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Under the article of making compensation for the effects of heat and cold, Mr. B. considers (wo effects; the alteration in the size of the balance, and the alteration both of the length and the strength of the spring, that is the alteration both of the Jaw and the degree of its force; the former of which he supposes to vary with the length of the spring, as was said before. For the particulars of this machinery, Mr. B. refers us the description of his marine clocks in the next part. The whole of it consists in applying, in various ways, the well known combination of brass and steel rods in the form of a grid-iron * We shall only add that in inquiring what are the most proper materials of which to make the balance, Mr. B. reckons gold the best, only too expensive, the balances of his machines being, very large. He rejects steel as being subject to rust, and to become magnetical, even by the process of forming it into a balance; and fixes

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brass. In the chapter of escapements, Mr. B. utterly rejects a}} those palets where the force of the clock is opposed to the motion of the pendulum in any part of the vibration; that is all those palets where the escapement wheel has a recoil, or retrograde motion t. After the wheel has given an impulse to the pendulum, he would then have the pendulum left to itself, and not checked in its motion. There are those who think it an advantage to have the motion of the pendulum checked and controuled by the palets. They think it gages the arch of vi- ' bration, and keeps the pendulum from flying out; that a force opposing the pendulum near the end of its ascent, and aiding it in the beginning of its descent, co-operates with the force of gravity, and tends to make the vibrations isochronous. For the force of gravity on a pendulum vibrating in a circular arch,

* Mr. B. is displeased with M. Le Roy for saying that Mr. B. made use of the gridiron of Mr. Harrison. Mr. B. in reply says, the gridiron was not employed in the marine watch of Mr. Harrison, and that he invented his method of compensation 20 years ago, before he knew the name of Harrison, and that his frame of compensation differed from Harrison's. See Reponse au Precis de M. Le Roy, p. 38. We shall observe on this controversy, that the only difference between Mr. Harrison's gridiron and Mr. B.'s frame of compensation is, that in the former the bars or rods were round, in the latter {quare : and though Mr. Harrison did not use this frame of compenfation in his last-made.time-keeper, yet he employed it in every one of his first great machines. Mr. Harrison's gridiron (as it was called) was publicly shown by him in 1736, and, fone time after, imitated by an itinerant teacher of experimental philosophy, who exhibited it in his public course of lectures.

+ The balance wheel is that which immediately acts upon the ba. lance. The fwing wheel is that which acts upon the pendulum. We want a word for the general idea including both. The French term is La roue de rencontre, or La roue d'échappement.

is too little towards the end of the arch, and requires an addition to give it that force which produces isochronous vibrations. We will not affirm these are Mr. Harrison's sentiments in the case of pendulums, but from some expressions in the very concise account of his principles, we are inclined to think so.' The case of balances moved by a spiral spring is very dif- . ferent I.

Mr. B. recounts some particular escapements. He mentions first the dead escapement of Mr. Graham, (L'é happement à repos) which has the eflential condition he requires of leaving the pendulum at liberty at the end of the vibration; but he didlikes it on account of friction, and for other reasons.

The next escapement is that of Mr. Cumming $; which Mr. B. says, in spight of the seeming advantages, had sufficient friction and difficulties in the execution, to deter him from making use of it.

Mr. B then mentions Mr. Harrison's escapement, made use of in his three first machines, and by which, as Mr. B. says, the whole force of the escapeinent wheel is communicated to

The escapement of Mr. H.'s last time-piece is a very unusual one. The crown wheel and its teeth are of the common form, only the straight side of the tooth is quite upright. As soon as one tooth has escaped, the upright face of the opposite tooth meets the edge of the other palet. The balance at that time has passed its point of rest, and is going on to the extremity of its vibration. The edge of the palet therefore goes up the face of the tooth till it arrives at the point of the tooth. The point of the tooth then goes round the edge of the palet, and acts against the back of the palet (which is circular or bump-backed) to the end of the vibration. While the tooth thus acts against the back of the palet (which it does near the extremity of each vibration, the force of the crown wheel affifts the balance in going forwards, and retards its return. The force of the wheel in most other escapements has either a contrary effect at the extremity of the vibrations, or none at all. In other parts of the vibration, the force of the wheel on the balance is nearly the same as in the very common crown wheel escapement.

