CAUSE OF MOTION IN AIR. 311 



that the absorption of the solar rays by the 

 strata of air to which we have access is consi- 

 derable in amount, even for moderate thick- 

 nesses. By calculation it has been found that 

 about one-third of the solar heat is lost by the 

 transmission of the rays through our atmosphere. 

 The watery vapour present in the air absorbs 

 these rays in a remarkable degree. We are thus 

 shaded, as it were, at all times from the full 

 influence of the solar heat. While, however, 

 this is true, it is also certain that the atmo- 

 sphere receives a much larger portion of heat 

 from its contact with the heated earth. In 

 consequence of this the particles of air expand, 

 become specifically lighter, and are rendered 

 sufficiently buoyant to rise upwards, their places 

 being immediately supplied by cooler particles of 

 air drawn from the vicinity. If, therefore, we 

 suppose a particular spot or tract of land to be 

 heated by the sun, while a neighbouring part 

 remains comparatively cool, the immediate result 

 is that an upward current rises from that spot 

 and ascends into the higher regions of the air. 

 But as it rises, its place below must be occupied 

 by an equal bulk of air which is necessarily 

 derived from the sides. A current is thus im- 

 mediately established, an ascending stream in the 

 one case, and a horizontal flow of greater or less 

 force in the second. 



