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 centre 

 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 

 called 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 w^hat he called the secondary forma- 

 tion of 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 

 \hefall 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 of air into which it rose, that is, of equal tension. He 



* A synopsis of Mr. Espy's philosophy of storms was published in Vol. 39, (pp. 

 120 — 132,) and we therefore omit it in this place. — Eds. 



