c i 3 i i 
$, I is to be imputed to the fuperior impulfe of the 
water at the head of 1 1 inches above that of 3 
inches: fo that if we reduce 8,1 to 8, on account 
of the impulfe of the 3 inch head, we Jhall have 
the ratio of the power , computed upon the height of 
the wheel only , to the eff 'edt at a maximum as 10 : 8, 
or as 5 : 4, nearly : and from the equality of the ratio 
between power and effedt, fubfifting where the con- 
ftrudtions are Similar, we muff: infer, that the effedfs, as 
well as the powers, are as the quantities of water and 
perpendicular heights multiplied together refpeclively. 
II. Concerning the mofi proper Height of the Wheel 
in proportion to the whole Defcent. 
We have already feen, from the preceding ob~ 
fervation,. that the effedt of the fame quantity of 
water, defeending thro’ the fame perpendicular Space, 
is double, when adting by its gravity upon an over- 
shot wheel, to what the fame produces when adting 
by its impulfe upon an underihot. It alfo appears, 
that by increasing the head from 3 inches to 1 1, that 
is, the whole defcent, from 27 inches to 35, or in the 
ratio of 7 to 9 nearly, the effedt is advanced no more 
than in the ratio of 8,1 to 8,4, that is, as 7 : 7,26 ; 
and confequently the increafe of effedt as not i-yth 
of the increafe of perpendicular height. Hence it 
follows, that the higher the wheel is in proportion to 
the whole defcent , the greater will he the eff'cdl be- 
cause it depends lefs upon the impulfe of the head, 
and more upon the gravity of the water in the 
buckets : and if we conffder how obliquely the 
water iffuing from the head muff Strike the buckets, 
we (hall not be at a lofs to account for the little ad- 
S 2 vantage 
r* 
