PROF. O. REYNOLDS ON COMETARY PHENOMENA. 195 



way on (and, in fact, are both proportional to) the extent 

 and nature of the earth's surface ; and the quantity of heat 

 received depends on (in addition to this) the distance of 

 the earth from the sun (it varies inversely as the square of 

 this distance) ; whereas the quantity of heat radiated away 

 depends on the temperature of the earth's surface as well 

 as on its extent and nature. Hence the ratio which the 

 heat received bears to that radiated away will decrease as 

 the distance of the earth from the sun increases, and also as 

 the temperature of the earth , s surface increases. 



If there were nothing to melt or evaporate on the earth 

 (or any other body whose distance from the sun is nearly 

 constant), then the heat radiated away would eventually 

 equal the heat received ; for the temperature at the surface 

 would continually rise until the quantity radiated away 

 equalled that received, and there was equilibrium. This 

 temperature I have in the remainder of this paper called 

 the temperature of equilibrium. It will depend simply on 

 the distance of a body from the sun, increasing inversely as 

 the square of the distance. Hence in the case of planets 

 this will be constant, whereas in the case of comets it will 

 vary. If there is any material on the body which evapo- 

 rates at a lower temperature than that of equilibrium, 

 then there will be evaporation until the material is all 

 gone or its conditions of boiling are altered. The tem- 

 perature at which the softest material will evaporate 

 will depend on the nature of that material and on the 

 pressure of the atmosphere surrounding the body. Any 

 increase in the pressure of the atmosphere will increase 

 the temperature required to evaporate the material. If 

 initially a body has no atmosphere, then we may assume 

 that its materials will evaporate until the vapour forms one 

 sufficient (if possible) to increase by its pressure the tem- 

 perature of evaporation to that of equilibrium. But the 

 possibility of this will depend on the size of the body and 



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