296 iNVEirrioTsr or cnuoiNoMETEilS, &c. 



of the voyage to Jamaica, added for the same purple the cy» 



cloidal pin, to regulate the balance spring ; but this method 



l^eterleBoy of adjustment never appeared satisfactory or certain. P. le 



fii-st distinctly Roy, in his Memoire siir la meilleure Maniere de mesurer le 



announced "" ' 



that isochron- Temps eiiMer, rewarded in 1766, first announced distinctly the 



bf'b^ ^1 "^"^' discovery of a general principle, proper to produce the iso- 

 spring. chronism, by means of the balance spring, which is expressed as 



He asserted, follows : That in every spring sufficiently long ^ a certain portion 

 Ien<^th otsririno- ^f ^^ '^^^^ ^^ isochronal, whether long or short ; that the length 

 viWihfihochxo- of this portion being foimd, if it be lessened, the long vibra-^ 

 leno'th will have ^^^"^ "^'^^^ ^^ quicker than the short ones ; and that on the con- 

 the wider vi- trary, if the length be increased, the small arcs will be performed 

 quicker, and *^ ^^^^ ^^'"^^ ^^'^" ^^'^ great arcs. This important property of the 

 a greater spring, enabled P. le Roy to bring to a happy issue his labours 



narrov/er vi- ^0' ^he improvement of chronometry ; and the art is indebted 

 brations quick' to him for the practical utility of that discovery, as much as 



for the invention of the detached escapement. 



P. Berthoud F. Berthoud appears again on this occasion as a rival of P. 



*^overv ^ * ^^ ^°y» ^"^ arrogates to himself the honour of the discovery 



of the isochronism, by means of the balance spring ; but his 



proofs are unsatisfactory, and the dates of the respective labours 



of those authors are too well established to admit of any pre- 



—but without sumption favourable to his pretensions. F. Berthoud did not 



any proof or publish any researches, or even ideas, upon the subject, till 



1773, which is the date of the Traiie des Horloges Marines; 



where (Premiere Partie, chap. iv. art. ii.), with the same want 



of candour, as we have already remarked in the case of the. 



detached escapement, he gives a very minute detail of his own 



inquiries and experiments, without even once mentioning those 



of his predecessor. When that author was, afterwards, obliged 



Berthoud's to take notice* of the accusations published against him by 



defence in an- p^ ]g j^^y (\^ j^jg Pf-^cis des differentes Recherches qui ont its 



swer to le Roy. . . ' ^ 



faites depuis plus de quarante Ans, pour parvenir a resoudre le 



fameux Probleme des Longitudes par le Secours de VHorluger 



rie), he refers to a passage of the Essai sur I'Horlogerie (vol. 



i. p. l68), to shew that he had, in 1760, laid the foundation 



— 1» unsatisfac- ^^ ^'^^ discovery ; but that passage signifies nothing, and can 



tory. 



* In the Eclaircissements, he. servant de suite a 1' Essai £ur 1' 

 Ilorlogeric. et au Traite des Horloges Marines, 1773. 



only 



