Boyal Institution of Great Britain, 423 



varying friction of the escape wheel and train, which are forced back. 

 The recoil is, however, by no means the worst objection to an esca{je* 

 raent ; and all other parts being good, very excellent timekeeper^ 

 may be made with escapements having a moderate recoil. 



It was at one time thought to be a considerable defect, and great 

 pains were taken to remcve it by variations in the form of the pal- 

 lets, on the principle before explained. By these the recoil is perr 

 fectly got rid of; but although the pendulum is not employed to 

 force back the escape wheel, the pallet rubs against the tooth during 

 pearly the whole of the vibration ; and this, of course, interferes with 

 the free motion of the pendulum. 



The next step was to avoid the recoil, and the greater part of the 

 rubbing, which was effected by the detached escapement, in which 

 the pallets and the escape wheel are in contact during only a small 

 part of each vibration. Still the pendulum received its impulse 

 directly from the escape wheel, and was liable on this account to 

 irregularities which were obviated by the invention of the remontoire 

 escapement, in which the escape wheel winds up a spring, whose 

 return gives an impulse to the pendulum. The impulse is, therefore, 

 always uniform, because the escape wheel can wind up the spring 

 only a certain quantity, whether the maintaining power be great or 

 small. 



One of the best escapements, on this principle, is that of Mr. 

 Hardy, published in the 38th volume of the ' Transactions of the 

 Society of Arts.' A large model of this was exhibited and ex- 

 plained; but the parts are too minute to form the subject of an in- 

 telligible diagram. 



Hitherto no mention has been made of the greatest source of error 

 in timekeepers, that which arises from the effect of temperature on 

 the pendulum, or on that which, in portable timekeepers, is the sub- 

 stitute for the pendulum, and is called the balance. This source of 

 error has given rise to a completely distinct series of inventions, 

 called compensation pendulums and compensation balances, which 

 will probably form the subject of some future illustration. 



Several very curious specimens of art were placed in the library, 

 the production of Signor Abbiati. They consisted of inlaid wood of 

 various colours, so arranged, as to give the outlines and colouring of 

 the subjects represented, the effect being a little assisted by linps 

 engraved upon the surface of the wood, and afterwards filled in 

 with black. 



