f 110 ] 
XVIII. Essay on the Gales experienced in the Atlantic States 
of North America. By Rovert Hare, M.D. Professor of 
Chemistry in the University of Pennsylvannia*. 
OF the gales experienced in the Atlantic States of North 
America, those from the north-east and north-west are 
by far the most influential: the one, remarkable for its dryness 
—the other, for its humidity. During a north-western gale, 
the sky, unless at its commencement, is always peculiarly clear, 
and not only water, but ice, evaporates rapidly. A north- 
east wind, when it approaches to the nature of a durable gale, 
is always accompanied by clouds, and usually by rain or snow. 
The object of the following essay, is to account for this striking 
diversity of character. 
When, by a rise of temperature, the lower portions of a 
non-elastic fluid are rendered lighter than those which are 
above them, an exchange of position must ensue. The par- 
ticles which were coldest at first, after their descent, becoming 
the warmest, resume their previous elevation; from which 
they are again displaced by warmer particles. Thus, the 
temperatures reversing the situations, and the situations re- 
versing the temperatures, a circulation is kept up, tending to 
restore the equilibrium. : 
Precisely similar would be the case with our atmosphere, 
were it not an elastic fluid, and dependent for its density on 
pressure as well as on heat. Its temperature would be much 
more uniform than at present—and all its variations would be 
gradual. An interchange of position would incessantly take 
place, between the colder air of the upper regions, and the 
warmer, and of course lighter, air, near the earth’s surface, 
where there is the most copious evolution of solar heat. Cur- 
rents would incessantly set from the poles to the equator below, 
and from the equator to the poles above. Such currents would 
constitute our only winds, unless where mountains might pro- 
duce some deviations. Violent gales, squalls, or tornadoes, 
would never ensue ; gentler movements would anticipate them. 
But the actual character of the air, with respect to elasticity, 
is the opposite of that which we have supposed. It is perfectly 
elastic. Its density is dependent on pressure, as well as on 
heat; and it does not follow, that air which may be heated, in 
consequence of its proximity to the earth, will give place to 
colder air from above. The pressure of the atmosphere vary-_ 
ing with the elevation, one stratum of air may be as much 
rarer by the diminution of pressure, consequent to its altitude, 
* From the Journal of the Academy of Natural Sciences of Philadelphia. 
as 
