ISrVENTIOJT OF CHRONOMETERS, &C. 281 



** require to be clone by the hand in the manner used for regu- 

 " lating a common watch." 



This kind of compensation has been since applied in other Objections to 

 ways; but the method, in genera), is liable to some material ^°"JPg"^^V^_^^ 

 objections, on account of its altering the length of the balance 

 sj>ring, and the difficulty, peihapsimpossibility, of effecting with 

 it an accurate adjustment. Mr. Harrison himself was aware Harrison pro- 

 of these objections, and expressed the well known observation,* fhe^compensa- 

 that if the provision for heat and cold could properly be m tion in the ba- 

 the balance itself, ---the -watch- - - would then perform to a few d^d^'not'con-^"'^ 

 seconds in a year. By the watch, ]\Ir. Harrison meant his own trive any mean 

 longitude timekeeper ; and we have now sufficient reason to *^ ^ ^^ '"° * ' 

 believe, that he overrated its merits, though the construction 

 had been improved with the desired invention, upon which he 

 set so great a value ; but that assertion, which has been repeat- 

 ed t and strengthened by men of learning and good judges of 

 mechanics, shews, at least, the importance of the desideratum, 

 which seeiTked to be still wanting to complete the perfection of ' 

 chronometry, 



Pierre le Roy, eldest son and successor to Julien le Roy, ^^'^^'^l^^i\^, 

 the companion and friend of Henry Sully, had the merit of fected this pur- 

 accomplishing that great desideratum. In the chronometer, P°^,*l^^ ^*''*'^' 



in 1766. 

 which was presented to the king of France the 5th August 



176'^, and obtained the prize of the Academy of Sciences of 

 Paris the 31st of the same month, that author executed a 

 compensation in the balance, which he has fully explained in 

 his description of that machine. J This compensation is com- 

 posed (Fig. ;, PL VH.) of two thermometers, 2* K ^ A K, of mer- —by mercurial 



, . . „ . , , . , ,. f thermometers 



cury and spirits of wine, made each m the form of a ;„ the balance 



parallelogram, except in the upper branch, which bears the of a chronome- 

 "" * ^^ ' ter, which ob- 



* A Description concerning sucli Mechanism as will afford a j^'"*:*^ ^^"^ ^^*' 

 ' . ^ a demical prize 



nice, or true Mensuration of time, &c. By James Harrison, 177i>, of that year. 

 p. 103. 



t See Mr. Ludlam's letter to Dr. Maskelync, in the Report from 

 the committee to which Mr. Mudge's petition was referred, pp. yo 

 and 97. 



X Memoire sur la meilleure Maniere de mesurer le Temps on 

 Mer, qui a remport6 le Prix double au Jugement de I'Academie 

 Royale des Sciences. Contenant la Description de la Montre a 

 Longitudes, piesentee a sa Majest6 le 5 Aout, 1766. Par M. le 

 Roy, Horloger du Roi. pp. 41 to 44. This Memoir accompanits 

 the account of Cassiui's voyage in 1768, published in 1770. 



Vol. XIV.— August, 180&. Oo ball 



