| Moving Foree. 143 
He does not say that the consideration of 
the time is necessarily excluded, he only says 
it is not necessarily included in the estimation 
of mechanical power; and he has (at the 
_ conclusion of the passage referred to by the 
reviewers) taken care to discriminate ‘the par- 
ticular cases in which the time may or may 
not be so taken into consideration. He says, 
“ but nofe all this, (relating to the quantity of 
power expended in raising a known weight 
with a uniform velocity to a known height) is 
to be understood in the case of slow or equable 
motions of the body raised ; for in quick, ac- 
celerated, or retarded motion, the vis inertia, 
of the body moved will make a variation.’’* 
He might indeed, consistently with his 
‘principles, have excluded altogether the con- 
sideration of the time in which any mechanical 
effect is produced. For, according to these 
principles, the same quantity of mechanical 
power is required to raise a given weight to a 
given height, in whatever time it may be 
effected, or whether the motion be equable 
or not, provided that the velocity of the weight 
at the beginning and the end of the operation 
be the same.t Accordingly he says, “from 
n 
* Philosophical Trans, 1759, p. 106. 
t It is, I presume, hardly necessary to say, that when the 
motion of the weight is so quick as to make the resistance of 
