310 THE CHEMISTRY OF CREATION. 



although where we are now placed they are 

 so soft and gentle that we do not feel them. 

 But some of the movements may be actually 

 seen. If we look attentively at certain objects, 

 as on a hillock, upon which the heat of the sun 

 has been directed all day long, we shall perceive 

 a number of minute undulations in the air just 

 above it. This is rendered still more apparent 

 if a telescope is employed, and so interferes 

 with the direction of the rays of light as to 

 make the objects appear distorted and indis- 

 tinct Upon the sea-shore this effect is some- 

 times curiously manifested, and at a little 

 distance off we may observe the whole shore- 

 line marked by the waving and trembling 

 stratum of air rising above it up to a certain 

 height. The cause of these tremulous move- 

 ments is undoubtedly the heat of the sun, since 

 on a cold or cloudy day they are not perceptible. 

 The grand cause, indeed, of most of the move- 

 ments of the air is the solar ray, and the mode in 

 which it acts is explicable in a simple manner. 



It is generally said that the air only receives 

 heat from its contact with the earth, and absorbs 

 but a very minute portion of the heat rays of 

 the sunbeam as they pass through it. Recent 

 experiments have shown this idea to be in some 

 measure erroneous. In a series of experiments 

 conducted by Professor Forbes, it has been found 



