80 REPOBT— 1886. 



conductors when heated by the sun, and hence, through a diminution 

 of resistance, the electric current will be increased. The curious 

 similarity detected by Senhor Capello between the diurnal variation of 

 the magnetic dip and that of the tension of aqueous vapour (see Appendix 

 II. to this report) might perhaps seem to point to a variation of 

 absorptive power, and hence of electric conductivity brought about in 

 certain atmospheric strata by the carriage of aqueous vapour. 



Again, Sir G. B. Airy (see appendix to the Greenwich observations, 

 1884) has expressed his opinion that the diurnal magnetic inequality is 

 due mainly, if not entii-ely, to the radiant heat of the sun, and he is also 

 led to imagine that the magnetic effect of the sun's heat upon the sea is 

 considerably greater than the effect on land ; while again Sir J. Henry 

 Lefroy (see Appendix III. to this report) having observed a difference in 

 the time of turning between the solar-diurnal variation of declination at 

 Toronto and at Greenwich, has expressed his belief that this difference is 

 due to the fact that these two places are differently situated with regard 

 to the Atlantic. 



9. The fact that the solar-diurnal variation is greater at times of 

 maximum than at times of minimum sun-spot frequency is explained by 

 the advocates of this hypothesis on the assumption that not only is the 

 sun most powerful on the former occasions, but that the solar radiation 

 then contains probably a larger proportion of such rays as are absorbed by 

 the upper strata of the atmosphere, while the composition of these strata 

 with respect to aqueous vapour may likewise be such as to cause 

 an increased absorption. This increased absorption means an increased 

 temperature, and hence an increased conductivity. 



10. It has moreover been adduced in favour of this hypothesis that 

 the tendency seems to be, as pointed out by Mr. William Ellis and by Pro- 

 fessor Stewart, that changes in the range of the daily variation of magnetic 

 declination lag behind corresponding solar changes in point of time. 

 This kind of behaviour is apparently inconsistent with direct magnetic 

 action of the sun operating as the chief cause, and points rather to some 

 indirect influence, probably caused by the radiant energy of the sun, 

 inasmuch as the changes and turning-points of such indirect influences 

 due to radiation are well known to lag, in respect of time, behind the 

 corresponding changes and turning-points in their cause. This subject 

 demands further attention. 



11. Hitherto we have been considering that portion of the motion of 

 the upper atmospheric currents which is from west to east in both hemi- 

 spheres. Let us now consider that portion of such motion which is from 

 south to north in the northern, and from north to south in the southern, 

 hemisphere.' 



Now, here it may be well to remark that it seems quite possible to 

 conceive a set of cui-rents to exist in the earth's atmosphere without 

 exhibiting a considerable diurnal variation. Let us take, for instance, an 

 ordinary electric current, say of a circular shape and horizontal, and heat 

 it by causing some source of heat, such as a lamp, to travel slowly round 

 it with a definite rate of progress. It will be evident that we shall have 

 (assuming the current to be otherwise constant) no variation in flow due 

 to this heating effect. In like manner, if there be electric currents in 



' The discussion of this point is almost identical in wording with a similar dis- 

 cussion brought by Professor Stewart before the Physical Society. 



