138 COSMOS. 



tain Younghusband. At Toronto the principal disturbances 

 generally occurred in the period from midnight to 5 A.M. ; it 

 was only occasionally that they were observed as early as from 

 10 P.M. to midnight, and consequently they predominated by 

 night at Toronto, as well as at Hobarton. After having 

 made a very careful and ingenious investigation of the 3940 

 disturbances at Toronto, and the 3470 disturbances at 

 Hobarton, which were included in the cycle of 6 years (from 

 1843 to 1848), of which the disturbed variations constituted 

 the ninth and tenth parts, Sabine was enabled to draw the 

 conclusion 75 that " the disturbances belong to a special kind 

 of periodically recurring variations, which follow recognisable 

 laws, depend upon the position of the sun in the ecliptic and 

 upon the daily rotation of the earth round its axis, and, 

 further, ought no longer to be designated as irregular 

 motions, since we may distinguish in them, in addition to a 

 special local type, processes which affect the whole earth." 

 In those years in which the disturbances were more frequent 

 at Toronto, they occurred in almost equal numbers in the 

 southern hemisphere at Hobarton. At the first-named of 

 these places these disturbances were, on the whole, doubly as 

 frequent in the summer, namely from April to September, as in 

 the winter months, from October to March. The greatest 

 number fell in the month of September, in the same manner as 

 at the autumn equinox in my Berlin observations of 1806. T6 

 They are more rare in the winter months in all places ; at 



75 Sabine, in the Phil. Transact, for 1851, pt. i, pp. 125127. " The 

 diurnal variation observed is in fact constituted by two variations 

 superposed upon each other, having different laws, and bearing different 

 proportions to each other in different parts of the globe. At tropical 

 stations the influence of what have been hitherto called the irregular 

 disturbances (magnetic storms), is comparatively feeble ; but it is other- 

 wise at stations situated as are Toronto (Canada) and Hobarton (Van 

 Diemen's Island), where their influence is both really and proportion- 

 ally greater, and amounts to a clearly recognisable part of the whole 

 diurnal variation." We find here, in the complicated effect of simul- 

 taneous but different causes of motion, the same condition which has 

 been so admirably demonstrated by Poisson in his theory of waves 

 (Annales de Chimie et de Physique, t. vii, 1817, p. 293). " Waves of 

 different kinds may cross each other in the water as in the air, where 

 the smaller movements are superposed upon each other." See Lamont's 

 conjectures regarding the compound effect of a polar and an equatorial 

 wave, in Poggend. Annalen, Bd. Ixxxiv. s. 583. 



< 6 See p. 134. 



