SOLAR-DIURNAL VARIATION OP THE DECLINATION. 511 



the additional complication which is occasioned by the 

 superposition of this disturbance-diurnal variation upon 

 the variations treated of in 1 and 2. Respecting 

 the disturbance-variation generally, it must be noticed 

 in the first place that, although it presents at each of 

 the stations where it has yet been examined, the same 

 characteristics of a regular, systematic, and easily deter- 

 minable progression, with well-marked hours of maxima 

 and minima, these hours, unlike the turning hours of 

 the variations treated of in the two preceding sections, 

 vary at different stations apparently without limit. It 

 had been at one time inferred from observations at a 

 European station, ee that the disturbance- variation has 

 always the same direction as the regular solar-diurnal 

 variation, and thus the disturbing forces tend only to 

 increase the action of the forces which produce the 

 solar-diurnal variation ; " this conclusion, perfectly well- 

 founded as regards the evidence derivable from one 

 station, may be met with equal truth by an exactly 

 opposite one at another station. For example, at two 

 places on the American continent, Toronto and Point 

 Barrow, the regular solar-diurnal variation has the same 

 turning hours, and the same direction at both stations. 

 But the disturbance-variation turns, indeed, at both 

 stations, nearly at the same hours, but does so in oppo- 

 site directions, the greatest westerly extreme of the 

 disturbance-variation at the one station being reached 

 nearly at the same hour as the greatest easterly at the 

 other station, and vice versa ; so that at one station the 

 two variations (as in the European case above referred to), 

 agree with and reinforce each other ; while at the other 

 station they oppose, and, to a certain extent, counteract 



