IN THE MONTH OF FEBRUARY, 1842. 175 
doubled, is 673° F., and the same quantity is withdrawn when air of the ordinary state 
expands into double its volume. According to the experiments of Desormes and Clement, 
the heat is greater than this. When we consider the extreme mobility of the atmosphere, 
it can hardly be doubted that the cold of expansion is almost identically that which 
prevails ordinarily in the higher regions of the atmosphere, and that, when vapour is 
condensed by the cold of diminished pressure, the heat evolved raises the temperature of 
the surrounding air above that which is proper to this height, and it is thus elevated 
still farther by its own buoyancy. 
In some parts of the Arctic regions, clouds are almost unknown in the winter. The 
reason is obvious. The temperature is constantly below zero, and the tension of vapour 
can seldom rise as high as 0.068. If all this vapour should be condensed, it would raise 
the temperature of the air only 7°; so that if air from the earth’s surface could be 
mechanically elevated to a great height, its temperature, notwithstanding the condensed 
vapour, could hardly exceed, perhaps not equal, that of the surrounding air. Hence 
there is little opportunity for ascending currents, and consequently no cloud. 
Let us begin with the first formation of cloud on the western border of the United 
States, which happened a little before sunrise of February 15. ‘The heat liberated in the 
formation of this cloud raises the thermometer, causing a more decided tendency of the 
air inward towards the region of condensation. ‘The whole heavens being covered with 
a veil, radiation from the earth’s surface is checked, while it is going on beyond the 
region of the cloud. This causes a relative elevation of temperature under the cloud, and 
gives increased velocity to the inward current of air. More cloud is thus formed, heat 
is liberated: the air, expanded, swells up beyond its usual limits, and flows over in 
every direction, carrying with it the snow formed, which, while suspended in the air, 
appears merely as cloud, causing the barometer to fall gradually in the centre of the 
storm. ‘Thus the storm gains violence by its own action, and being floated eastward by 
the prevalent westerly current, it ceases in the western states, while it is still raging 
with increased violence in the east. ‘The winds might then be expected to flow inward 
towards the centre of the storm. As, however, the storm covers a large area and is some- 
what irregular in shape, the winds could not all blow towards one point. Moreover, such 
a central tendency, whether in air or water, uniformly causes the fluid to approach the 
centre, not exactly in radii, but circuitously. This effect can hardly be detected on the 
chart for the fifteenth, at sunrise, but at sunset a slight tendency may be perceived, to 
sirculate against the sun. On the sixteenth, at sunrise, the phenomenon is more strongly 
marked, and at sunset it is impossible to overlook it. There is a physical cause for this 
rotation, and for its being uniform in direction, in the case of large storms. The 
southerly wind has a greater motion eastward than the parallels upon which it succes- 
sively arrives, arising from the rotation of the earth; and the northerly wind a less 
motion eastward, or a relative motion westward. Hence in this region the circulation, in 
great storms, is probably invariable in direction. 
On the chart of February 15, at sunset, we see ina striking light, the effect of the 
upper current. The condensed vapour as it rose spread out in every direction, but only 
slightly westward, while eastward it extended nearly four hundred miles in advance of 
the snow. On the southern border, the temperature is such as to melt the flakes of 
