118 HEAT OF EAUTH's OUTER CRUST; 



a warming influence, which is very unequal, vary- 

 ing according to the direction in which they arrive 

 (their incidence). Every one knows how feeble 

 is the effect of the morning and of the evening sun, 

 compared with that of noon-day ; and every one is 

 aware that the heat of our summer is intimately 

 connected with the height, which the sun attains 

 above the horizon. Yet the inferior effect, of these 

 slanting rays, depends in but a very small degree 

 on their passing through thicker layers of an 

 atmosphere, which is usually more charged with 

 mists. 



To obtain a clear insight into the cause of this 

 peculiarity, conceive a stream of water into which 

 you plunge a plank, at various inclinations suc- 

 cessively, but always to an equal depth, so that it 

 is always struck by an equal quantity of the run- 

 ning liquid. 



You will immediately understand that the im- 

 mersed portion of the plank will be the smaller, 

 the more nearly the direction in which it encoun- 

 ters the current approaches the perpendicular. 

 Just so, then, do the rays of the sun form a stream 

 flowing towards the earth ; beams of them, equal 

 in section, strike smaller surfaces on the earth, 

 and therefore exert a greater heating influence on 

 equal surfaces, the more perpendicularly they fall. 

 This explains why the power of the sun at any 

 place is less, the greater its distance from the 



