1871. ] Proceedings of the Asiatic Society. 39 
eurrents. There is not much difficulty in perceiving one great rea- 
son why the problems furnished by the extra-tropical parts of the 
globe are of especial complexity. The volume of vapour-bearing 
air which, rising from the equatorial belt and escaping away north- 
wards and southwards, constitutes the anti-trades, must, so to speak, 
shrink in volume as it proceeds towards the poles. It advances or 
flows away from the place of ascent in consequence of the superio- 
rity of the horizontal pressure which is represented by the sum of 
its own tension and that of its contained vapour at the height, 
where the lateral escape occurs over that of the adjacent portion 
of atmosphere. And it is enabled to pass into and fit itself to the 
gradually lessening spherical space which, as the result of gravita- 
tion, corresponds to the higher latitude of the terrestrial globe, 
because it gradually cools by radiation on its journey and as it 
cools contracts. The necessary result of this process is, that the 
onward flowing mixture of air and vapour comes to be at some 
point specifically heavier than the comparatively dry air which 
feeds the trades below it, and which is itself undergoing a converse 
process. Consequently the upper stream falls, or rather (for it is 
of course at any considerable distance from the equator generally 
moving with a high relative velocity) drives through the lower 
stratum, and makes its appearance on the earth’s surface as a steady 
southwest wind in the northern hemisphere and as a northwest 
wind in the southern hemisphere. The downcoming in this way 
of the anti-trades determines the outside edge of the belt, over 
which the trades prevail, so that on the polar side of this edge the 
atmospheric phenomena are the resultants of a totally new order of 
things, namely, a conflict of currents of equatorial westerly winds 
on the one side with currents of polar easterly winds on the other, 
the currents constantly shifting beds cnter se and always varying 
greatly in hygrometrical condition. We, therefore, see ample rea- 
son here for the complexity and variableness of the atmospheric 
phenomena in the extra-tropical zones. 
IT have so far entered upon these details, notwithstanding that 
every one present is probably more or less familiar with them, sim- 
ply by way of leading the members of this Society and indeed 
through them, persons outside our body to consider the singular ad- 
