Or WATER MILLS, &c. 145 
«ton in his paper on mechanic power (publithed in the 
*Philofophical Tranfacions for the year 1776) allows, 
“that the theory ufually given will not correfpond with 
“« matter of faét, when compared with the motion of ma- 
“chines; and feems to attribute this difagreement, rather 
** to deficiency in the theory, than to the obflacles which 
“‘ have prevented the application of it to the complicated 
““ motion of engines, &c. In order to fatisfy himfelf con- 
** cerning the reafon of this difagreement he conftruted a 
“© fet of experiments, which, from the known abilities 
‘and Ingenuity of the author, certainly deferve great con+ 
** fideration and attention from every one who is inter- 
‘**efted in thefe inquiries.” And notwithftanding the fame 
“¢ learned author fays, “* The evidence upon which the 
* theory refts is fearcely lefs than mathematical.” I am 
forry to find, in the prefent ftate of the f{ciences, one of 
his abilities concluding (page 380) “It is not probable 
that the theory of motion, however inconteftible its prin- 
ciples may be, can afford much affiftance to the practical 
mechanic,” although indeed his theory, compared with 
the above cited experiments, might fuggeft fuch an infer- 
ence. But to come tothe point, I would juft premife 
thefe 
Definitions. 
Tf a ftream of water imping againit a wheel in motion, 
there are three different velocities to be confidered, apper= 
taining thereto, viz. 
Firft, the abfolute velocity of the water: 
Second, the abfolute velocity of the wheel: 
Third, the relative velocity of the water to that of 
the whecl, z ¢. the difference of the abfolute velocities ; 
or the velocity with which the water overtates or ftrikes 
the wheel, 
VOL, UTI. T Now 
