566 



THE HUMAN BODY. 



the to-and-fro movement of the pendulum, writing sometimes 

 on one side of the line 0-1-2 and sometimes on the other. 

 Starting at a moment when the pendulum crosses the middle, 

 0, we would get described the curve 0, a^a^a^ at first sepa- 

 rating fast from the vertical line, then slower, then return- 

 ing, at first gradually, then faster, until it crossed the vertical 

 again at the end of a second, and commenced a similar ex- 

 cursion on its other side, at the end of which it would be 

 back at 1, and in just the same position, and ready to repeat 

 exactly the swing, with which we commenced. A pendulum 

 thus executes similar movements in equal 

 periods of time, or its vibrations are periodic. 

 A full swing on each side of the position of 

 rest constitutes a complete vibration, so the 

 vibrational period of a seconds pendulum is 

 two seconds: at the end of that time it is 

 precisely where it was two seconds before, 

 and moving in the same direction and at the 

 same rate. It is clear that by examining 

 such a curve ^we could tell exactly how the 

 pendulum moved, and also in what period, if 

 we knew the rate at which the paper on 

 which its point wrote was moving. The 

 vertical line 0-1-2 is called the abscissa; 

 perpendiculars drawn from it and meeting 

 the curve are ordinates : equal lengths on 

 the abscissa represent equal times; where an 

 ordinate from a given point of the abscissa 

 meets the curve, there the writing point was 

 at that moment; where successive ordinates 

 increase or decrease rapidly the pendulum 

 moved fast from or towards its position of 

 rest, and vice versa. Similarly, any other 

 periodic movement may be perfectly repre- 

 sented by curves; and since the form of the 

 curve tells us all about the movement, it is 

 common to speak of the " form of a vibration," meaning the 

 form of the curve which indicates its characters. Periodic 

 vibrations (Fig. 169), whose ordinates at first grow fast, then 

 more slowly, next diminish slowly and then faster, and 

 represented by a symmetrical curve on one side the abscissa, 



