1814.] Dr. JVells' Essay on Dew. 301 



21. BiogTaphical account of Peter Jacob Hjelms. A translation 

 of this will be inserted in the next Number of the Annals. 



II. An Essay on D^w, and several Appearances connected with 

 it. By William Charles Wells, M D. F.R.S. London. Taylor 

 and Hessey. 1814. 



Dr. Wells was induced, in consequence of a rude experiment 

 made in IJ84, to think it probable that the formation of dew is 

 attended with the production of cold Mr. Wilson and JVlr. Six 

 entertained the same opinion : and Dr. Wells, in the course of his 

 reading, met with many facts that appeared to confirm it; but he 

 made no experiments on the subject till the autumn of 1811. 

 The facts then observed induced him to begin a series of observa- 

 tions in 1812; these were prosecuted in 1813, and gradually led 

 to a knowledge of the facts -contained in this publication, which 

 may be considered as a theory of the formation of dew — a theory 

 whi< h appears to me both satisfactory and complete. 



The essay is divided into three parts. In the fiist we have an ac- 

 count of the phenomena of dew. It appears only in calm and 

 serene nights. In cloudy nights the quantity is small, and the 

 same observation applies to windy nights; and when the night is 

 both cloudy and windy, no dew whatever is deposited. It usually 

 begins to make its appearance on grass towards the decline of the 

 day, when the sun has ceased to shine on it; but it does not appear 

 in any considerable quantity till "after sun-set. It continues to be 

 deposited during the whole of the nifiht till after sun-rise, supposing 

 no change in the weather to have taken place. There is usually a 

 greater quantity deposited between midnight and sun-rise than, 

 between sun-set and midnight. Supposing every thing else the 

 same, the deposition of dew is more abundant the more the atmos- 

 phere is loaded with moisture* 



Whatever has a tendency to diminish the aspect of the sky to 

 any body diminishes at the same time the quantity of dew deposited 

 on it. Thus, a flat board was placed in a horizontal position four 

 feet above b grass plat; 10 grains of wool placed on the under sur- 

 face of it gained only four grains of weight, while the same quan- 

 tity placed on the up]; r surface gained 11 grains. In another trial 

 the wool on the under side gained six grains, while that on the 

 upper side gained 19 grains. In another tnal the wool on the 

 under side gained two grains, while that on the upper side gained 

 11 ; and in another the wool below gained lour grains, while that 

 above gained _'0. This difference was not owing. to the moistuie 

 falling Irotn above in a form similar to that of rain, otherwise the 

 wool oti the under surface uould have acquired no increase of 

 Weight, Besides, a quantity of wool placed at the bottom of a tall 



stone-ware cylinder, open at top, acquired just as little increase of 

 weight as the wool on the under surface of the board. 



When wool is lajd upon a large horizontal body, as a board, it 



acquires more dew than when suspended ia die air at a distance 



i 



