174 ON TWO STORMS EXPERIENCED THROUGHOUT THE UNITED STATES, 
The prevalent westerly winds glancing up over the Alleghany ridge deposit a portion 
of their vapour, forming cloud on the western slope, particularly in winter, when these 
winds are usually strongest, and the dew point differs least from the temperature of the 
air. It is thought that the operation of this principle may be traced in the storms of 
February, 1842. On the fourth, at sunset, the clouds were very much broken, particu- 
larly on the east slope of the Alleghanies, while cloud, and in some places rain, still lin- 
gered on the west slope. As the westerly wind pours over the mountains and descends 
to the level of the sea, it comes under greater pressure, and heat is developed, which dis 
solves the precipitated vapour, producing clear sky. Thus clear sky succeeds a storm much 
Sooner on the eastern than on the western side of the mountains. The same is seen at 
sunset on the sixteenth, the clear sky advancing with great rapidity on the southern 
border of the storm. 
I do not regard the configuration of the earth’s surface as the main cause of storms in 
the United States, but only as an auxiliary and modifying cause. The same effect 
which is produced by an interposed mountain is produced by the apposition of surface 
winds, by which means a mass of air is elevated into a region of greater cold. 
iI. THE MOTIONS OF THE WIND. 
That an easterly wind should spring up on the morning of February 15, in the region 
of Ohio, was the necessary result of the greater weight and density of the air to the east- 
ward. But a westerly wind, at the same time, prevailed a little beyond the Mississippi 
river. ‘These two winds were partially opposed, and from this opposition the air between 
them was elevated somewhat above the surface of the earth. Being cooled by diminished 
pressure, its vapour is condensed, a portion of it falls as snow, and the remainder forms 
cloud which expands and covers the surrounding country. . The condensation of the first 
vapour developed heat, which diminished the specific gravity of the surrounding air, 
thereby causing a more decided tendency towards the storm, which increased the preci- 
pitation and the development of heat, so that the storm increased in violence as it con- 
tinued. In the region of the greatest condensation, the rise of temperature was probably 
greater than at the surface of the earth. As the result of this rarefaction, the air swells 
up above its usual height, and flows off in every direction, carrying with it the cloud 
already formed, and causing the barometer to fall steadily as the storm continues to rage. 
Let us examine particularly the case of a body of air, from the earth’s surface, forced up 
to a considerable elevation. 
According to Poisson, “‘ Nous manquons des données nécessaires pour la solution com- 
plete de ce probléme;” nevertheless, I propose to inquire to what extent the problem is 
undetermined. Gay Lussac, in his aeronautic ascent to the height of twenty-three thou- 
sand feet, found the thermometer fall 1° F. for three hundred and seventeen feet clevation. 
According to Professor Forbes, the mean decrease, in Europe, is 1° for three hundred and 
fifty-two feet; according to Pouillet, three hundred and sixty-four feet in the equatorial 
regions, and three hundred and nineteen in Europe; according to Lubbock, 1° for three 
hundred and forty feet. If we assume 1° for three hundred and thirty feet, we cannot be 
far from the truth. According to Leslie, the heat liberated, when air has its density 
