3G OF CLOUDS. CHAP. 1. 10. 



of this chapter, I may observe, that if, agreeable 

 to the experiments of modern chemists, the con- 

 needs to be insisted upon. Glass, and all other hard, smooth 

 substances, I have tried, when cooled to a degree below what 

 the surrounding aqueous vapour can support, cause it to be 

 condensed on their surfaces into water. The degree of cold 

 is usually from 1 to 10 below the mean heat of 24 hours; in 

 summer I have often observed the point as high as 58 or 

 59, corresponding to half an inch of mercury in force; and 

 once or twice have seen it at 62"; in changeable and windy 

 weather it is liable to considerable fluctuation: but this is not 

 the place to enlarge upon it. 



" For the purpose of observing the evaporation in atmo- 

 spheric temperatures I got two light tin vessels, the one six 

 inches in diameter and half an inch deep, the other eight 

 inches diameter and three fourths of an inch deep, and made 

 to be suspended from a balance. When any experiment, 

 designed as a test of the theory, was made, a quantity of 

 water was put into one of these (generally the six-inch one, 

 which I preferred), the whole was weighed to a grain; then 

 it was placed in an open window or other exposed situation 

 for 10 or 15 minutes, and again weighed to ascertain the loss 

 by evaporation ; at the same time the temperature of the 

 water was observed, the force of the aqueous atmosphere 

 ascertained as above, and the strength of the current of air 

 noticed. From a great variety of experiments made both in 

 the winter and summer, and when the evaporating force was 

 strong and weak, I have found the results entirely conformable 

 with the above theory. The same quantity is evaporated with 

 the same evaporating force, thus determined, whatever be the 

 temperature of the air, as near as can be judged; but with the 



