THE HEAT OF THE SUN. 7 



around him. "We know that the fire which gladdens our 

 hearth requires occasional renewal by fuel else it will go 

 out ; so too that glowing globe in the heavens which glad- 

 dens and beautifies the earth must also go out unless its 

 energies be recuperated from some source or other. 



The amount of heat squandered by the sun is truly pro- 

 digious. The earth intercepts only an extremely small 

 proportion of the total radiation of sunbeams. It would be 

 easy to show that the sun distributes sufficient light and 

 heat to maintain two thousand million planets in the same 

 comfortable circumstances as those in which our earth is 

 placed. The greater part of that radiation is entirely 

 lost, or, at least, lost in so far as any of the planets are 

 concerned. Our fellow-worlds Jupiter, Saturn, Yenus, 

 Mercury, and Mars do, no doubt, intercept a little of the 

 heat that would otherwise depart altogether from our 

 system, but the total amount of radiant solar energy that 

 all the planets together can utilise is utterly inappreciable 

 when compared with that which streams away into space, 

 and has gone from us for ever. The quantity of heat 

 radiated from the sun is one of the most astounding facts 

 in nature. Let us consider with the help of a few illus- 

 trations the wealth of radiation which our great central 

 hearth pours forth. We shall make use of some of the 

 facts collected in Professor Young's well-known treatise 

 on the sun. 



When an engineer is designing the furnaces to supply 

 a steam engine he has to arrange that the heating surfaces 

 of the boilers shall have an extent duly proportioned to 

 the work which the engine has to do. Each square foot 

 of boiler exposed to the flame is capable of generating so 



