[ 101 ] 
truth than towards it. Hence the common obferva- 
tion, that a thing may do very weli in a model, that 
will not anfwer in large. And indeed, tho’ the ut- 
moft circumfpe&ion be ufed in this way, the belt 
ftrudture of machines cannot be fully ascertained, 
but by making trials with them, when made of their 
proper fize. It is for this reafon, that, tho’ the mo- 
dels referred to, and the greatefi: part of the follow- 
ing experiments, were made in the years 175*2 and 
17 f3, yet I deferred offering them to the Society, 
till I had an opportunity of putting the deductions 
made therefrom in real practice, in a variety of cafes, 
and for various purpofes j fo as to be able to allure 
the Society, that I have found them to anfwer. 
PART I. 
Concerning Undershot Water-Wheels. 
Plate IV. Fig. 1. is a perfpe&ive view of the ma- 
chine for experiments on water-wheels ; wherein 
ABCD is the lower ciftern, or magazine, for re- 
ceiving the water, after it has quitted the wheel $ 
and for fupplying 
D E the upper ciftern, or head ; wherein the water 
being raifed to any height required, by a pump, 
that Height is (hewn by 
F G, a fmall rod, divided into inches and parts ; 
with a float at the bottom, to move the rod up 
and down, as the furface of the water riles and 
falls. 
HI is a rod by which the fluice is drawn, and ftopt 
at any height required, by means of 
K a pin or peg, which fits feveral holes, placed 
hi 
