22 LECTURE III. 



the second ; and it has been supposed that if we could discover any similar 

 impulse that might be the cause of gravitation, we should have a perfect 

 idea of its operation. But the fact is, that even in cases of apparent 

 jj impulse, the bodies impelling each other are not actually in contact j| 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 Profesor Robison t has estimated this force at a thousand pounds for 

 every square inch. These extremely minute intervals have been ascer- 

 tained by observations 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 microscope, 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 dimen- 

 sions accordingly. Hence it is obvious, that whenever two pieces of glass 

 strike each other, without exerting a pressure 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 acknowledge our total 

 ignorance of the intimate nature of forces of every kind ; and we are 



/ first to examine the effect of forces, considering only their magnitude and 



L direction, without any regard to their origin. 



It was truly asserted by Descartes,^ that the state of motion is equally 

 natural with that of rest. When a body is once in motion, it requires no 

 foreign power to sustain its velocity. If, therefore, a moving body is sub- 

 jected to the influence of any force, which acts upon it in the line of its 

 direction, its motion will be either accelerated or retarded, accordingly as 

 the direction of the force coincides with that of the motion, or is opposed 

 to it. A stone, for instance, beginning to fall, or projected downwards, 

 by no means retains the same velocity throughout its descent, but acquires 

 more and more motion every instant. We well know that the greater 

 the height from which a body falls, the more danger there is of its destroy- 

 ing whatever opposes its progress. In the same manner, when a ball is 

 thrown upwards, it gradually loses its motion by the operation of gravita- 

 tion, which is now a retarding force, and at last begins again to descend. 



It may here be proper to inquire what is the precise meaning of the 

 term velocity ; we appear indeed to understand sufficiently the common use 

 of the word, but it is not easy to give a correct definition of it. The velocity 

 of a body may be said to be the quantity or degree of its motion, indepen- 

 dently of any consideration of its mass or magnitude ; and it might 

 always be measured by the space described in a certain portion of time, for 



* Optics, Book II. See also Huygens, Ph. Tr. No. 86. 

 f Robison's Mechanical Philosophy, Brewster's Ed., i. 250. 

 J Principia Philos. Part ii. 26. 



