SOLAR-DIURNAL VARIATION OF THE DECLINATION. 515 



and special cause, and how the residual portion of the 

 variation would be affected by its separation ; and this 

 interest is increased when, even on a slight examination, 

 we perceive that the hours which are chiefly affected by 

 disturbance are those hours of the night when the con- 

 tinuous, and otherwise regular, easterly progression of 

 the diurnal variation suffers interruption, occasioning a 

 double, and even sometimes in the winter months a 

 triple, alternation of easterly and westerly movement." 

 (Tor. Obs. vol. i. p. xxi.) 



The view thus early taken has been strengthened by 

 the results of the observations in succeeding years. The 

 third volume of the same work contains a discussion of 

 the diurnal variation obtained from five years of hourly 

 observation at Toronto, wherein it is shown (pp. Ixxxvi 

 Ixxxix) that when all the disturbances are allowed to 

 remain in the body of the observations, the nocturnal 

 westerly retrogression commences a little after 9 P.M., 

 and the easterly point which the declination had reached 

 at that hour is not regained until after 4 A.M., having 

 been at 2 P.M. 0'.7 west of that point : but that, when the 

 larger disturbances, or those which equalled or exceeded 

 Z on either side of the mean or normal at the same hour 

 in the same month, are eliminated, the westerly retrogres- 

 sion is diminished in duration to the interval between 1 1 

 P.M. and 3 A.M., and in value to 0''19 at 2 P.M. ; and fur- 

 ther, that if we proceed on the assumption that by ab- 

 stracting the disturbances equalling or exceeding 5', we 

 may have eliminated half of the whole influence of the 

 causes to which the larger disturbances are due (which 

 is certainly no unreasonable supposition), the westerly 

 retrogression becomes almost wholly obliterated, and the 



