1842.] THE NATIONAL INSTITUTION. 141 



condensation of vapor. Mr. Espy showed, that, a densa 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 outhnc of the elements of his theory or philosophy 

 of storms, nearly in the following words : 



Up-moving currents of air maybe formed tith r 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 cun'ent, at about as many hundred yards high as the dew. 

 point at the time is below the temperature of the air in degrees. 



Whan 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 bo 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 outsidu ; 

 and consequently their relative specific gravities will thus bo known. On making 

 Buch comparison, Mr. Espy showed, that when the dew-point is very high, and th(j 

 cloud thus formad 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 9tli of October, 1827, there fell over a small territory 

 thirty-one inches of rain in twenty-two hours, and the latent caloric given out by 

 the condensing vapor producing this rain, would be sufficient to beat the lo.ver 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 atmospliere two hundred degrees ; 

 or, in other words, would produce about the same effect in prociucing an up.moving 

 as ucvun thousand tons of anthracite coal burnt in half an hour, o^er each square 



