 3 P.M.: the 

 change of direction is most rapid from 10 to 11 A.M. 

 The hours of extreme deflection, or, as they are fre- 

 quently called, the " turning hours," do not appear to 

 depend, as some physicists have supposed, on the angle 

 which the magnetic direction may make with the astro- 

 nomical meridian (viz. on the declination); as the 

 hours are approximately the same in places which 

 differ very widely from each other in that respect. For 

 example, at Dublin, where the declination is 29 west; 

 at Prague, where it is 17 west; at Toronto, where it is 



