38 " ^JN PERcirssioy. 



Considcrntions P^^^^*^ ^^^^ *^® centre of motion ; and it should not past 



respecting me- unobserved that, although the intensity of any vis vwirix 



chamc force, jg increased by being placed at what is called a mechanical 



advantage, yet on the contrary, any quantity of mechanic 



force is not liable to either increase or diminution by any 



such variation in the mode of its application. 



Since we can by means of any mechanic force consist- 

 ing of a vis matrix exerted through a given spaccy give 

 motion to a body for the purpose of employing its irnpe* 

 tits for the production of any sudden effect, or can, on 

 the contrary, occasion a moving body to ascend, and 

 thus resolve its impetus into a movingiorce ready to exert 

 itself through a determinate space of descent, and capa- 

 ble of producing precisely the same quantity of mechanic 

 effect as before, the force depending on impetus may 

 justly be said to be of the same kind as any other mecha- 

 nic force, and they may be strictly compared as to 

 quantity. 



in this manner we may even compart; the force of a 

 body in motion to the same kind of force contained in a 

 given quantity of gunpowder, and may say that we have 

 the same quantity of mechanic force at command whether 

 we have Hb. 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 gradually, or the same weight pos- 

 sessing its original velocity to be employed in any sudden 

 exertion. 



By making use of the same measure as in the former 

 ca^es, 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 thait 

 one or more horses may have performed in a day, each 

 being expressed by the space through which a given mov- 

 ing force is exerted. Tft the case of animal exertion how- 

 ever, considerable uncertainty always prevails in conse- 

 quence 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 



different 



