1842 .] 
THE NATIONAL INSTITUTION. 
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condensation of vapor. Mr. Espy showed, that a dense cloud is actually produced 
in the nephelescope at the moment of expansion when moist air is used, and that 
the quantity of expansion thus obtained by experiment actually agrees with the 
result obtained by calculation founded on the well known laws of latent caloric of 
steam, specific caloric of atmospheric air, and expansion of air by heat. 
Mr. Espy then gave a brief outline of the elements of his theory or philosophy 
of storms, nearly in the following words: 
Up-moving currents of air may be formed either by heat or moisture. In ascend¬ 
ing they will come under less pressure and expand ; in expanding they will become 
colder about one degree and a quarter for every hundred yards of ascent; and as the 
dew-point sinks by this expansion about one quarter of a degree, cloud will begin 
to form in the up-moving current, at about as many hundred yards high as the dew¬ 
point at the time is below the temperature of the air in degrees. 
When the vapor begins to condense into cloud, the latent caloric will begin to 
be evolved, and the higher the column ascends the more vapor will be condensed, 
and the more latent caloric will be evolved, and above the base of the cloud the air 
in ascending one hundred yards will cool only about one half as much as it would 
do if no vapor was condensed. Now, as the law of cooling on the outside of the 
ascending column is known to be about one degree for every hundred yards of 
ascent into the atmosphere, the temperature of the air in the inside of the ascend¬ 
ing column may be compared, at all its different heights, with the air on the outside ; 
and consequently their relative specific gravities will thus be known. On making 
such comparison, Mr. Espy showed, that when the dew-point is very high, and the 
cloud thus formed of great perpendicular diameter, its specific gravity would be so 
much less than that of the surrounding air as to cause the barometer in extreme 
cases to fall nearly three inches, or about as much as it is known to do in great 
storms. The evolution of latent caloric, then, in the formation of cloud, Mr. Espy 
contends, is not merely a vera causa, but the sole cause of the fall of the barometer 
in storms, unless it shall be shown that this instrument sometimes actually falls 
more than three inches, which it has never yet been known to do. Mr. Espy then 
went on to show that his theory was demonstrated by three independent methods. 
First, by calculations founded on well known physical laws; second, by experi¬ 
ments with the nephelescope; and, third, by its ability to explain all the pheno¬ 
mena. Under the last head a few were mentioned which he deemed decisive of the 
question. The great degree of cold suddenly produced necessary to condense such 
immense quantities of vapor as are known to be condensed, in particular cases, can 
not be accounted for on any supposition but the up-moving current of air in the 
cloud. At Joyeuse, on the 9th of October, 1827, there fell over a small territory 
thirty-one inches of rain in tv/enty-two hours, and the latent caloric given out by 
the condensing vapor producing this rain, would be sufficient to heat the lower half 
of the atmosphere over the region where the rain fell about six hundred degrees. 
At the mouth of the Catskill, on the 26th July, 1819, there fell over a region of 
nine miles in diameter, ten inches of rain in half an hour, which would give out 
latent caloric enough to heat the lower half of the atmosphere two hundred degrees; 
or, in other words, would produce about the same effect in producing an up-moving 
as seven thousand tons of anthracite coal burnt in half an hour, over each square 
