HOROLOGY. 



in 1766, first announced distinctly the 

 discovery of a general principle, proper 

 to produce the isochronism, by means of 

 the balance spring, which is expressed as 

 follows : that in every spring sufficiently 

 long, a certain portion of it will be iso- 

 chronal, whether long or short ; that the 

 length of this portion being found, if it be 

 lessened, thelong vibrations willbe quick- 

 er than the short ones ; and that, on the 

 contrary, if the length be increased, the 

 small arcs will be performed in less time 

 than the great arcs." This important 

 property of the spring enabled P. le Roy 

 to bring to a happy issue his labours for 

 the improvement of chronometry : and 

 the art is indebted to him for the practi- 

 cal utility of that discovery, as much as 

 for the invention of the detached escape- 

 ment. 



Berthoud found that the spiral spring, 

 in order to be isochronal, must have an 

 ascending force in arithemetical progres- 

 sion, and that this property may be affect- 

 ed, not only by the length of the spring, 

 but by the number of coils, and the taper- 

 ing or decreasing thickness from the cen- 

 tre to the extremity, &c. He adds, be- 

 sides, the proportions of the tapering in 

 many springs, which he had actually 

 tried, and gives minute accounts of the 

 experiments made with them in several 

 time-keepers. 



The late Mr. Arnold applied to the ba- 

 lance the cylindrical or helical spring, 

 which had been employed long before to 

 a variety of purposes instead of the spiral, 

 which had been constantly used in watches 

 since the time of Dr. Hooke and M. Huy- 

 gens. This is one of the articles of his 

 patent of 1782, whence it would appear, 

 that provided the spring be made of that 

 form, the vibrations cannot fail to be iso- 

 chronal; but experience is contrary to 

 that, notion, and artists are obliged to at- 

 tend to a variety of circumstances in the 

 application of the helical, as well as that 

 of the spiral spring. At present, some 

 watchmakers think thatthe helical spring 

 does not possess any advantage with re- 

 gard to that property ; but as the opi- 

 nion of other persons is in the affirmative, 

 while all the manufacturers, as far as our 

 knowledge goes, agree in considering the 

 cylindrical form as more easily managed 

 than the other, its application seems en- 

 titled to the merit of a practical improve- 

 ment. 



Mr. Earnshaw, in the explanation of his 

 time-keepers presented to the board of 

 longitude, after noticing the insufficieny 

 of the cylindrical spring, states, that he 



had, by long preseverance, found how to 

 make springs increasing in thickness to 

 the outer end, in order to effect the iso- 

 chronism of the vibrations. This method 

 of obtaining isochronal vibrations had 

 been long before explained by Berthoud, 

 with regard to the spiral spring, in that 

 part of his treatise on marine time-pieces 

 which we have already quoted. 



This artist states as a considerable dis- 

 covery, that the balance spring falls off or 

 tires in its strength, and he gives an al- 

 lowance for it; but the fact is neither so 

 constant nor so equable as to admit of his 

 general remedy. 



Fig. 7. represents the balance of a chro- 

 nometer, or time-piece, as usually made 

 by our artists. A circular groove is turn- 

 ed in the flat face of a piece of steel, and 

 into this groove apiece of good brass is 

 driven, and a little of the solution of bo- 

 rax is applied, to prevent oxydation. This 

 compound piece being then put into a 

 crucible, is made sufficently hot to melt 

 the brass; which in these circumstances 

 adheres firmly to the steel without re- 

 quiring any solder. The face of the steel 

 is then cleaned, andbyproperapplication 

 of the mechanical means of turning, bor- 

 ing, and filing, the superfluous steel is 

 taken away, and the balance is left, con- 

 sisting of two or sometimes three radii, 

 and a rim, the external part of which is 

 brass, and the internal part steel, the for- 

 mer metal being about twice the thick- 

 ness of the latter. Some artists solder the 

 metals together ; and others plunge the 

 steel balance into melted brass, and suffer 

 them to cool together, but the method we 

 have described appears to be the best. 

 In this state the arcs of the rim are then 

 cut through and diminishedin theirlength 

 as in the figure ; and near that extremity 

 of each arc which is farthest from its ra- 

 dius, a piece or weight is put on, which 

 can be slided along the arm so as to be 

 adjusted at thr.t distance, which upon 

 trial shall be found to produce a good 

 performance, under the different changes 

 of temperature. For it scarcely need be 

 observed, that the flexure of these arms, 

 by change of temperature, will carry the 

 weights nearer to the centre in hot than 

 in cold weather ; and the more, the 

 greater the distance of the weights from 

 the radius. The small screws near the 

 ends of the radii afford an adjustment 

 for time, as the balance will vibrate 

 more quickly, the further these are 

 screwed in ; and the contrary will be the 

 case, if they be unscrewed or drawn fur- 

 ther out. 



