112 Prof. Hare on the Gales experienced 
vapour, being much lighter than the atmospheric air, renders 
it more buoyant by its admixture. 
Yet the density, arising from inferiority of situation in the 
stratum of air immediately over the Gulf, compared with that 
of the volumes of this fluid lying upon the mountainous coun- 
try beyond it, may to a certain extent more than compensate 
for the influence of the heat and moisture derived from the 
Gulf: but violent winds must arise, as soon as these causes 
predominate over atmospheric pressure, sufficiently to ren- 
der the cold air of the mountains heavier. : : 
When, instead of the air covering a small portion of th 
mountainous or table land in Spanish America, that of the whole 
north-eastern portion of the North American continent is ex~ 
cited into motion, the effects cannot but be equally powerful, 
and much more permanent. The air of the adjoining country, 
first precipitates itself upon the surface of the Gulf, and after- 
wards, that from regions more distant. ‘Thus a current from 
the north-eastward is produced below. In the interim, the 
air displaced by this current rises, and being confined by the 
table land of Spanish America, and in part, possibly, by the 
trade-winds, from passing off in any southernly course, it is, 
of necessity, forced to proceed over our part of the continent, 
forming a south-western current above us. At the same time 
its capacity for heat being enlarged, by the rarefaction arising 
from its increased altitude, much of its moisture will be pre- 
cipitated ; and the lower stratum of the south-western cur- 
rent, mixing with the upper stratum of the cold north-eastern 
current below, there must be a prodigious condensation of 
aqueous vapour. 
The reason is obvious why this change is productive only 
of north-eastern gales—and that we have not northern gales, 
accompanied by the same phenomena. The course of our 
mountains is from the north-east to the south-west. Thus no 
channel is afforded for the air proceeding to the Gulf, in any 
other course, than that north-eastern route which it actually 
ursues. 
That the table lands of Mexico are competent to prevent 
the escape over them of the moist warm air displaced from 
the surface of the Gulf, must be evident from the peculiar dry- 
ness of their climate—and the testimony of Humboldt. Ac- 
cording to this celebrated traveller, the clouds formed over 
the Gulf never rise to a greater height than four thousand 
nine hundred feet; while the table land, for many hundred 
leagues, lies between the elevation of seven and nine thousand 
feet. Consistently with the chemical laws which have been 
experimentally ascertained to operate throughout aaa: 
whic 
