312 Mr. Daniell on the Radiation of Heat. 



or his talents and vast acquirements I have the most unfeigned 

 respect, and I beg to assure him that, if he should hereafter con- 

 descend to redeem his pledge of analyzing the remaining Essays 

 of my book, in the true spirit of philosophic inquiry, I shall es- 



eem myself most highly honoured. 



Art. X. On Evaporation ; by John Bostock, M.D.,F,R.S., 

 in a Letter to J.F. Daniell, Esq. 



Dear Sir, 



In perusing the account of your Experiments on 

 Evaporation, in No. XXXIII. of the Quarterly Journal, I was 

 struck with the coincidence of their results with some which I ob- 

 tained several years ago, on the same subject. ]My object was to 

 ascertain the absolute amount of evaporation, from a given surface 

 of water, in different states of the atmosphere. I employed, for this 

 purpose, a shallow silver dish, of two inches in diameter, with per- 

 pendicular sides. The dish, containing 100 grains of distilled 

 water, was accurately weighed ; the thermometer, barometer, di- 

 rection of the wind, and other atmospherical phenomena, which 

 might be supposed to affect the results, were noticed. A small 

 China cup of water was placed near the dish, in which a delicate 

 thermometer remained immersed during the experiment. The dish 

 and cup were inclosed in a glass cylinder with an open top, in order 

 to prevent the current of air from accelerating the evaporation, 

 •while, at the same time, the surface of the water was freely exposed 

 to the atmosphere. After a certain inten^al the dish was again 

 weighed, and the loss of weight noted. 



I was induced to undertake the experiments for the purpose of 

 ascertaining how far the rise and fall of the barometer was affected 

 by the quantity of water dissolved, or suspended in the atmosphere. 

 They commenced in July, 1812, and were continued, occasionally, 

 until January, 1815, when the results were found so little to sup- 

 port the hypothesis that I had formed, and appeared altogether so 

 anomalous, that I discontinued them, and laid them aside, as of no 

 value. I can, however, venture to assert, that they were made 



