C I2 5 3 
the level from whence it fell ; * one would therefore 
fuppofe, that a cubic inch of water, let fall thro’ a 
fpace of 30 inches, and there impinging upon an- 
other body, would be capable of producing an equal 
effed by collifion, as if the fame cubic inch had de- 
fended thro’ the fame fpace with a flower motion, 
and produced its effects gradually : for in both cafes 
gravity ads upon an equal quantity of matter, thro’ 
an equal fpace -f* ; and confequently, that whatever 
was the ratio between the power and effed in under- 
fhot wheels, the fame would obtain in overfhot, and 
indeed in all others : yet, however conclufive this 
reafoning may feem, it will appear, in the courfe of 
the following dedudions, that the effed of the gra- 
vity of defending bodies is very different from the 
effed of the ffroke of fuch as are non-elaftic , tho” 
generated by an equal mechanical power. 
The alterations in the machinery already defcribed, 
to accommodate the fame for experiments on over- 
fhot wheels, were principally as follows. 
Plate V. Fig. 2. The fluice I b being fhut 
down, the rod HI was unff rewed and taken off. 
The underfhot water-wheel was taken off the axis, 
and inflead thereof an overfhot wheel of the fame 
* This is a confequence of the rifing of jetts to the height of 
their refetvoirs nearly. 
f Gravity, it is true, ads a longer fpace of time upon the body 
that defcends flow than upon that which falls quick ; but this can- 
not occalion the difference in the effed : for an elaffic body falling 
thro’ the fame fpace in the fame time, will, by collifion upon an- 
other elaffic body, rebound nearly to the height from which it fell; 
or, by communicating its motion, caufe an equal one to afcend 
to the fame height. 
diameter 
