SIXTH ORDINARY MEETING. 45 



universe is invariable and can neither be increased or diminished, 

 yet by virtue of laws of which we have a particular case in Clausius' 

 " Second Main Principle of the Mechanical Theory of Heat," the 

 amount of what may be termed available energy is being constantly 

 exhausted. 



The truth of this, together with many very important conse- 

 quences which follow from it, was first pointed out by Sir W. 

 Thomson in a remarkably able paper on a " Universal Tendency in 

 Nature to the Dissipation of Mechanical Energy." It is simply 

 another method of saying that no known natural processes are per- 

 fectly reversible. 



A few moments reflection will suffice to show that the main 

 sources of energy available for man are (1) Food; (2) Fuel; (3) 

 Water Power; (4) Wind. Of these food and fuel are of the same 

 nature, food being utilized by means of animal machines, such as 

 men, horses, &c, while fuel is converted into mechanical motion by 

 means of engines of various kinds. The mechanical energy which 

 is thus produced by means of food and fuel is evidently, for the 

 most part, derived from the heat and light radiated from the sun. 

 Water power and wind even more obviously obtain their energy 

 from the same source. Solar radiation is therefore the grand source 

 whence nearly all the energy available for man is derived. 



Various theories have been advanced to account for the enormous 

 amount of energy in the form of heat and light annually sent forth 

 by the sun, and of which the earth intercepts a very small portion. 

 It was, for instance, supposed by some that the sun's heat was pro- 

 duced by the combustion of its materials. A very few facts will 

 show that this hypothesis is utterly untenable. The mass of the 

 sun, estimated from the most reliable determinations of the solar 

 parallax, has been found to be about 4(10) 30 lbs. The consumption 

 of a pound of coal is known to produce an amount of heat equiva- 

 lent to 9,200,000 foot-pounds. 



Combining these, we see that if the materials of the sun were 

 supposed to be capable of producing by their combustion as much 

 heat as equal masses of coal, an assumption eminently favorable to 

 the hypothesis in question, the total mass of the sun would be con- 

 sumed in producing a quantity of heat whose mechanical equivalent 

 is 368(10) 35 foot-pounds. In estimating the probable density of the 

 ether, it was found that the quantity of energy radiated from the 



