CHAP, i.] OSCILLATIONS OF THE PENDULUM. 23 



the pile and the iron ring with which the head is fitted to resist 

 the lateral strain which, without it, would split the piece of wood 

 into fragments. 



III. CLOCK PENDI/LUMS. 



Galileo, after discovering that the oscillations of the same pen- 

 dulum took place sensibly in equal times, when their amplitude 

 was very small, thought to utilize this valuable property in measur- 

 ing the exact number of beats of the pulse. The instrument called 

 , the pulsilogium, which is simply a pendulum, was, it is said, invented 

 by him. 



But it appears certain that Huygens was the first inventor of 

 the application of the isochronism of the pendulum to clockmaking 

 (1656). For nearly three centuries and a half clocks with cogged 

 wheels had been used, but they were as yet very imperfect instru- 

 ments, not having a constant regulator of theix movement. Huy- 

 geiis solved the problem in the following manner : It is known that 

 in clocks the motive power is sometimes a weight, which, under the 

 influence of gravity, unwinds the cord by which it is suspended, and 

 thus continuously turns the axis of a cogged wheel ; and sometimes 

 it is a steel spring, which unbends gradually, and, since by a special 

 mechanism its action is rendered almost regular, this spring also 

 causes the cogged wheel to move in a continuous manner. In both 

 cases this wheel transmits its movement to all the other parts of the 

 clock. 



In both cases, also, the difficulty was to establish a perfectly 

 regular and uniform movement, notwithstanding all the causes of 

 error and the variety of resistances presented by the action of so 

 many parts. This was accomplished in different ways, by trans- 

 forming the continuous movement given to the wheel work by the 

 motive power into an oscillatory or periodical one, by using a 

 regulator. The most simple and at the same time the most exact 

 regulator of clocks is the pendulum. Huygens's arrangement is 

 shown in Fig. 5. 



E is a cogged wheel with oblique teeth, to which a movement is 

 communicated by the spring or weight of the clock. This motion it 



