[ r 38 ] 
water ftrikes at a certain point below the furface of 
the head, and after that defcends in the arch of a 
circle, prefling by its gravity upon the wheel ; the 
effect of Jitch a wheel will be equal to the effect of an 
under '[hot, whofe head is equal to the difference of 
level between the Jurface of the water in the refervoir 
and the point where it fir ikes the wheel , added to that 
of an overjhot , whofe height is equal to the difference 
of level, between the point where it ftrikes the wheel 
and the level of the tail-water. It is here fuppofed, 
that the wheel receives the fhock of the water at 
right angles to its radii ; and that the velocity of its 
circumference is properly adapted to receive the ut- 
moft advantage of both thefe powers ; otherwife a 
reduction mufl be made on that account. 
Many obvious and confiderable improvements up- 
on the common praftice naturally offer themfelves, 
from a due confideration of the principles here eftab- 
lifhed, as well as many popular errors fhow them- 
felves in view : but as my prefent purpofe extends 
no farther than the laying down fuch general rules 
as will be found to anfwer in practice, I leave the 
particular application to the intelligent artift, and to 
the curious in thefe matters. 
PART III. 
On the Conftrufiion and E fields of Windmill- 
Sails. 
Read 3 1 May & 
14 June, 1759. 
I N trying experiments on windmill- 
fails, the wind itfelf is too uncertain 
to anfwer the purpofe : we muft therefore have re- 
courfe to an artificial wind. 
This 
