386 TJTVENTION OF THE BALANCE SPRING, &.€, 



1748. It may be said to operate thus : After one of the ba-* 

 lance wheel teeth has given isnpulse to the pallet, and has just 

 quitted it, there is a detent, which is made to come in and 

 catch or stop the wheel, by interposing itself in the way of one 

 of the wheel teeth. Plere the wheel rests during this vibra- 

 tion ; and, on the balance returning, there is on its axis an arm 

 er pallet, which gets into a sort of fork fixed on the arbor of 

 the detent, and which serves, by means of this fork, both to 

 lock and unlock the wheel. The unlocking is never com- 

 pletely done until such time as the pallet (which receives 

 the impulse) has got near or against the point of the next 

 impelling tooth, which then gets a recoil from the pallet, 

 by the mom3ntuni of the balance on its return, and on the 

 wheel being free, and again pressing the pallet forward, it again 

 becomes locked, and so on. 



Le Roy and How has it happened, that both M. Berthoud and M. 



both^have"""'^ Le Roy have overlooked this 'scapement of M. Thiout's ? 



known Thiout's Jt could hardly be unknown to them in the course of their 



escapemen . ^jjgp^g concerning the priority of invention of the free or 

 detached 'scapement. 



Description. In the figure, Plate X. Fig. 1., A B is the balance wheel ; 



G the pallet in which the teeth act; d the arbor of the detent; 

 c the locking and unlocking pallet, which is done by means of 

 the fork f', h the hook which locks the wheel teeth ; i is the 

 verge or axis of the balance. M. Le Roy made the balance 

 wheel a sort of contrate one, having the teeth standing upright. 

 This is a small improvement on 1\L Thiout's, which seems to bo 

 also the origin of the Echappcment d Virgule* 



* Some of the information contained in the above letter has 

 been, by a singular coincidence, anticipated in an hrticle ia the 

 iast number of this Journal. It may, therefore, be necessary to 

 mention, that what regards Dr. Hooke in the present letter, was 

 composed fourteen years ago, and that tlie whole was in possession 

 of Mr. Nicholson pi-evious to the publication of that essay. — "Note 

 of the author. 



