THE CONSTITUTION OF NA TURti. 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. 
AVhen, however, we spoke of the conservation of force, iu 
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. He 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 
