8 IN STARRY REALMS. 



much steam per hour, and may thus be regarded as the 

 equivalent of so much horse-power. Let us consider an 

 area of a single square foot on the sun's surface, and sup- 

 pose that all the heat which passes through it on its way 

 to outer space could be collected and applied to the gene- 

 ration of steam in a boiler. The generation of steam in 

 that boiler would be so copious that a mighty engine of ten 

 thousand horse-power might be maintained in continuous 

 action. Indeed a great Atlantic liner could be driven at 

 full speed at a heat expenditure not larger than that 

 which flows out through each square foot of the sun. It 

 would be easy to show that if the heat from an area on the 

 sun of an acre or two in extent could all be utilised by a 

 system of boilers, they would generate as much steam as 

 would suffice to sustain in full work every steam engine in 

 the world. 



We may exhibit the quantity of heat radiated from the 

 sun in another way. Let us suppose that it is to be 

 entirely applied to the melting of ice, and that this ice 

 is disposed in a shell closely enveloping the whole sun. 

 Even if the ice had a thickness of 48^ feet the radiation 

 would be sufficient to reduce it all to water in one minute. 

 Statements like this give some conception of the profuse 

 expenditure with which the sun pours forth its stores of 

 heat ; they also raise a desire to study the method by 

 which the exhaustion that would seem the obvious con- 

 sequence of such monstrous extravagance can be avoided. 



In the first place it should be noticed that the enor- 

 mous size of the sun is a very important element in the 

 inquiry. A large body cools much more slowly than a 

 tma.ll one. The loss of heat by radiation takes place 



