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fach multiplication, are equal : for if a power can 
raife twice the weight to the fame height ; or the 
fame weight to twice the height, in the fame time 
that another power can, the firft power is double the 
fecond : and if a power can raife half the weight to 
double the height ; or double the weight to half the 
height, in the fame time that another can, thofe two 
powers are equal. But note, all this is to be under- 
flood in cafe of flow or equable motion of the body 
raifed ; for in quick, accelerated, or retarded mo- 
tions, the vis inertia of the matter moved will make 
a variation. 
In comparing the effects produced by water- 
wheels, with the powers producing them ; or, in 
other words, to know what part of the original 
power is neceffarily loft in the application, we mu ft 
previoufly know how much of the power is fpent 
in overcoming the friction of the machinery, and 
the refinance of the air ; alfo what is the real velo- 
city of the water at the inflant that it flrikes the 
wheel ; and the real quantity of water expended in 
a given time. 
From the velocity of the water, at the inflant that 
it flrikes the wheel, given ; the height of head pro- 
ductive of fuch velocity can be deduced, from ac- 
knowleged and experimented principles of hydrofla- 
tics : fo that by multiplying the quantity, or weight 
of water, really expended in a given time, by the 
height of head fo obtained $ which mufl be confi- 
dered as the height from which that weight of wa- 
ter had defcended in that given time j we fhall have 
a produdl, equal to the original power of the water ; 
and clear of all uncertainty, that would arife from 
the friction of the water, in palling fmall apertures y 
