INVENTION OF CHRONOMETERS, &:C. 301 



deep researches upon the subject of chronometers, ascribed* 

 the principal cause of the imperfections of watches to the wide- 

 ness of the vibrations ; and, while he assigned the small power 

 of the balance, as the second cause of error, he found himself 

 under the necessity of recommending f very short vibrations, 

 in order to effect, in that way, the required equality of their 

 duration. 



Another great advantage, proceeding from the isochronism of This isochron- 

 the balance spring, combined with the detached escapement, is l^die^esca t^" 

 the destruction of those variations which would arise from the ment do also 

 main spring. From the first appearance of the invention of f,^r?™?arity 

 the detached escapement, we find P. le Roy attentive to that arising from 

 important object ; and in 174-8, when he presented his model *^ first mover, 

 to the Academy of Sciences, he tried it before that assembly 

 with the main spi'ing, at the two extremes, when the difference 

 in the rate of going was found to be very small. J The author 

 shewed how to destroy even that small difference, and explam- 

 ed how all the vibrations might be rendered of the same dura- 

 tion. || P. le Roy having afterwards discovered the method, Peter le Roy, 



of effecting the isochronism, by means of the balance spring, j'^° l'^^^ ]f°" 

 ° ' •' f DJ duced both so 



trusted so confidently to that property, combined with his im- very early, 



proved detached escapement, that in the chronometer for 



xvhich he was rewarded in 17 66, he laid aside the chain and —did reject the 



fusee, and made the maintaining power act immediately on ^'^^"^ ^"^/i^" 

 ' = ^ -^ see, in 1706. 



the train of wheels. In the '< Succinct Account of the Attempts 



*' of Messrs. Harrison and Le Roy, for finding the Longitude 



" at Sea, by P. le Roy ;" an English translation of which, by a 



Fellow of the Royal Society, was published in 17^8, in London ; 



the author expressly states, that his watches, according to that 



construction, go thirty hours without winding up ; and upon 



examining their rate of going in the first, and in the last fifteen 



* Recherches, Mechaniques et Astronomiques, sur la Question 

 proposee par 1 ' Academie Royale des Sciences, pour 1 ' Annee, 

 1745 : la meilleure Maniere de trouver 1' Heure en Mer. § xxii. 



f The same. § xxviii. 



X Journal des Machines, 1748. 



II See the description of that chronometer, in the Meraoire sur 

 la meilleure Maniere de mesurer le Temps en Mei", par P. le Roy; 

 attached to Cassini's voyage. 



hours 



