In a life of Peter le Roy, the son of Julien, the method of compensation is erroneously ascribed to that artist's father, to whom " we are indebted," says the writer of the article, " for the method of compensating the effects of heat and cold on the balances of chronometers, by the unequal expansion of different metals ; a discovery which has been brought by our English artists to a state of great perfection, although it had been laid aside by the inventor's son Peter." It is difficult to understand how tl.: -; mistake should have been committed ; for the following account of the discovery, given by Peter le Roy himself, is in direct contradiction to the preceding statement.
" Observation V.—Sur la compensation des effets de la chaleur et du froid " Scion la gazette du commerce, et la rapport signe Luclelam, envoye a Pacademie : pour remedier aux irregu larites produites clans les montres marines par Is chalcur et par he froid, 111. Harrison se sert d'une barre composee de deux pieces minces de cuivre et d'acier de longeur de deux ponces, rivees ensemble dens plusieurs endroits, „fixees par tin bout. et avant de rautre deux goupillos an travers des quelles passe le ressort du balancier. Si eerie barre reste droite dans le tents tempere (comme le cuivre recoit plus &impression de la chaleur que racier.) le rote de cuivre deviendra convexe au tents c/zaud, et le cote d'acier le sera an tcms froid. ?linsi les goupilles fixent tour-a tour les parties du ressort, selon les differens degres de chaleur, et l'alongent ou le racourcissent : (rat nail la compensation des effets du chaud et du froid.
" Si j'avois comut cette ingenieuse methode avant d'avoir peme a roes thermometres, je n'aurois vraisem blablement point hest 6 a en usage dans ma ma chine.
" J'ai balance quelque terns, si je ne dcvois pas lui dormer la p.eference. J'ai metric fait quelques essais clans cette vuc. Yen parleroi bientot : mais apres y avoir pense murement, et avoir mis a part, autant qu'il m'a vossible, ce penchant qui nous pile en faveur de nos mes thermometres m'ont paru eferable," &c. &c.
See Armoire sur la meillcztr nzaniere, de mesurer le tents en mer, p 55. 56, inserted at the end of Voyage fait par ordre du Roi, in 1768, pour eprouver les montres ma rines inventees par 1W. le Roy, &c. par M. Cassini, fils, Paris, 1770.
As must always be the case in the infancy of any branch of science, various methods are fallen on before it arrives at its most improved state. Berthoud, Arnold, and others, had recourse to different modes of compensation before they arrived at the one which gave them complete satis faction. The former, in his first machines, used small
wires or brass and steel, combined nearly like the gridiron pendulum, to effect the purpose of compensation ; to those of a later date, was applied a straight piece, Plate CCCIV. Fig. 2 ; composed of latrine: of brass and steel pinned together, acting on the short arm of a lever. In the end of the other arm, which was long, the curb pins were fixed. Even with those balances which were after wards made, and composed of brass and steel pinned toge ther, he adopted as a supplementary aid, the straight com pensation piece with the moveable arm and curb pins. Fig. S. Considering the talents he possessed, and the great experience which he mt ,t have had, this seems a little curious; as we think, whet e there is a complete com pensation in the balance, it alone should be sufficient, and that the curb pins would tend to disturb the pendulum spring rather than give any aid to the compensation. The more free the pendulum spring is, the chance is more in favour of good performance, when the compensation ba lance is supposed to be fit and complete for what is requir ed. In some of Arnold's balances, two pair of laminm were placed parallel to the diametrical arms ; on the mid dle of them was fixed a small wire which came through the rim, outside of which, and on the end of the wires, a small ball was fixed to each. Fig. 4. These balls were push ed out or drawn in by changes of temperature. The argu ments given in favour of such as Earnshaw's, which are turned first on the steel, and again on the brass, after being melted on the steel, are certainly very strong and convinc ing, and nothing can well be said against them, and yet there appears to be a softness in such a balance, that cannot exist in those where the lamina; are set or turned up by hand. Fig. 5. There is undoubtedly a great deal more trouble in making the last, and though it has been said that they cannot be true or round when made in this way, yet we have seen some of those done by Owen Robinson, as round and true as arty turning could make them, and possessing a degree of stiffness that cannot exist in the other ; they have been even unscrewed, or taken to pieces, and again put together, without altering the rate of the chronometer. After all, we are not aware that cronometers with the one balance actually perform better than those with the other.