492 Meteorology. 
and this is, I imagine, the cause of the m- 
crease of its weight in that season. 
I am aware that another conclusion, the 
very opposite to this, may be deduced from 
the premises. Aqueous vapour, it may .be 
said, is specifically lighter than dry air 5 and 
on that account, the greater the quantity, of 
aqueous vapour, the less is the weight in- 
any given volume of air of given elasticity. 
But it must be remembered that the aqueous 
vapour at the most, constitutes but 2, part 
of the atmosphere, and any excess of this 
which may prevail in any one place, cannot 
be supposed. powerful enough to’ move the 
rest of the atmosphere towards any other 
place, where the vapour is deficient. . Now, 
we have no reason to believe that much inter- 
course takes place between the atmospheres 
of the northern and southern hemispheres. 
The great and unceasing currents of air are 
between the equator and the polar regions; 
but that any large volumes of air cross the 
equator from one hemisphere to the. other, 
does not appear from any phenomena we are 
acquainted with. And if the air does not 
cross the equator, the vapour cannot, being 
so intimately blended with the air. ‘Thus, 
although there may be a constant pressure or 
tendency of the atmosphere in the northern 
