STATICS 



Of Uniform Motion* 



21. A body is said to have a uniform motion when it passes 

 continually over the same space in the same time. 



In order to compare the motions of two bodies which move 

 uniformly, it is necessary to consider the space which each des- 

 cribes in the same determinate time, as one minute, one second, 

 &c. This space is what is called the velocity of the body. 



22. The velocity of a body therefore is, properly speaking, 

 only the space which this body is capable of describing uniform- 

 ly in the interval of time which we take for unity. 



Thus in the uniform motion of two bodies, the time being 

 reckoned in seconds, if one passes over five feet in a second, and 

 the other six feet in a second, we say that the velocity of the 

 first is five feet, and that of the second six feet. 



23. But if, the second being always taken as the unit of time, 

 I am told that a body passes over 100 feet in 5 seconds, 100 feet 

 does not express the velocity, since this space is not that which 

 answers to the unit of time, a second ; but it will be perceived, 

 that in each second it would pass over a fifth part of this 100 

 feet, or 20 feet ; that is, in order to find the velocity, I divide the 

 number 100, the parts of the space passed over, by 5, the num- 

 ber of units in the elapsed time. Hence universally, the velocity is 

 equal to the space divided by the time ; for it is clear, that if we 

 divide the whole space into as many equal parts, as there are 

 units in the time elapsed, each part will be the space described 

 during this unit of time, and will consequently be the velocity 

 according to our definition. Thus calling v the velocity and s 



