178 Relation of the Laws of Mechanics to Perpetual Motion. 
the revolution of the heavenly bodies. This is owing to the fact, 
endowed, like animals, with a principle of voluntary motion, it 
would be as easy for it to cease to move, at pleasure, as to begin ; 
to move. It would then be as difficult to esas perpetual mo- 
tion in a machine, as in a horse or an ele 
Some of the eenaaies which really edd in the way of the 
SP eo = perpetual motion are the following. 
ery machine, there is some loss of motion, from fric- 
~ tion and ‘the resistance of the air; and commonly a much greater 
Pee loss, from the expenditure of force, in producing the effect for 
which the machine is designed. en a certain momentum is 
given to a body, it will continue the same, till there is some 
cause to vary it. A wheel suspended freely on an axis wou uld 
of the air could be entirely removed. Perpetual motion would 
b: . , then be a thing of course. But these obstructions it is impos- 
2" sible ee ae ‘and Aeay necessarily occasion a continual loss 
: “See of m 
eg 4 Tn py een to this” a ‘machine, to answer any practical purpose, 
must not only go, but it must also be capable of a constant ex- ; 
* penditure of force upon the effect to be produced. A slitting mill, 
. Bi hee! instance, must not only be kept in motion, but must furnish 
a continual supply of force,.to separate the iron. 'The loss of mo- 
‘on this account is fargreater than that occasioned by friction 
ance of the air. Both must be compensated by a re- 
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