129 



sation of vapour, counteracted the reduction of temperature produced 

 by the expansion in a ratio which increased with the increase of tempe- 

 rature. Thus, he stated as the result of experiments, that an expan- 

 sion occurring in air saturated with aqueous vapour, at a temperature 

 of about 71°, produced an increase of temperature half as great as in 

 dry air; and at a temperature of 102°, a similar expansion increased 

 the temperature only one third as much as when the air was dry. 



Mr. Espy went on to show, that by experiments made with this in- 

 strument, he had been able to make out a law, from which, when the 

 temperature of the air and the dew point at the surface of the earth 

 under the base of a forming cloud are known, the decrease of tempe- 

 rature can be determined up to the base of the cloud, and even to its 

 top, though that should be ten miles high, as some great clouds in the 

 summer are. And as the temperature of the air on the outside of the 

 cloud is nearly known, being about one degree colder for every hun- 

 dred yards in height, the specific gravity of the cloud can be known, 

 when compared with that of the air surrounding it. Mr. Espy en- 

 tered into a calculation to show that the air under the base of a form- 

 ing cloud is colder about one degree and a quarter for every hundred 

 yards above the surface of the earth, and that from the base of the 

 cloud upwards it gets colder about one degree and a quarter for each 

 two hundred yards of increased elevation. This calculation is found- 

 ed on the supposition that there is an up-moving column of air under 

 and in every forming cloud, as established in his Philosophy of 

 Storms. 



Mr. Espy went on to state, that it is ascertained, both by experi- 

 ments made with the nephelescope and by calculations founded on 

 the well known laws of latent heat in vapour, and specific caloric of 

 air, that the latent caloric, given out into air by the vapour which 

 condenses into cloud, expands the air in the cloud about 8000 cubic 

 feet for every cubic foot of water generated in the cloud ; and it is 

 known, that it requires about 1300 cubic feet of vapour in the air to 

 make one cubic foot of water. The difference between these quanti- 

 ties, or 6700 cubic feet, is therefore the actual expansion for every 

 cubic foot of water generated from the condensing vapour. This 

 great expansion of the air in a forming cloud, should evidently cause 

 the air to spread out above, around the cloud, causing the barometer 

 to rise around it, by the increased quantity of gravitating matter, and 

 also causing the barometer to fall under the cloud, especially near 

 the middle of the base of the ascending column, as it is known to do 

 under great storm-clouds. 



