The Science of the Weather 785 



dense rain-cloud of the cumulus form. It is very unstable and is 

 the source of all our heavy showers of rain and hail in the summer 

 months. Often it is accompained by storms of thunder and 

 lightning, the latter caused by the disruptive discharge of the 

 electricity that has accumulated upon the raindrops in the cloud, 

 and the former being the audible effect of the discharge. Light 

 travels nearly a million times as fast as sound, and therefore we 

 see the lightning before we hear the thunder; both are, of course, 

 actually simultaneous. Lightning flashes may take place between 

 cloud and earth, and also from cloud to cloud. 



There are many clouds in other parts of the world which we 

 have not portrayed here. There is not always a simple answer to 

 the question, why they do this, and why they do that. 



The huge cloud which overhangs a volcano (Humboldt's 

 Volcanic Cloud) as it rolls in huge masses, with the lightning 

 flashing through it, sometimes descends to earth with vio- 

 lence, and rolls down the mountain side with a velocity for 

 which neither gravitation nor wind can account. It so moves 

 because it is attracted to the earth, by the direct action of 

 its high electrical charge. In various degrees similar electri- 

 cal phenomena doubtless play their part in the movements 

 of more ordinary clouds. Again, the gossamer threads of 

 cirrus clouds, the highest clouds of all, sometimes run criss- 

 cross in complex entanglement, and do other things which 

 wind, or wind alone, is not enough to account for. The 

 puzzles which the clouds set us, one way and another, are 

 endless, and our study of them, like that of so many other 

 interesting things in the world, has only begun. 



The formation of clouds is not easy to explain; to do so re- 

 quires on the part of the reader a knowledge of the laws of ther- 

 modynamics, and these are rather complex. But anyone can 

 observe the clouds intelligently, and the changes they undergo, 

 and from these changes learn to anticipate the weather that is 

 likely to follow. Such a study will be found as interesting as it 

 is useful. 



