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particles into the larger drops which fell, and the precipitation of these by gravitation 

 and by the blast, aided, it may be, by the co-operation of electric force, the process being 

 essentially the same whether the blast have come on as an onward moving cold wave or 

 have advanced as an advancing whirlwind which raised the air through which it passed 

 -to an elevation at which, it may be in consequence of sudden expansion, the temperature 

 was too low to retain all the moisture in solution. 



" With the copious evaporation going on from the leaves of a forest, there is nothing 

 surprising in any change of wind producing a cloud or mist above a forest, where 

 formerly the air had been perfectly transparent, and everything known in regard to such 

 phenomena makes it probable that in general, if not invariably, the cloud is produced 

 there, and not attracted thither by the forest." 



Let us go on with the explanation, premising that it is founded on the one given 

 by Herschel, and published in the " British EncyclopsBdia ;" that it has been adopted :? 

 "by Flammarion, one of the leading French meteorologists, who published an exhaustive 

 work containing it in 1879 ; and that this work is edited by Mr. Glaisher, one of the 

 leading British authorities of to-day. 



It is aa follows : — At the equator, where, as we all know, it is always very warm, 

 -the broad heated ocean sends up, as is the nature of water when heated, vast quantities of 

 itself in the form of vapour. At the same time the air there is always being heated, and 

 rises as the vapour does, with great force, but not with nearly so much as the vapour. 

 Vapour of water is the lightest of all known vapours, and except hydrogen and ammonia, 

 the lightest even of gases. How much lighter than air it is you can see for yourself, if 

 you notice how fast it climbs through the air from the pot to the ceiling. True, the air 

 has not quite its chance ; it, too, would climb fast if it were hot. But heat it as you will 

 it could not climb like that. Now, the quantity of water sent into the air in the tropics 

 by evaporation is immense. It is calculated that throughout the whole great equatorial 

 region there rises thus annually a body of water sixteen feet deep. That is over half an 

 inch a day. That does not sound large, but it will sound larger when you think how 

 it grows. Turned into vapour, even at only fifty degrees temperature, it takes a 

 space many thousand times larger than before, or some thousand feet high over the 

 whole region. Add to this that the air it has been forced into when thrown upwards 

 from the ocean is itself expanding largely, and therefore becoming lighter and rising also, 

 you will see that there is an immense body of air and vapour being sent upwards very 

 rapidly and constantly over the whole great equatorial region. Now this uprising leaves 

 no large vacuum of its own size, as a body of that dimension would if sent upward in 

 some circumstances. There is no vacuum left whatever, for the water is below and con- 

 tinually affords fresh vapour. But north and south there is abundance of air ; air, too, 

 which has not been heated as has that of our central body which is going up ; and as the 

 central body over that vast space — remember we are speaking of a belt round the world 

 thousands of miles wide — as it gets heated, rarefies and rises, the great cooler and more 

 solid bodies of air north and south rush in, themselves become dilated in the scorching 

 heat, and rise upwards in their turn along with the immense volume of vapour, which 

 being still more inclined to rise than they 'are, hurries them aloft ; so that at the equator, 

 or rather in the great equatorial regions, are two great masses of air rushing from the 

 temperate regions, north and south, towards the equator, meeting, rising, and going 

 upwards together with the vapour arising from the sea. 



