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)* 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- 
‘sSumed in producing a quantity of heat whose mechanical equivalent 
is 368(10)* foot-pounds. In estimating the probable density of the 
ether, it was found that the quantity of energy radiated from the 
