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Proceedings of the British Association. 329 
that the following statements had never met the eye of Sir John, 
or he would at least have hesitated before he gave it as his opin- 
ion, that the air could not blow in towards a common ceutre 
without causing the barometer to rise above the mean. Mr. 
Forth says in the second volume of the Philosophical 'Transac- 
tions, (abridged,) that during a great depression of the barometer, 
January 8, 1735, he observed that the wind in the northern parts 
of the island blew from the N. E., and on the southern parts of 
the island from the S. W. And Mr. Howard says, in a great _ 
storm of 1812, the wind on the north of the Humber blew from 
the E. N. E., and on the south of the Humber from the S. W. 
‘Mr. Espy then stated that he found by calculating according to 
well-known chemical laws, that the caloric of elasticity given out 
in the air in which a cloud is formed, would expand the air in the 
cloud about 8000 cubic feet for every cubic foot of water formed 
in a cloud by condensation of the vapor; and he exhibited an in- 
strument which he called a nepheloscope, which enabled him to 
measure the expansion with great accuracy, and he found it to 
agree with the calculations made on chemical principles. He 
then proceeded to give an outline of his theory, premising that the 
numbers he should introduce were not intended to be strictly ac- 
curate, and would be subject to many corrections, one in particu- 
lar, in which no notice had been taken of the specific heat of air 
under different pressures.* . 
This paper gave rise to a very interesting conversation, but 
from the great length of the paper itself, we can only direct at- 
tention to the leading points of the discussion. Prof. Stevelly 
ealled the attention of the Section to the fact that he had at the 
Edinburgh meeting in 1834, used the principle of cold, produced 
by rarefaction, to explain what he called the secondary forma- 
tionof clouds, and thus the propagation of storms; and even as- 
signed this rarefaction as the cause of the summer hail. He ob- 
jected: to the main position, however, in Mr. Espy’s theory, that 
the fall of temperature caused by the expansion of any body of 
air rendered light by being loaded with moisture as it rose in the 
atmosphere, was the same as the constituent temperature of the 
Strata A air into which it ae that is, of equal tension. He 
* A synopsis of Mr. Espy’s philosophy of storms = 7 in Vol. 39, (pp. 
120—132,) and we therefore omit it in this place.— 
