[ ] 
vantage that arifes from the impulfe thereof ; and (hall 
immediately fee of how little confequence this impulfe 
is to the effebt of an overfhot wheel. However, as 
every thing has its limits, fo has this : for thus much is 
defirable, that the water Jhould have J'omewhat greater 
velocity , than the circumference of the wheel , in 
coming thereon ; otherwife the wheel will not only 
be retarded, by the buckets finking the water, but 
thereby dafhing a part of it over, fo much of the 
power is loft. 
The velocity that the circumference of the wheel 
ought to have, being known by the following de- 
ductions, the head requifite to give the water its pro- 
per velocity is eafily computed from the common 
rules of hydroftatics ; and will be found much lefs 
than what is generally prabtifed. 
III. Concerning the Velocity of the Circumference of 
the Wheel , in order to produce the greatejl Effeel. 
If a body is let fall freely from the furface of the 
head to the bottom of the defcent, it will take a 
certain time in falling ; and in this cafe the whole 
ablion of gravity is fpent in giving the body a certain 
velocity : but if this body in falling is made to abt 
upon fome other body, fo as to produce a mechani- 
cal effebt, the falling body will be retarded ; becaufe 
a part of the adtion of gravity is then fpent in pro- 
ducing the effebt, and the remainder only giving 
motion to the falling body : and therefore the flower 
a body defcends , the greater will be the portion of the 
a Cl ion of gravity applicable to the producing a me- 
chanical efl'eCl ; and in confequence the greater that 
effebt may be. 
If 
