THE ACCURATE MEASUREMENT OF TIME. 663 



Assuming tliat the power for an accurate clock must be a weight, 

 we are ready to pass to the application of this power to the propelling 

 of the pendulum, save this one consideration, that, unless there is some 

 special provision, the clock will not advance while it is in process of 

 winding. This provision is made in fine clocks by means of what is 

 termed a retaining-click — an ingenious contrivance which brings the 

 power of a small spring to bear while the key withdraws the power 

 of the weight. This device is also found in the English — that is, the 

 fusee-watch. Other watches need no such contrivance, for, as one end 

 of the spring is fast upon the winding-post and the other upon the 

 outside of the barrel, winding tends rather to stimulate and not to 

 stop its going. The same is true of marine clocks. But common 

 pendulum-clocks have the outer end of the spring attached to the 

 frame of the clock, and hence the application of the key takes off the 

 power, and the scape-wheel does not advance while the clock is. in 

 process of winding. 



Supposing ourselves now possessed with a uniform power, uniformly 

 applied to the scape-wheel, the problem arises as to the mode of com- 

 municating the power of the scape-wheel to the pendulum, in such a 

 way as to sustain its beat, but not affect the time of it. Without go- 

 ing minutely into the discussion of the escapement, it may be remarked 

 that there are two current forms of it. One of them is termed the 

 " recoil " escapement, and its peculiarity is that it at no time arrests 

 wholly the power of the scape-wheel — that is, it recoils by its own 

 action after a tooth has passed one side of the verge, and returns 

 toward the other beat. This is the escapement of the common kitchen- 

 clock, and the chief objection to it is that, according to no definite 

 law, the swing of the pendulum is made more rapid when the power of 

 the spring is increased, as by winding. 



A better escapement by far, and the one used in fine clocks, is the 

 " dead " escapement, of which the characteristic is that, after it makes 

 one beat, the pallet must be thrown off by the return of the pendulum 

 before the scape- wheel can again apply its power. An example of this 

 may be found in the ordinary marine clocks as well as in most watches, 

 in which the lever is at rest after the balance has been thrown in one 

 direction, until the return of the balance again trips it, and then its 

 power is applied in the opposite direction. 



This is found to be a satisfactory escapement for fine clocks which 

 are not to be disturbed by any outside influences, but, for tower-clocks, 

 which are affected by the wind, still another form is employed called 

 the "gravity," or "remontoire" escapement, the principle of which is 

 that the power of the clock merely lifts a small weight which is then 

 unlocked by the swing of the pendulum and falls upon it, applying the 

 uniform amount of its weight to propel the pendulum. There are, of 

 course, two such small weights lifted alternately at each beat of the 

 pendulum. The pendulum, therefore, has nothing to do with the 



