in the study of rapid movements, and we call to our aid what is termed the graphic method of registering movement.
Suppose I hold my pen between my thumb
and first two fingers in the ordinary way, and
extend the thumb and fingers so as to move
the pen upwards on the paper. I produce a line thus—
Fig. 20.
The length of the line from a to b will show the amount of movement of the pen point, but it will not tell me anything about the rate of movement, nor whether the pen moved With uniform velocity in passing from a to b. Suppose now that I repeat the experiment of drawing the line from a to b; but, on this occasion, suppose the paper is moving with uniform velocity from right to left while I draw the line. It will then be found that I describe a curve something like this—
Fig. 21.
The paper was moving in the direction of
