HOROLOGY. 



Pierre le Roy, eldest son and successor to 

 Julien le Roy, the companion and friend 

 of H. Sully, had the merit of accomplish- 

 ing the great desideratum of making the 

 compensation on the balance itself. In the 

 chronometer, which was presented to the 

 king of France the 5th August, 1766, and 

 obtained the prize of the Academy of Sci- 

 ences of Paris the 31st of the same month, 

 that author executed a compensation in 

 the balance, which he has fully explain- 

 ed in his description of that machine. 

 (" Memoires sur la meilleure Maniere de 

 mesurer le Temps en Mer, qui a rempor- 

 tc le Prix double au judgement de 1'Aca- 

 demie Royale des Sciences. Contenant 

 la Description delaMontre a Longitudes, 

 presentee a sa Majeste le 5 Aout, 1766." 

 Par M. le Roy, Horloger du Roi, pp. 41 

 to 44. ) This memoir accompanies the ac- 

 count of Cassini's voyage in 1668, pub- 

 lished in 1770. This compensation is 

 composed (fig. 4.) of two thermometers, 

 t K t A K, of mercury and spirits of .wine, 

 made each in the form ofaparallelgogram, 

 except in the upper branch, which bears 

 the ball containing the spirits of wine, and 

 is a little bent downwards ; the mercury 

 is in the lower part, and the vertical 

 branch of the tube, A K, is open at the 

 upper end. These two thermometers 

 are placed opposite one another, the axis 

 of the balance being in the same plane 

 with the centrical lines of the tubes ; and 

 the thermometers and balance are solidly 

 attached together, and form a well pois- 

 ed and steady regulator. M. the middle . 

 temperature of the atmosphere, the 

 quicksilver stands at K A t K ; but, when 

 an increase of heat occurs, the alcohol, 

 by its expansion, forces the mercury from 

 the exterior branch, t K t, towards A K, 

 and a portion of the mass of the regulator 

 contracts by approaching the centre. On 

 the contrary, if the variation consist of an 

 additional degree of cold, the mercury 

 moves towards the exterior branch, and 

 the weight towards the circumference of 

 the balance becomes greater. Thus, if 

 the thermometers are well adjusted, the 

 effects of all the changes of temperature 

 in the balance will be compensated, and 

 the regulator will act with the same uni- 

 formity as if its figure were not liable to 

 such alterations. 



Peter le Roy mentions his knowledge 

 of Harrison's expansion curb, acquired 

 soon after he made his thermometrical 

 balance ; and he constructed a balance 

 accordingly on the principle of Harrison's 

 curb, the arms or arcs of which act by 

 flexure, and are adjusted by moveable 

 weights. He proved_the effect of such 



arms by experiment ; but gave the pre- 

 ference to his own mercurial compensa- 

 tion. Peter le Roy's second invention is 

 the same as is now used : but though so 

 publicly declared in the face of the 

 French government and academy in 1766, 

 Arnold took an English patent for it in 

 1782. 



We have not yet taken any notice of 

 improvements made in the escapement, 

 because after all the plans proposed for 

 this most essential part of chronometers, 

 the principle of what is called the de- 

 tached escapement is the only one now 

 used ; and, being established upon long 

 experience, seems to merit the prefer- 

 ence given to it over all the constructions 

 proposed till now. We shall content our- 

 selves with stating in a general manner 

 the beginning and progress of that es- 

 capement. 



In all the escapements known till the 

 middle of the last century, the escape 

 wheel was in continual contact with the 

 pallets belonging to the axis of the ba- 

 lance wheel; and the friction proceeding 

 from this circumstance may be consider- 

 ed as a principal source of irregularity in 

 the going of the watches. Suppose that 

 a regulator should be made so perfect as 

 to be exactly isochronal, while vibrating 

 in a free situation, that advantage would 

 be diminished or lost as soon as it was pla- 

 ced in connection with a train of wheels; 

 and the errors would be more or less,'ac- 

 cording to the nature and quantity of fric- 

 tion in the escapement. It would be, 

 therefore, extremely useful to secure to 

 the regulator a perfect liberty of vibra- 

 tion, except during the short intervals of 

 time which may be necessary for the ac- 

 tion of the escape wheel, to give it a new 

 impulse. This ingenious idea was also 

 started by P. le Roy, and carried into exe- 

 cution by the same artist, in a model which 

 he presented in 1748 to the Academy of 

 Sciences of Paris, and is described in the 

 collection of machines approved by that 

 society for the same year, vol. vii. p. 385. 

 The effect or action of le Roy's escape- 

 ment in few words is this : An escape 

 wheel is kept in repose by a lever detent. 

 The balance unlocks the detent, and re- 

 ceives an impulse or stroke on a pallet 

 through a part of every second vibration ; 

 and during great part of its course it is 

 free and detached. 



About the year 1755, according to 

 Count de Bruhl, the late Mr. Thomas 

 Mudge invented a detached escapement, 

 and applied it to a watch which he made 

 for the king of Spain, Ferdinand VI. 

 This is the same escapement that was 



