100 THE EEASON WHY. 



'And God did so that night : for it was dry upon the fleece only, and there was 

 dew oh all the ground." JUDGES vi. 



Because as they result from the temperature of the air, which is 

 less likely to vary than that of the earth, there is a probability 

 that the coldness thereof will last for some time. 



415. What benefits result from the radiation of heat, fyc, ? 



But for the radiation of heat, we should be subjected to the most 

 unequal temperatures. The setting" of the sun would be like the 

 going out of a mighty fire. The earth would become suddenly 

 cold, and its inhabitants would have to bury themselves in warm 

 covering;, to wait the return of day. By the radiation of heat, 

 an equilibrium of temperature is provided for, without which we 

 should require a new order of existence. 



The amount of heat which our earth receives from the sun, and the economy 

 of that heat by the laws of radiation, reflection, absorption, and convection, 

 are exactly proportionate to the necessities of our planet, and the living things 

 that inhabit it. It is held by philosophers that any change in th e orbit of our 

 earth, which would either increase or decrease the amount of heat falling upon 

 it, would, of necessity, be fallowed by the annihilation of all the existing races. 

 The planets Mercury and Venus, which are distant respectively 37 millions of 

 miles, and 63 millions of miles, from the great source of solar heat, possess a 

 temperature which would melt our solid rocks; while Uranus (1,800 millions of 

 miles), and Neptune (whose distance from the sun has not been determined), 

 must receive so small an amount of heat, that water, such as ours, would become 

 as solid as the hardest rock, and our atmosphere would be resolved into a liquid ! 

 Yet, poised in the- mysterious balance of opposing forces, our orb flies unerringly 

 on its course, at the rate of 65,01)3 miles an hour ; preserving, in its wonderful 

 flight, that precise relation to the sun, which takes from his life-inspiring rays 

 the exact degree of heat, which, being shared by every atom of matter, and 

 every form of organic existence, is just the amount needed to constitute the heat- 

 life of the world ! 



CHAPTER XX. 



416. What is rain ? 



Rain is the vapour of the clouds which, being condensed by a 

 fall of temperature, forms drop? of water that descend to the earth. 



It is the return to the earth in the form of water, of the moisture 

 absorbed by the air in the form of vapour. 



417. Does rain ever occur without clouds ? 



It sometimes, but rarely happens, that a sudden transition from 



