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alfo the wheel lofes its power ; for tho’ here is the 
exertion of gravity upon a given quantity of water, 
yet being prevented by a counterbalance from mov- 
ing, is capable of producing no mechanical effe5l y 
according to our definition. But, in reality, an over- 
fhot wheel generally ceafes to be ufeful before it is 
loaded to that pitch ; for when it meets with fuch a 
refijlance as to diminijh its velocity to a certain degree , 
its motion becomes irregular ; yet this never happens 
till the velocity of the circumference is lefs than 2 feet 
per fecond, where the refijlance is equable , as appears 
not only from the preceding fpecimen, but from ex- 
periments on larger wheels. 
Scholium. 
Having now examined the different effects of the 
power of water, when afting by its impulfe , and by 
its weight , under the titles of underfoot and overfoot 
wheels j we might naturally proceed to examine the 
effects when the impulfe and weight are combined, as 
in the feveral kinds of breaf -wheels, &c. but, what has 
been already delivered being carefully attended to, the 
application of the fame principles in thefe mixt cafes 
will be eafy, and reduce what I have to fay on this 
head into a narrow compafs : for all kinds of wheels 
where the water cannot defcend thro’ a given fpace, 
unlefs the wheel moves therewith, are to be con- 
fidered of the nature of an overfhot wheel, accord- 
. ing to the perpendicular height that the water de- 
fcends from ; and all thofe that receive the impulfe 
or fhock of the water, whether in an horizontal, per- 
pendicular, or oblique direction, are to be confidered 
as underfhots. And therefore a wheel, which the 
Vol. LI. T water 
