18 FRAGMENTS OF SCIENCE 



in the amount of work tTiat 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 elevation 

 in opposition to the force of gravity, without being actu- 

 ally 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 pre- 

 cisely 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 starting with 

 any velocity, would, if wholly unopposed or unaided, con- 

 tinue to move forever with the same velocity. But when, 

 as in the case before us, the body is thrown upward, it 



