22 Dr. Wollaston’s Lecture on the Force of Percussion. 
quantity of mechanic force at command whether we have i lb. 
of powder, which by its expansion could give to 1 ton weight 
a velocity sufficient to raise it through 40 feet, or the weight 
actually raised to that height and ready to be let down gra- 
dually, or the same weight possessing its original velocity to 
be employed in any sudden exertion. 
By making use of the same measure as in the former cases, 
a distinct expression is likewise obtained for the quantity of 
mechanic force given to a steam-engine by any quantity 
of coals ; and we are enabled to make a comparison of its 
effect with the quantity of work that one or more horses may 
have performed in a day, each being expressed by the space 
through which a given moving force is exerted. In the case 
of animal exertion however, considerable uncertainty always 
prevails in consequence of the unequal powers of animals of 
the same species, and varying vigour of the same animal. 
The information which I have received in reply to inquiries 
respecting the weights raised in one hour by horses in dif- 
ferent situations, has varied as far as from 6 to 15 tons to the 
height of 100 feet. But although the rate at which mechanic 
force is generated may vary, any quantity of work executed 
is the same, in whatever time it may have been performed. 
In short, whether we are considering the sources of extended 
exertion or of accumulated energy, whether we compare the 
accumulated forces themselves by their gradual or by their 
sudden effects, the idea of mechanic force in practice is always 
the same, and is proportional to the space through which any 
moving force is exerted or overcome, or to the square of the 
velocity of a body in which such force is accumulated. 
