Oct. 26, 1876] 



NATURE 



573 



PRINCIPLES OF TIME-MEASURING APPA- 

 RATUS'- 



III. 



Clock Escapements. 



AN escapement in general is to be considered a good 

 one just in proportion as it prevents variations of 

 friction in the clock-train from reaching the pendulum. 



Fig. 14. 



The first form of escapement with which the pendulum 

 was used is that early form mentioned in our description 

 of the clock from Dover Castle ; but this was speedily 

 abandoned on account of the unduly large arc through 



except that the pendulum need only swing 2° or 3° in 

 order that the teeth may pass. 



SW is the escape-wheel. The tooth T is now being 

 held by the right-hand pallet, P ; in point of fact, as the 

 pendulum is swinging to the left, the pallet is actually 

 recoiling or driving it back a little. By-and-by the 

 pendulum will return, lift the pallet, and allow the 

 tooth to escape, when the same action will take place 

 upon the opposite pallet. You can readily see what 

 the effect would be, supposing a little more force to be 

 occasionally transmitted by the clock-train ; it is obvious 

 that the pendulum would be beaten backwards and for- 

 wards by the action of the pallets, and the time of the 

 clock would be greatly accelerated. The reverse action 

 would take place supposing a little less force to be trans- 

 mitted. This escapement is called the recoil escapement. 



We now come to the dead escapement (see Fig. 15), 

 invented by the same Graham who discovered the mer- 



which the pendulum had to swing in order to liberate the 

 teeth of the escape-wheel. The form next employed is 

 that shown in Fig. 14. The principle is nearly the same, 



I Lectures by Mr. H. Dent Gardner, at the Loan Collection, South 

 Kensington. Continued from p. 556. 



Fig. 16 



curial pendulum. The escapement is so called because, 

 during the greater part of the swing of the pendulum, the 

 seconds hand lies motionless or dead, upon the division 

 of the clock dial 



That tooth T of the escape-wheel has just got clear of 

 the left-hand pallet, and v has fallen upon the face C D of 

 the right hand, w A and C D are portions of circles 

 described from Y, the axis of motion of the pallets, and 

 you see they have therefore no tendency to drive back or 

 recoil the escape-wheel. 



In order to understand its advantage, I must ask you 

 to follow very carefully what I am now going to say. As 

 I told you when we were discussing barometric compen- 

 sation, any force acting upon the pendulum in the same 

 direction as gravity, will cause it to swing faster, and any 

 force against gravity to swing slower. The force of the 

 clock train, when it gives impulse to the pendulum, may 

 act either -with or against gravity ; that is to say, it may 



