56 LECTURE VI. 



draws or presses another, it is itself as much drawn or pressed. If any one 

 presses a stone with his finger, his finger is also pressed by the stone. If a 

 horse is drawing n M'eight tied to a rope, the horse is also equally drawn back- 

 wards towards the weight : for the rope, being distended throughout, will iu 

 the same endeavour to contract, urge the horse towards the weight, and the 

 weight towards the horse, and will impede the progress of the one as much as 

 it promotes the advance of the other." Now although Newton has always 

 applied this law in the most unexceptionable manner, yet it must be confessed 

 that the illustrations here quoted are clothed in such language as to have too 

 much the appearance of paradox. When we say that a thing presses another, 

 we commonly mean, that the thing pressing has a tendency to move forwards, 

 ^ into the place of the tiling pressed, but the stone would not sensibly advance 

 into the place of the finger, if it were removed ; and in the same manner we 

 imderstand, that a thing pulling another has a tendency to recede further from 

 the thing pulled, and to draw this after it; but it is obvious that the weight 

 which the horse is drawing would not return towards its first situation, with 

 the horse in its train, although the exertion of the horse should intirely cease; 

 in these senses, therefore, we cannot say, that the stone presses, or,tliat the weight 

 pulls, and we have no reason to ofi^end the just prejudices of a beginner, by 

 introducing paradoxical expressions without necessity. Yet it is true in both 

 cases, that if all friction, and all connexion with the surrounding bodies, could 

 be instantaneously destroyed, the point of the finger and the stone would re- 

 cede from each other, and the horse and the weight would approach each 

 other, with equal quantities of motion. And this is what we mean by the re- 

 ciprocality of forces, or the equality of action and reaction. 



The quantity of action of two attractive or lepulsive bodies on each other 

 is partly dependent on their magnitude. When the bodies are of the same 

 kind, their mvitual action is in the compound ratio of their bulks; that is, in 

 the ratio of the products of the numbers expressing their bulks. For instance, 

 if two bodies, each containing a cubic inch of matter, attract or repel each 

 other with a force of a grain, and there be two other bodies, the one contain- 

 ing two inches, the other ten, of the same matter, then the mutual attraction 

 or repulsion of these will be expressed by 20 grains ; for each of the 10 inches 

 is attracted by each of the two with a force of a grain. And the mutual ac- 

 tion of 3 and 10 will be 30, of 4 and 10, 40; so that when one of the bodies 



