SOLID AND LIQUID PARTICLES IN CLOUDS. 317 



extremely small, they evaporate so quickly, that never more than two or three are ever 

 visible at a time on one square of the micrometer. As the cloud gradually thins away, 

 the number of drops diminishes and their size at the same time decreases. 



The maximum number of water particles actually observed in a cloud is four times as 

 great as the maximum yet observed in a fog. We cannot, however, from this draw any 

 conclusion, as the recorded observations are too few to found upon. 



There is an interesting point connected with the conditions existing in some clouds, 

 which I have not yet been able to study as fully as is desirable. The point to which I 

 refer is, What is the state of the air with regard to humidity in a cloud when all exposed 

 surfaces are dry ? We have seen, that while the fog-counter showed the air to be full of 

 water particles, showering down at the rate of thousands of drops to the square inch in a 

 minute, yet all exposed surfaces were frequently quite dry. Not only were they 

 dry, but if wetted they soon dried again, showing that the air was absorbing moisture 

 while it was at the same time packed full of water drops. Further, wet and dry bulb 

 thermometers hung in the open may show a difference of a degree or two, proving that 

 evaporation is going on. 



What, then, is the explanation of this apparent contradiction ? Simply this : radiant 

 heat. The sun's rays, falling on the upper surface of a cloud, are partly absorbed and 

 spent in heating it and evaporating some of the suspended water, but a good deal of 

 the heat penetrates the cloud, and falling on the surface of bodies heats them ; while 

 these heated surfaces in turn heat the air in contact with them, and the small cloud 

 particles, when they fall into this hot stratum of air, are either evaporated before reach- 

 ing the surface of the bodies, or are rapidly evaporated after they touch. 



In all cases in which I had an opportunity of testing the humidity of the air in a 

 cloud, it was found to be saturated. Though the wet and dry bulb thermometers 

 may have shown a difference of a degree or more, when not properly protected from 

 radiation, yet they read alike when radiation was completely cut off. That a vast 

 amount of radiant heat may penetrate through clouded air is easily proved by exposing a 

 thermometer with black bulb in vacuo. An instrument of this kind exposed while 

 these observations were being made, indicated 40 and 50 degrees above the tempera- 

 ture of the air, and it was always above the temperature of the air when surfaces were 

 dry. Further, when the clouds overhead get thin, a glow of heat can be distinctly 

 felt on the hands and face. It is this radiant heat passing through the clouded air, 

 and absorbed by exposed surfaces, which heats them and keeps them dry, though 

 surrounded by saturated air and exposed to a continuous shower of fine rain. 



Although this conclusion has been arrived at by the aid of the fog-counter and the 

 vacuum thermometer, yet the same conclusion was drawn from some observations I 

 made on the summit of Pilatus, which I visited last year, while it was in cloud. The 

 result of the observations made at that time I had incorporated into my dust observa- 

 tions of last year, which are only yet in preparation, but as they properly belong to the 

 present subject I have transferred my remarks to this paper. 



