ON THE SUN'S HEAT.. 



385 



branch of solar science to which only allusion 

 can be made at the present time. 



What concerns us as to the explanation of 

 sun-light and sun-heat may be summarised in 

 two propositions : 



(1) Gigantic currents throughout the sun's liquid 

 mass are continually maintained by fluid, slightly 

 cooled by radiation, falling down from the sur- 

 face, and hotter fluid rushing up to take its 

 place. 



(2) The work done in any time by the 

 mutual gravitation of all the parts of the fluid, 

 as it shrinks in virtue of the lowering of its 

 temperature, is but little less than (so little 

 less than that we may regard it as practically 

 equal to) the dynamical equivalent of the 

 heat that is radiated . from the sun in the 

 same time. 



The rate of shrinkage corresponding to the 

 present rate of solar radiation has been proved 

 to us, by the consideration of our dynamical 

 .model, to be 35 metres on the radius per year, 

 or one ten-thousandth of its own length on the 



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