TRANSACTIONS OF THE SECTIONS. 7 



the hottest and coldest parts of the earth's surface. We may expect them, there- 

 fore, 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 imagine 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 par- 

 ticular place, 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 

 dm'ing 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 wron^- in supposing tliat they 

 also, as regards any particular place, exhibit a daily variation in the intensity with 

 which they blow. 



Again, we are aware that the earth is a magnet. Let us uot now concern our- 

 selves about the origin of its magnetism, but rather let us talce 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 conductors 

 crossing Hues of magnetic force ; we may therefore expect them to be the vehicle 

 of electric currents. Such electric currents will of course react on the magnetism 

 of the earth. Now, since the velocity of these upper currents has a daily variation, 

 their influence as exhibited at any place upon the magnetism of the earth may be 

 expected to have a daily variation also. 



The question thus arises. Have wo possibly here a cause which may account for 

 the well-known daily magnetic variation r Are the peculiarities of this variation 

 such as to correspond to those which might be expected to belong to such electric 

 currents? I think it niay be said tliat, as far as we can judge, there is a likeness of 

 this kind between the pecidiarities of these two things ; but a more prolonged 

 scrutiny will of course be essential before we can be absolutely certain tliat such 

 currents are fitted to produce the daily variation of the earth's magnetism. 



Besides the daily and yearly periodic changes in these upper convection-cun-ents, 

 we shoidd also expect occasional and abrupt changes forming the counterparts of 

 those disturbances in the lower strata with which we are familiar. And these mav 

 be expected in like manner to produce non-periodic occasional disturbances of the 

 magnetism of the earth. Now it is well known that such disturbances do occur, 

 and, further, that they are most frequent in those years when cyclones are most fre- 

 quent — ^that is to say, in years of maximum sun-spots. In one word, it appears to 

 be a tenable hypothesis to attribute at least the most prominent magnetic changes 

 to atmospheric motions taking place in tlie upper regions of the atmosphere, where 

 each moving stratum of air becomes a conductor moving across lines of magnetic 

 force; and it was Sir William Thomson, I believe, who first suggested that the motion 

 of conductors across the lines of the earth's magnetic force must be taken into 

 account in any attempted explanation of terrestrial magnetism. 



It thus seems possible that the excessive magnetic disturbances which take place 

 in years of maximum sun-spots may not be directly caused by any solar action, 

 but may rather be due to the excessive meteorological disturbances which are like- 

 wise characteristic of such years ; on the other hand, that magnetic and meteoro- 

 logical influence which Mr. Broun has found to be connected with the sun's rotation 

 points to some unknown direct effect produced by our luminary, even if we 

 imagine that the magnetic part of it is caused by the meteorological. Mr. Broun 

 is of opinion that this effect of the sun does not depend upon the amount of spots 

 on his surface. 



In the next place, that influence of the sun in virtue of which we have most 

 cyclones and greater meteorological disturbance in the years of maximum spots, 

 cannot, I think (as far as we know at present), be attrijjuted to a change in tlie 

 heating-power of the sun. We have no doubt traces of a temperature effect which 

 appears to depend upon the sun-period ; but its amount is very small, whereas the 



