t 45 ° ] 
XXVII. An Experimental Examination of the Quantity 
and Proportion of Mechanic Power necejjary to be em- 
ployed in giving different Degrees of Velocity to PLeavy 
Bodies from a State of Reft . By Mr. John Smeaton, 
F. R. S. 
R. April 25, BOUT the year 16S6 Sir Isaac new- 
J - ^ ton firft publifhed his Pr in dpi a, and, 
conformably to the language of mathematicians of thole 
times defined, that “ the quantity of motion is the 
u meafure of the fame, arifing from the velocity and 
“ quantity of matter conjointly.” Very loon after this 
publication, the truth or propriety of this definition was 
difputed by certain philofophers, who contended, that 
the meafure of the quantity of motion Ihould be ehi- 
mated by taking the quantity of matter and the fquare 
of the velocity conjointly. There is nothing more 
certain, than that from equal impelling powers, acting 
for equal intervals of time, equal increafes of velocity 
are acquired by given bodies, when unrefifted by a me- 
dium; thus gravity caufes a body, in obeying its impulfe 
during one fecond of time, to acquire a velocity which 
would carry it uniformly forward, without any addi- 
tional impulfe, at the rate of 32 ft. 2 in. per fecond; and 
if gravity is fuffered to a£t upon it for two leconds, it 
3 will 
