176 ON TWO STORMS EXPERIENCED THROUGHOUT THE UNITED STATES, 
snow as they fall, producing rain; and as the heat of the storm increases, the breadth of 
the rain increases to beyond the limits of the United States, and this, although the storm 
was continually advancing to a higher latitude. As the storm continued, the north-west 
wind which succeeded it set in with increased violence towards this region of rarefac- 
tion, producing a sudden depression of the thermometer, amounting to more than twenty 
degrees below the mean. 
‘T'wo questions here naturally arise. First, What caused the formation of the first cloud 
on the morning of the fifteenth? and secondly, Why should the storm once organized 
ever cease? ‘This first cloud I ascribe to the superior weight and density of the air in 
the central parts of the United States. This superior weight was the result of a storm 
experienced in the same region two days previous, the margin of which is seen upon the 
extreme east border of chart 1. So also the low temperature which succeeded the pre- 
sent storm, as exhibited in chart 5, produced a similar easterly wind, causing a cloud 
which is seen upon the extreme left of the chart, which grew into a storm of nearly 
equal violence with that we are now considering. Thus, one storm begets its successor. 
The undulations thus excited in the atmosphere bear considerable analogy to the waves 
of the ocean agitated by a tempest, and which are propagated by mechanical laws long 
after the first exciting cause has ceased to act. 
Secondly, It may be thought, if the explanations here given are well founded, that 
when a storm is once organized it should go on increasing in violence, and never cease 
raining while there was vapour to be precipitated. By inspecting the annexed charts, 
I think we shall see a principle in operation which must prevent any considerable 
further increase of violence, and soon cause it to decline. On chart 3, the line of — 20° 
is distant about seven hundred miles from that of + 5°; on chart 4, it is distant only five 
hundred miles from that of + 10°; and on chart 5, it is less than five hundred miles from 
the line of + 20°. ‘The line of mean temperature is only one hundred and sixty miles 
from that of + 20°. This cold north-west wave advanced more rapidly than the centre 
of the storm. It was beginning to counteract the increase of temperature arising from 
the condensed vapour, and after that the violence of the storm must rapidly decline. 
Let us now turn to the storm of February 4. Beginning with February 1, sunset, we 
find the barometer: at its mean height near the valley of the Mississippi, and .3 inch 
above the mean in the vicinity of New York. The thermometer stood 10° above the 
mean near the Mississippi, while farther east it stood at the mean. For both of these 
reasons there should be an easterly wind near the Mississippi. This easterly wind 
encountering the prevalent westerly wind beyond the United States, which is seen on 
the chart the next morning, is lifted from the surface and a portion of its vapour is con- 
densed, forming cloud. This is the origin of the two clouds near the Mississippi. No 
great elevation is requisite to produce this effect. Although the dew point at the earth’s 
surface may be low, there is always a point, not very elevated, where the air is saturated 
with vapour. Any increased elevation of this stratum will cause a precipitation of mois- 
ture. Cloud being once formed in considerable quantity, contains within itself the prin- 
ciple of rain. ‘The easterly winds become more general and strong, and the precipitated 
vapour soon reaches the earth in the form of rain. In the eastern states the winds were 
strong from the north-west, the result of the storm which had prevailed the day previous. 
