17G 
MOVING FORCJE. 
Crises of diffi- 
culty in the 
doctrines of 
moving force. 
ture or channel, than through a small one; and just so we 
know, that by a large and strong machine, a given quantity of 
moving force may be conveyed in less time than by a small and 
weak one. But if the whole, or any determinate portion of 
the moving force be properly applied, the whole or propor- 
tionate effect, must jrevertheless be the same^ whatever may be 
the portion of time occupied in the operation. And the same 
principle bolds good in the application of the elastic force of 
steam, or of any other moving force, to produce a mechanical 
effect. 
In objection, however, to this, the reviewers observe as fol- 
lows : ' 
“■ When it is said, for example, that a bushel of good coals 
will give to a steam engine the power required to grind eleven 
bushels of wheat, this must always imply a rate of burning 
included within certain limits ; for the fuel might be applied 
so slowly that the steam generated would not be of sliength 
sufficient to work the mill ; or it might be made to turn so 
fast, that very little effect would be produced. In the same 
way, when Mr, Smeaton says, if 1000 tons of water-be let out 
on an overshot wheel, and descend thfough twenty feet, it will 
grind the same quantity of corn, at whatever rate it be ex- 
pended*, the extreme cases of very great slowness, or very 
great rapidity, must surely be excepted. But if the extreme 
cases must be excepted, it is a proof that, even in the interme- 
diate cases, the effect is not constant or invariable in its mag- " 
ritude, though the differences may be inconsiderable ; this, at : 
least, is what one would be disposed to infer from that conti- 
nuity in the variation of causes and effects, to which there is, 9 
perhaps, no exception, either among the works of nature or of i) 
artf.” 
To these objections it may be replied, that however slow or n 
quick the combustion of the coals may be if they be effectually 
burnt, the full quantity of heat must be given out, If the q, 
heat be allowed to escape without being communicated to the 
• Phil. Trans. 1776, p.474: !j 
+ Edinburgh Review, vol. l‘i, p. 129. 
water; 
