passes over a space of thousands of feet and often does great 

 damage where it strikes. 



The thunder is a sound which may be compared to the crack 

 heard when a spark passes from the trolley, though of course the 

 noise is very much louder. The crack of the lightning echoes 

 and reverberates among the clouds, often changing to a great 

 rumble ; but this rumbling is mainly caused by the echo, the 

 sound from the lightning being a loud crack or crash like that 

 which we sometimes hear when the lightning strikes near by. 



Some of the vapor of the air on condensing gathers on solid 

 objects like grass, or glass ; but some, as fog, floats about in the air. 

 Really this too is often gathered around solid objects. Floating 

 about in the air are innumerable bits of " dust " which you can 

 see dancing about in the sunlight when a sunbeam enters a dark 

 room. Some of these "dust" particles are actual dust from 

 the road, but much of it is something else, as the pollen of 

 plants, microbes, and the solid bits produced by the burning of 

 wood or coal. 



Each bit serves as a tiny nucleus on which the vapor con- 

 denses ; and so the very "dust" in the air aids in the formation 

 of rain by giving something solid around which the liquid can 

 gather. The great amount of dust in the air near the great city 

 of London is believed to be one of the causes for the frequent 

 fogs of that city. 



That there is dust in the air, and that the rain removes it, is 

 often proved when a dull hazy air is changed to a clear, bright air 

 by a summer shower. Watch to find instances of this. Indeed, 

 after such a hazy day, when the rain drops first begin to fall, if 

 you will let a few drops fall upon a sheet of clean white paper, 

 and then dry it, you will find the paper discolored by the dust 

 that the rain brought with it. So the rain is important because 

 it purifies the air by removing from it the solids that are float- 

 ing in it. 



These are only a few of the things of interest that you can 

 see for yourself by studying the air. Watch the sky ; it is full 

 of interest. See what you can observe for yourself. Watch 

 especially the clouds, for they are not only interesting but beau- 

 tiful (Fig. 9). Their forms are often graceful, and they change 



