194 PROF. 0. REYNOLDS ON COMETARY PHENOMENA. 



continually pouring into the earth or any of its sur- 

 rounding bodies is expended in one of the three follow- 

 ing ways : — 



1. By external radiation from the body. 



II. By the evaporation and liquefaction of the materials 

 of the body. 



III. By producing changes in the body such as the 

 formation of coal and the growth of living things. 



The amount of heat expended in the third way may be 

 considered very small in any such body as the earth ; for 

 the amount of energy given out by the combustion of fuel 

 and the work of animals must be nearly equal to that 

 stored in the growing plants. 



Therefore the heat which the earth receives from the 

 sun during any period (a day or a year) is nearly all spent 

 in evaporation and liquefaction or radiated away into 

 space. Hence the quantity of heat spent in evaporation 

 &c, is the difference between the heat received and that 

 radiated away ; and consequently it follows : — 



i. If these are equal, there will be no evaporation. 



2. If the heat received is greater than that radiated 

 away, there will be evaporation &c. 



3. If less, there will be condensation &c. 



That is to say, if over any definite period of time the 

 heat which the earth receives from the sun is equal to that 

 which it radiates into space, then the amount of ice and 

 vapour will be unchanged (unless there be some interchange 

 between these). 



If, on the other hand, the heat received is in excess of 

 that radiated away, the vapour in the atmosphere will 

 increase and the ice diminish, and vice versa. Now the 

 relation which the heat radiated away bears to that received 

 will depend on two things, viz. the temperature of the 

 earth's surface, and its distance from the sun. For both 

 the heat received and that radiated depend in the same 



