THE CONSTITUTION OF NA TUHE. 9 



the attraction of gravity a force, without any reference to 

 motion. A body resting on a shelf is as much pulled by 

 gravity as when, after having been pushed off the shelf, it 

 falls toward the earth. We applied the term force also to 

 that molecular attraction which we called chemical affinity. 

 When, however, we spoke of the conservation of force, in 

 the case of elastic collision, we meant neither a pull nor a 

 push, which, as just indicated, might be exerted upon 

 inert matter, but we meant force invested in motion the 

 vis viva, as it is called, of the colliding masses. 



Force in this form has a definite mechanical measure, in 

 the amount of work that it can perform. The simplest 

 form of work is the raising of a weight. A man walking 

 uphill, or upstairs, with a pound weight in his hand, to 

 an elevation say of sixteen feet, performs a certain 

 amount of work, over and above the lifting of his own 

 body. If he carries the pound to a height of thirty-two 

 feet, he does twice the work; if to a height of forty-eight 

 feet, he does three times the work; if to sixty-four feet, 

 he does four times the work, and so on. If, moreover, he 

 carries up two pounds instead of one, other things being 

 equal, he does twice the work; if three, four, or five 

 pounds, he does three, four, or five times the work. In 

 fact, it is plain that the work performed depends on two 

 factors, the weight raised and the height to which it is 

 raised. It is expressed by the product of these two 

 factors. 



But a body may be caused to reach a certain eleva- 

 tion in opposition to the force of gravity, without being 

 actually carried up. If a hodman, for example, wished to 

 land a brick at an elevation of sixteen feet above the 

 place where he stood, he would probably pitch it up to 

 the bricklayer. lie would thus impart, by a sudden 

 effort, a velocity to the brick sufficient to raise it to the 

 required height; the work accomplished by that effort 

 being precisely the same as if he had slowly carried up the 

 brick. The initial velocity to be imparted, in this case, is 

 well known. To reach a height of sixteen feet, the 

 brick must quit the man's hand with a velocity of thirty- 

 two feet a second. It is needless to say, that a body start- 

 ing with any velocity, would, if wholly unopposed or 

 unaided, continue to move forever with the same velocity. 

 But when ; as in the case before us, the body is thrown 



