MOVEMENT 53 



off from each time-division a distance which represents 

 the space traversed by the ball in the corresponding 

 interval of time. The small crosses mark the lengths 

 of such a series of ordinates. Taken together they 

 form the line V, which is the curve of velocity. Such 

 a line is a straight one, but obliquely inclined, and 

 expresses a velocity of uniform increment. 



Finally, the curve of acceleration is obtained by 

 marking off on the ordinates, under each of the time- 

 divisions, the excess of velocity of each period over 

 and above that of the one which precedes it, that is 

 to say, the excess of the second over the first, and 

 the third over the second ; in other words, the incre- 

 ment of velocity, or acceleration, in a series of time 

 intervals. A large black dot marks the length of each 

 of these ordinates ; the dots united together by the 

 line A constitute a straight horizontal line showing 

 the acceleration was uniform.* 



In constructing these figures, the unit of time was 

 represented as any interval. This method answers 

 very well in comparing the relative degrees of speed 

 and acceleration ; but to ascertain the exact degree, 

 these indefinite intervals must represent a second, the 

 recognized unit of time. 



The chronometric dial provides us with the means 

 of doing this in the same manner as the divided scale 

 measured in meters and fractions of meters the space 

 traversed by a falling body. Thus chronophotography 

 provides us with the means of constructing the curves 

 of movement. 



* On account of a mistake in the diagram (Fig. 35), the degree of 

 acceleration is half what it ought to be. 



