OF NATURAL PHILOSOPHY. 149 



(142.) Here, then, we see the great importance 

 of possessing a stock of analogous instances or phe- 

 nomena which class themselves with that under 

 consideration, the explanation of one among which 

 may naturally be expected to lead to that of all 

 the rest. If the analogy of two phenomena be 

 very close and striking, while, at the same time, the 

 cause of one is very obvious, it becomes scarcely 

 possible to refuse to admit the action of an analogous 

 cause in the other, though not so obvious in itself 

 For instance, when we see a stone whirled round in 

 a sling, describing a circular orbit round the hand, 

 keeping the string stretched, and flying away the 

 moment it breaks, we never hesitate to regard it as 

 retained in its orbit by the tension of the string, 

 that is, by a force directed to the centre ; for we 

 feel that we do really exert such a force. We have 

 here the direct perception of the cause. When, 

 therefore, we see a great body like the moon cir- 

 culating round the earth and not flying off, we 

 cannot help believing it to be prevented from so 

 doing, not indeed by a material tie, but by that 

 which operates in the other case through the in- 

 termedium of the string, a force directed con- 

 stantly to the centre. It is thus that we are con- 

 tinually acquiring a knowledge of the existence 

 of causes acting under circumstances of such con- 

 cealment as effectually to prexfent their direct dis- 

 covery. 



(143.) In general we must observe that motion, 



wherever produced or changed, invariably points 



out the existence of force as its cause; and thus 



the forces of nature become known and measured 



L 3 



