480 



SCIENTIFIC RECREATIONS. 



prayer he could repeat in sixty seconds. At the appointed hour he then 

 awoke the monastery to matins. 



Nature also marks time for us as, for example, the age of trees by means 

 of rings one for each year; and horses' teeth will guide the initiated to a guess 

 at the ages of the animals, while the horns of deer or cattle serve a like purpose. 

 But man required accuracy and minute divisions of time. He had recourse 

 therefore, to machinery and toothed wheels. Till the mechanical measurement 

 of time was adopted, the sunrise and sunset only marked the day, and the 

 Italians as well as Jews counted twenty-four hours from sunset to sunset. 

 This was a manifestly irregular method. To this day we have marked 

 differences of time in various places ; and at Geneva we have Swiss and 

 French clocks keeping different hours accord- 

 ing to Paris or Berne "time." This, of 

 course, is easily accounted for, and will be 

 referred to subsequently. 



We have read that the first clock in 

 England was put up in Old Palace yard in 

 1288, and the first application of the toothed- 

 wheel clock to astronomical purposes was in 

 1484, by Waltherus, of Nuremberg. Tycho 

 Brahe had a clock which marked the minutes 

 and seconds. If we had had any force inde- 

 pendent of gravitation which would act with 

 perfect uniformity, so that it would measure 

 an equal distance in equal spaces of time, 

 all the various appliances for chronometers 

 would have been rendered useless. In the 

 supposed case the simple mechanism, as shown 

 in the margin (fig. 5 1 7), would have sufficed. 

 The same effect would be produced by the 

 spring, were it possible that the spring by it- 

 self would always uncoil with the same force. 

 But it will not do so : we therefore have to 

 check the unwinding of the cord and weight, for left to itself it would 

 rapidly increase in velocity ; and if we likewise make an arrangement of 

 wheels whereby the spring shall uncoil with even pressure all the time, we 

 shall have the principle of the watch. 



It is to Huygens that the employment of the pendulum in clocks is due, 

 and the escapement action subsequently rendered the pendulum available 

 in simple clocks, while the manner of making pendulums self-regulating 

 by using different metals, has rendered timepieces very exact Of course 

 the length of a pendulum determines the movement, fast or slow ; a long 

 pendulum will cause the hands of the clock to go slower, for the swing will 

 be a fraction longer. A common pendulum with the escapement is shown 

 (fig. 5 1 8). Each movement liberates a Jooth of the escapement. The 



Fig. 517. Clock movement. 



