iJ8 LECTURE III. 



The exertion of an animal, the unbending of a bow, and the commun?Ca- 

 tion of motion by impulse, are familiar instances of the actions of forces. Wc 

 must not imagine that the idea of force is naturally connected with that of la- 

 bour or difficulty; this association is only derived from habit, since our vo- 

 luntary actions are in general attended with a certain effort, which leaves an 

 impression almost inseparable from that of the force that it calls into action. 



It is natural to inquire in what immediate manner any force acts, so as ta 

 produce motion; for instance, by what means the earth causes a stone to gra- 

 vitate towards it. In some cases, indeed, we are disposed to imagine that we 

 understand better the nature of the action of a force, as, when a body in mo- 

 tion strikes another, we conceive that the impenetrability of matter is a suffi- 

 cient cause for the communication of motion, since the first body cannot con- 

 tinue its course without displacing the second; and it has been supposed that 

 if we could discover any similar impulse that might be the cause of gravita- 

 tion, we should have a perfect idea of its operation. But the fact is, that 

 even in cases of apparent impulse, the bodies impelling each othcF are not ac- 

 tually in contact; and if any analogy between gravitation and impulse be ever 

 established, it will not be by referring them both to the impenetrability of 

 matter, but to the intervention of some common agent, perhaps imponderable. 

 It was observed by Newton, that a considerable force was necessary to bring 

 two pieces of glass into a degree of contact, which still was not quite perfect; 

 and Professor Robison has estimated this force at a thousand pounds for every 

 square inch. These extremely minute intervalsiiave been ascertained by ob- 

 servations on the colours of the thin plate of air included between the glasses ; 

 and when an image of these colours is exhibited by means of the solar mi- 

 croscope, it is very easily shown that the glasses are separated from each 

 other, by the operation of this repulsive force, as soon as the pressure of the 

 screws which confine them is diminished; the rings of colours dependent on 

 their distance contracting their dimensions accordingly. Hence it is obvious, 

 that whenever two pieces of glass strike each other,, without exerting a pres- 

 sure equal to a thousand pounds on a square inch, they may affect each other's 

 motion without actually coming into contact. Some persons might perhaps 

 be disposed to attribute this repulsion to the elasticity of particles of air 

 adhering to the glass, but I have found that the experiment succeeds equally 

 well in the vacuum of the air pump. We must therefore be contented to ac- 



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