COMMON THINGS BLOCKS AND WATCHES. 



The direction: in wHch the wheel c D is turned by the force of 

 the descending weight is indicated by the arrow, and in that 

 direction it will continue to turn so long as the weight acts upon 

 the coil of the cord upon the drum. But so soon as the cord, 

 by the continued descent of the weight, shall have been dis- 

 charged from the drum, the rotation imparted to CD must cease. 

 It is then that the key must be applied to the square end, p, of 

 the axis of the drum, and turned continually in the direction 

 contrary to that in which the weight would turn the drum in 

 descending. 



29. It will no doubt be perceived by the attentive reader, that, 

 in this case, the hands of the clock would be always turned back- 

 wards while the clock is being wound up, unless some special 

 provision were made against such an effect ; for it is evident, that 

 if the wheel c D, when turned by the descent of the weight w, in 

 the direction of the arrow, give a progressive motion to the hands, 

 the motion imparted to CD, by the ascent of the weight w, while 

 the clock is being wound up, must necessarily impart to the 

 hands a motion in the contrary direction, that is a backward 

 motion. 



In all clocks this is prevented by an expedient called a ratchet 

 wheel and catch, the one being attached to the barrel A B, and 

 the other to the face of the wheel c D, the effect of which is to 

 allow the barrel A B, to be turned while the clock is being wound 

 up in the direction contrary to that indicated by the arrow with' 

 out turning the wheel c D ; but when the barrel A B is turned 

 by the descent of the weight w, in the direction of the arrow, the 

 catch acting in the teeth of the ratchet-wheel, the motion of A B is 

 imparted by the action of the catch on the ratchet-wheel to the 

 wheel c D, and by it to the hands. 



Fie. 12. 



The form and mode of application of the ratchet-wheel will be 

 presently more clearly explained. 



30. The moving power of a weight can only be applied to time- 

 22 



