190 THE CONTINUITY OF MOTION. 



varies in a totally-different manner: decreasing as the 

 square of the distance increases, but being practically con- 

 stant for terrestrial distances. These differences being rec- 

 ognized, there is nevertheless to be recognized a truth com- 

 mon to both cases. The weight of something held in the 

 hand shows that there exists between one body in space 

 and another, a strain : this downward pull, ascribed to grav- 

 ity, affects the hand as it might be affected by a stretched 

 elastic string. Hence, when a body projected upwards and 

 gradually retarded by gravity, finally stops, we must re- 

 gard the principle of activity manifested during its upward 

 motion but disappearing at its turning-point, as having be- 

 come latent in the strain between it and the Earth a strain 

 of which the quantity is to be conceived as the product of its 

 intensity and the distance through which it acts. Carrying 

 a step further our illustration of the stretched string, will 

 elucidate this. To simulate the action of gravity at terres- 

 trial distances, let us imagine that when the attached moving 

 body has stretched the elastic string to its limit, say at the 

 distance of ten feet, a second like string could instantly be 

 tied to the end of the first and to the body, which, continuing 

 its course, stretched this second string to an equal length, 

 and so on with a succession of such strings, till the body 

 was arrested. Then, manifestly, the quantity of the prin- 

 ciple of activity which the moving body had displayed, 

 but which has now become latent in the series of stretched 

 strings, is measured by the number of such strings simi- 

 larly stretched the number of feet through which this 

 constant strain has been encountered, and over which it still 

 extends. Now though we cannot conceive the tractive 

 force of gravity to be exercised in a like way though the 

 gravitative action, utterly unknown in nature, is probably 

 a resultant of actions pervading the ethereal medium; yet 

 the above analogy suggests the belief that the principle 

 of activity in a moving body arrested by gravity, has not 

 ceased to exist, but has become so much imperceptible or 



