358 
ON THE FINE REGIONS OF THE TRADE WINDS. [June 14, 1858 
resides in the influence of certain vast mountainous tracts, which 
produce an indraught of air from all sides, owing to causes we will 
proceed to indicate. These mountainous districts condense the 
moisture of the air that impinges upon them in large quantities, and 
Mr. Hopkins considers that condensation of moisture lightens the 
air, and therefore causes it to rise, for two distinct reasons : the 
one, that it has become specifically lighter owing to its loss of 
water ; the other, that it is warmer, and therefore more rarefied, than 
other air at the same elevation as itself, owing to heat given out by 
the act of condensation. If these postulates are acceded to, it 
follows that we have a system of ascending currents, an indraught 
of winds to feed them, and a consequent escape and overflow of dry 
air in the higher regions. The present paper suggests, rather than 
undertakes an enquiry into the localities, where the dried air again 
reaches the earth in fulfilling its circuit. 
“ The best evidence we have of the immediate source of the air In the trade 
winds of the Pacific is to be found in its degree of dryness. All accounts re- 
present them as being at first, and to a considerable degree, accompanied by a 
clear sky, but I have met with no register of the humidity of the air. The 
researches that are now in progress by Americans, as well as English, to 
improve our knowledge of nautical geography will probably give fresh informa- 
tion ; but it is particularly desirable that there should be registrations of the 
wet and of the dry bulb-thermometer in the parts treated of. If we were in 
possession of such registrations over a few lines of the Pacific from e. to w., a 
fair inference might no doubt be arrived at respecting the sources of the great 
masses of air which constitute the trade winds of this ocean.” 
Quotations are then made from various well known authors, Dana, 
Melville, and Darwin, to show the extent over which the fair 
weather of the trade winds extends, both in the Pacific and the 
Atlantic. 
Over both these oceans Mr. Hopkins considers that the currents 
in the higher regions pass in an opposite direction to those below, 
but that elsewhere, as in the Indian Ocean, the case is probably 
different, 
“ It is not necessary that we should presume that the air which descends in 
a fine locality has come from the terminus of the same wind that it feeds. 
The proposition is that air which has been dried by condensation of some of its 
vapour, in high regions, descends in some other parts to the lower regions as 
dry air, making the locality fine, but it does not follow that it must descend 
to feed the same stream that had furnished it.” 
“ Condensation of vapour, by irregularly disturbing the atmosphere at 
various heights, puts the air in motion at those heights, making it ascend in 
one part, and it must come down in another. Some of the areas of ascension 
have been described, and a few of descent, but there are innumerable others 
spread over the surface of the globe, every hill or place, where heavy rain falls, 
being to some extent an area of ascent, with the wind that blows towards it 
coming directly or indirectly from a region of descent. The whole aerial 
