168 On the Measure of 
might be applied so slowly that the steam 
generated would not be of strength sufficient. 
to work the mill; or it might he made to turn 
so fast, that very little effect would be pro- 
duced. In the same way, when Mr. Smeaton 
says, if 1000 tons of water be let out on an 
overshot wheel, and descend through twenty 
feet, it will grind the same quantity of corn, at 
whatever rate it be expended,* 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 intermediate cases, the effect 
is not constant or invariable in its magnitude, 
though the differences may be inconsiderable ; 
this, at least, is what one would be disposed to 
infer from that continuity in the variation of 
causes and effects, to which there is, perhaps, — 
no exception, either among the works. of 
nature or of art.” ¢ 
‘ 
To these objections it may be replied, that — 
however slow or quick the combustion o the 
coals may be, if they be effectually burnt, the 
full quantity of heat must be given out. Ff 
the heat be allowed to escape without being 
communicated to the water; or, if after being — 
communicated to the water the pressure of the 
* Philos. Trans. 1776, p. 474. i 
+ Edinburgh Review, vol, 12, p, 129. sam) 
