COMPENSATION against the effects of heat and cold in time-keepers, has been one of the greatest improvements that could have been applied to thorn. Without this they would have been far from keeping time, and would have varied continually with the temperature of the atmosphere, so that no fixed or settled rate could have been obtained. The detached 'scapement will show this more than any other ; for if there is no compensation to it, the watch will vary nearly thirty seconds in twenty-four hours. The in fluence of oil on the cylinder 'scapement becomes in itself a sort of compensation, and the effect of changes of tempe rature is much less obvious in it than in the detached 'scapement. There is very great reason to believe that Harrison was the first who applied a compensation ; but there are no written documents to warrant us in ascribing to him the honour of the invention, to which, however, we think he has a just title. Some very imperfect hints had been given by Martin Folkes, Esq. President of the Royal Society in the year 1749, of Ilarrison's having some me chanism of this sort in the three time-pieces which had been made prior to the fourth, which gained the reward voted by parliament. But as no description of it was ever made public, the French artists have had it in their power to claim a priority in the invention. In Harrison's fourth ma chine, it is known that the compensation piece in it was composed of laminae of brass and steel, pinned and rivetted together in several places. Perhaps those in the three for mer differed little or nothing from this.
The first pocket watch made in Europe with a compen sation was by F. Berthoud. It was begun in 1763, and fi nished in the beginning of 1764 ; and was sold in London in 1766, to Mr Pinchbeck, for his Britannic Majesty King George the Third. The compensation was effected by lamina of brass and steel pinned together ; one end of which being fixed to the potence plate, the other acted on a short arm from a moveable arbor, a longer arm having the curb pins in it, being made to move nearly in the cir cle of the outer coil of the spiral spring. It had a com mon crown wheel and verge 'scapement, and a going in time of winding. The balance was so heavy as to set, be ing sixteen grains in weight, and the vibrations were four in a second. Mr Kendal adopted this mode of compensa
tion in some of his pocket watches.
Mudge, some time before the year 1770, made a watch for Mr Smeaton, in which the compensation was effected by two long slips of brass and steel soldered together : being dressed up, it was turned up into a spiral, as close toge ther in the coils as to be free, and no more. The inner end was fixed to a circular curb•wheel, a short portion of the outer end had a pivot, bent in the circle of the outer coil, and supported by a small stud, through a hole of which it moved freely ; at this end was the curb pins, between which the spiral or pendulum spring passed; the effect of heat or cold on it was counteracted by the spiral compensation piece. The 'scapement of this was the cylindrical one ; and so long as the oil kept clean and fresh, the compensation might be useful to it. In 1774, we made one or two of the same sort for horizontal watch es. At that time no better 'scapement and compensation were known, at least so far as came under what was then the common practice.
In the voyage undertaken for the trial of Le Roy's Time keeper in 1768, and published by Cassini in 1770, along with the description and drawings of it, Le Roy has given that of a compensation balance, which is exactly like those of the present day, only the lamit.re arc pinned together, in place of the brass being melted on the steel. Plate CCCIV. Fig. 1. The compensation of the time-keeper, however, was not of this kind ; it consisted or two glass tube thermo meters, bent nearly into the form of a parallelogram, with a small ball at one end to each, the other open, and filled partly with mercury, partly with spirit of wine, fixed to the axis of the balance opposite one another: the balls lay very near to the axis. It would appear that Lc Roy had not thought of a metallic compensation, until the return of the time-keeper from the voyage of trial. fie had taken the idea of this from getting an account of Hai rison's, which was sent to the Royal Academy, signed by Ludlam, who was one of the scientific gentlemen appointed by the Com missioners of the Board of Longitude to take Harrison's ac count of his time-keeper, previous to any part of the reward being paid him.