METEOROLOGY OF THE SUN AND EARTH, 83 



nection between the meteorology of our luminary and that of our 

 planet, even although we are in complete ignorance as to the exact 

 nature of this bond. 



If "we now turn to terrestrial magnetism, the same connection be- 

 comes apparent. 



Sir Edward Sabine was the first to show that the disturbances of 

 the magnetism of the earth are most violent during years of maximum 

 sun-spots. Mr. Broun has shown that there is likewise a reference in 

 magnetic phenomena to the period of the sun's rotation about his axis, 

 an observation recently confirmed by Hornstein ; and still more re- 

 cently, Mr. Broun has shown that the moon has an action upon the 

 earth's magnetism which is not altogether of a tidal nature, but de- 

 pends, in part, at least, upon the relative position of the sun and 

 moon. 



I must trust to your forbearance if I now venture to bring forward 

 considerations of a somewhat speculative nature. 



We are all familiar with the generalization of Hadley,that is to 

 say, we know there are under-currents sweeping along the surface of 

 the earth from the poles to the equator, and upper-currents sweeping 

 back from the equator to the poles. "VVe are likewise aware that these 

 currents are caused by the unequal temperature of the earth ; they 

 are in truth convection-currents, and their course is determined by the 

 positions of the hottest and coldest parts of the earth's surface. We 

 may expect them, therefore, to have a reference not so much to the 

 geographical equator and poles as to the hottest and coldest regions. 

 In fact, we know that the equatorial regions, into which the trade- 

 winds rush and from which the anti-trades take their origin, have a 

 certain annual oscillation depending upon the position of the sun, or, 

 in other words, upon the season of the year. We may likewise ima- 

 gine that the region into which the upper-currents pour themselves is 

 not the geographical pole, but the pole of greatest cold. 



In the next place we may imagine that these currents, as far as 

 regards a particular jilace, have a daily oscillation. This has, I believe, 

 been proved as regards the lower-currents or trade-winds, which are 

 more powerful during the day than during the night, and we may 

 therefore expect it to hold good with regard to the upper-currents or 

 anti-trades ; in fact, we cannot go wrong in supposing that they also, 

 as regards any particular place, exhibit a daily variation in the inten- 

 sity with which they blow. 



Again, we are aware that the earth is a magnet. Let us not now 

 concern ourselves about the origin of its magnetism, but rather let us 

 take it as it is. We must next bear in mind that rarefied air is a good 

 conductor of electricity ; indeed, according to recent experiments, an 

 extremely good conductor. The return-trades that pass above from 

 the hotter equatorial regions to the poles of cold, consisting of moist 

 rarefied air, are therefore to be regarded in the light of good conduct- 



