It may be observed, that on the rim of the turned ba lances, which is separated into two, there are small pieces of brass, made to slide backwards and forwards, accord ing as it may be found requisite, when under the process of adjusting for heat and cold. In the other, this is done by screwing backwards and forwards the pieces of brass which turn spring tight on small screws left at the outer end of the laminm, and are bent in the same circle with them. Both balances have what are called meantime screws, placed in the vertical line of the hanging position, which serve to adjust them to their rate of time, &c. We have seen and made compensation balances with three arms, and three pieces, and three meantime screws, which were stiff, and answered very well.
On Balance or Pendulum Springs.
THE invention and application of the spiral spring to the balance of a watch is, by the foreign artists in general, ascribed to Huygens, while they admit the idea of a straight spring having been long before this applied by Dr Hooke. It was in 1658 that Dr Hooke observed the restorative action of springs, when he put one to the ba lance of a watch, and applied for a patent to secure his right of invention. The proff s were to be divided be tween himself, Sir Robert Moray, Mr Boyle, and Lord Brouncker. It was not carried into effect, in consequence of a quarrel between the parties, on account of a very unreasonable demand on the part of these gentlemen. Nor was it till 1674 that Huygens claimed the same discovery. Hooke charged Huygens with plagiarism, through the intervention of Mr Oldenburg!), Secretary of the Royal Society, who communicated to him, when he was twice in England, the registers of the society, and also cor responded with him upon the subject. In 1665, Sir Robert Moray requests Oldenburgh to tell Huygens that such watches had already been made in England, and to ask him if he does not apply a spiral spring to the ba lance arbor ? It may be asked, where had Sir Robert seen a spiral spring ? The natural answer is, that he must have seen it in the hands of Dr Hooke.
On nothing does a chronometer depend so much as in the good quality of the pendulum spring : great as the power of the rudder is, in controuling and regulating the motion of a ship, it is not more extraordinary than that of this spring, in regulating the motions of a chronome ter, and we may be allowed to say, that it possesses some thing like invisible properties. It may be set so as to make the machine go fast or slow, in any position requir ed, while neither its length, nor the weight of the balance, are in any way altered. Le Roy thought that he had made a great discovery, and it must be granted to be one, when he found, " that there is in every spring of a sufficient extent, a certain length, where all the vibra tions, long or short, great or small, are isochronous ; that this length being if you shorten the spring, the great vibrations will be quicker than the small ones ; if, on the contrary, it is lengthened, the small arcs will be performed in less time than the great ones." Notwith standing this condition of sufficient extent, the isochronous property will remain no longer than while the form of the spring is preserved, as it originally was. Should the coils be more compressed or taken in, the long vibrations will now be slower than the short ones ; and, on the con trary, if they are more let out or extended, the long vibra tions will be faster than the short ones. A more general principle for obtaining the quality of isochronism may be applied, by making the spring act proportionally, in an arithmetical progression, according to its tension. Every five degrees of tension should make an equilibrium with a given force or weight of ten grains, that is 5, 10, 15, 20, &c. degrees of tension, should he balanced by 10, 20, 30, 40, &c. grains. To try small springs by this process would require a very nice and delicatir tool. In order to obtain these properties in pendulum springs for his time keepers, Berthoud made them thicker gradually from the outer to the inner end ; our old English way is the re verse of this. Whatever may be the form of the spring, whether flat or cylindrical, the best and most direct way is to try them in the timekeeper itself, by taking four hours going, with the greatest force the main spring can give, and then four hours with the least. It is of consequence
to have these springs hard, or well tempered.
On the Jewelling of Pivot Holes.
OUR chronometers, from the art of jewelling the pivot holes, may be said to have acquired a durability and character, which they would not have otherwise received.
It must not be imagined that there is any time-keeping principle or improvement in a jewelled hole more than in a brass one ; and, notwithstanding what has been said in favour of the last, few will be hardy enough to run the hazard of having the balance, and balance wheels, to move in brass holes. It is very well known, that in a common verge watch, where the balance holes are jewel led, its motion will he kept up for a longer time than when it runs in brass holes. The friction at the balance holes cannot be supposed to be less than at those of the fusee; for, in the time of one-fourth of a turn of the fusee, the balance must make more than what is equivalent to 90U0 revolutions Berthoud regretted much that he had not an opportunity of getting the pivot holes of his time keepers jewelled ; yet, from that versatility of genius which he possessed, he supplied admirably the want of this, in a manner that very lew could have equalled. Some of the balances in his time-keepers were made to give six vibrations in a second. while others gave only one. His number eight made one vibration in a second, and was the one which gave the best performance of all those that he had constructed. It seems to have been considered as a wonderful discovery, that jewelled holes wore down the pivots, and thickened the oil, after they had been used for upwards of a hundred years. How came this not to be sooner observed, when so many were engaged in making chronometers, and that too in con siderable numbers ? That pivots, from a length of time, even with good oil, and with greater probability from bad oil, may have got, as it were, glued in their holes, there is little reason to doubt ; but this never arose from par ticles wearing away from either the steel or the stone, by the friction of the pivot. Let any one try to whet a graver, which requires some degree of force, on a polish ed Scotch pebble, for instance, and they will find that no exertion whatever will make the graver bite the stone, or the stone the graver: for where any effect of this kind takes place, it must be nearly mutual. The hardness of the Scots pebble is well known to be much inferior to that of the ruby or sapphire. After being exposed to the air for a considerable time, oil gets viscid and thick, which arises, as has been observed by chemists, from its absorbing or attracting oxygen. \Ve suspect that oil, from this cause alone, may become more glutinous at a jewelled hole than at a brass one. By its application to brass it soon acquires a bluish green tinge, as if some thing acted upon it. This is owing to the metal becom ing oxidated by the joint action of the oil and air. The oxide thus formed combines with the oil, and forms a me tallic soap, which is much less tenacious than that form ed at a jewelled hole. By the continuation of this pro cess, the hole in brass in time becomes wider, and the oil disappears, leaving the pivot and hole in a greater or less degree wasted ; and, instead of the oil, we have the metallic soap, which has hitherto been considered as rust. To be convinced, however, that this is not the case, we need only attempt to wipe it off from the pivot, from which it easily parts, and which it would not do were it really rust. Oil, however, can have no action on the jewelled hole, and any change that is effected by the oil must be confined to the steel pivot, on which its action is so exceedingly slow, that a great length of time must elapse before the oil is decomposed and disappears ; and hence what has been called rust in a brass hole, is sel dom or never met with in a jewelled hole. If a little fine Florence oil is put into a small phial for about two-tenths of an inch deep, and remain for a few years, it will be come exceedingly viscid and glutinous, and will be in termixed with parts tinged with red of various shades.