EVAPORATION. 265 



has a velocity less than its own, and by this pressure, and 

 perhaps also by its greater coldness, condenses the vapor 

 contained in it, and thus forms a cloud. This cloud, be- 

 ing so dense that little or no light can pass through it, 

 appears black. And the degree of darkness on the den- 

 sity of the vapor, or in other words, on the velocity of 

 the wind, and the quantity of water in the portion of air 

 compressed. The beautiful colors, that often adorn the 

 sky at sunset, are caused by the clouds reflecting the sun's 

 light. That redness of the sky in the morning, which is 

 often regarded as the precursor of a storm, probably re- 

 sults from the red rays of the sun passing through the 

 vapor collected in the air. Light is composed of seven 

 different colored rays, possessing different degrees of 

 force. These may be seen separate from each other in 

 the rainbow. Of these the red rays have the greatest 

 force or momentum. Hence, when the air is very full 

 of vapor, the red rays have sufficient power to penetrate 

 it, while the others have not. Many of the red rays, 

 however, do not come directly from the sun, but are scat- 

 tered in various directions on striking the vapor, and thus 

 the redness is diffused over a considerable space. 



Thunder-clouds exhibit an appearance peculiarly strik- 

 ing. To many they are objects of terror ; in a greater 

 or less degree they arrest the attention of almost every 

 one. These clouds are collections of vapor strongly 

 electrified. They are generally very dense, and very 

 near the earth. Frequently two clouds rise in different 

 parts of the horizon, and move towards each other, till 

 they meet, at the same time rising up towards the zenith. 

 When clouds in different electrical states approach near 

 each other, or when a strongly electrified cloud approaches 

 near to the earth, the electricity is discharged in vast 

 quantities and with tremendous violence, thus constitut- 

 ing what is termed lightning, while the concussion given 

 to the surrounding air by its force, and the rushing to- 

 gether of the portions of air separated by its motion causes 

 thunder. This sound, reflected and reverberated among 

 the clouds, produces the lonsf-coritinued and solemn roll, 

 which forms one of the sublimest characteristics of a 

 thunder-storm. It is often imagined that lightning always 



