452 Mr. smeaton on Mechanic Power . 
that, whichever we take for the proper definition of the 
term quantity of motion, by paying a proper regard to 
the collateral circumftances that attend the application of 
it, the fame conclulion, in point of computation, will re- 
fult. I fhould not, therefore, have thought it worth 
while to trouble the Society upon this fubjedt, had 1 not 
found, that not only myfelf and other practical artifts, 
but alfo fome of the moft approved writers, had been 
liable to fall into errors, in applying thefe dodlrines to 
pradtical mechanics, by fometimes forgetting or neg- 
ledting the due regard which ought to be had to thefe 
collateral circumftances. Some of thefe errors are not 
only very conftderable in themfelves, but alfo of great 
confequence to the public, as they tend greatly to millead 
the pradtical artift in works that occur daily, and which 
often require very great fums of money in their exe- 
cution. I fliall mention the following inftances. 
des aguliers, in his fecond volume of Experimental 
Philofopliy, treating upon the queftion concerning the 
forces of bodies in motion, after taking much pains to 
fhew that the diipute, which had then fubfifted fifty 
years, was a difpute about the meaning of words; and 
that the fame conclufion will be brought out, when 
things are rightly underftood, either upon the old or 
new opinion, as he diftinguillies them; among other, 
things, tells us, that the old and new opinion may be 
eafily reconciled in this inftance : that the wheel of an 
underfhot water-mill is capable of doing quadruple work 
when the velocity of the water is doubled, inftead of 
4 double 