Cumming's Elements of Clock and Watch-work, page 75. Mr. B. does not tell us that Mr. Cumming had published a description and drawing of these palets, nor does he tell us that the former escapement was Mr. Graham's, nor the next Mr. Harrison's; for the last escapement he mentions (that of free vibrations) he does indeed say Mr. Mudge showed him one of that sort in 1766 that had been made a long time. An escapement of this kind was also in the hands of the late Mr. Ellicott about the year 1746.-It is posible that ingenious men may hit on the same invention. We have here four in. ventions, every one of which has been made by Englishmen, and published to the world long ago.

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the regulator : but he rejects this efcapement as having a recoil, also being complicated and difficult to execute.

The last escapement he mentions, and which he prefers to all others, is that in which the pendulum receives a pulh, or rather a blow in the middle of its vibration, and is then left to itself for the remaining part of that vibration, and also for the subsequent returning vibration, so that this impulse is given only every other vibration. Between one impulse and the next, the escapement wheel is locked up by a detent, and is unlocked by the pendulum juft before the next impulse. The pendulum being thus at liberty to swing out at the end of each vibra ion, he calls in the escapement of free vibrations. Notwithftanding a long encomium upon this, Mr. B. says he is so little fatisfied with the trial of all these inventions, that he is now employed in improving the common dead escapement by means of ruby palets, and a wheel of steel made perfectly hard; but laments greatly the indispensable neceslicy of ufing oil in such a construction t.

There is nothing in the chapter on the wheel-work but what is common ; our author recommends high numbers, and direas stopping the pivot holes with pot brass.

The 7th chapter is on the first mover of the clock. Mr. B. makes a comparison between the advantages of ufing a weight or a fpring; and is against using a spring even for marine time

# This will or will not be, according as the several centers of mo. tion are placed. Upon the placing of these centers it likewise de. pends, whether the force of the escapement wheel on the regulator fhall be uniform, increasing or decreasing during the time of the acuion of the wheel on the palets.

• The manner of doing this is circumftantially described part ii. Chapter xii.

#Mr. H. in his three first machines, endeavoured to avoid the necessity of ufing oil, by lessening as much as poslible the relatie motion of those parts which touched each other. In the action of the balance wheet on the palets the relative motion of the tooth and palet was wholly taken away, and the palet was made of wood. In Bis last time-keeper (sent to the West Indies) the use of oil was ab. kolutely necessary, but no peculiar provision made against its effects. Indeed all the contrivances to procure isochronops vibrations, even though the force should vary, may be considered as one remedy; and the making the balance-wheel of steel and hard, and the palets of diamond as another. It was remarkable that though this timepiece had been in constant going for a long while, and the nature of ibe escapement such as occafioned a great relative motion of the taoth and palet, yet there was not in 1765 the least trace or mark of the palet having worn the tooth,-it was not to be expected the tooth thould make any impression on the palet,

pieces. His objections are, that a spring is liable to break, that it changes its force by heat and cold, that it is apt to set, and that the coils should always be kept oiled. Experience shows we need not fear the first of these objections. The others are all obviated by applying a very tender spring to the escapement wheel, to give to that wheel its force. When this spring has unrolled itself a little way, it is then to be wound up by the force of the main-spring. This little spring in Mr. Harrison's time piece is wound up eight times in a minute I.

Our Author, has at the end of this chapter, a short article on the proportion the moving force of the clock ought to have to that of the regulator. We did expect he would have considered this at the beginning of his theory. What he says in chis place is loose, and confused; he here, and in many other places, uses the words force and motion as synonimous terms, and standing for precitely the same idea.

The last chapter in this part, is on the suspension of marine clocks, that is, the manner of supporting or carrying them at sea. He lays down two principles, one that the clock should be so suspended, as to keep its position always horizontal, without partaking of the motion of the thip, the other that the sufpension fhould have (with this freedom in yielding to the motion of the ship) great firmness and solidity.--Such a solidity that the balance may not let in motion the case by which the sime-keeper is defended from the external air. Our Author recommends Cardan's method of suspension g; he would have the clock kept in a sort of closet to defend it from the sea- air, and placed as near as conveniently may be to the center of gravity of the ship. He also uses spiral springs to break the force of sudden shocks 1.

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I The machinery for doing this is called a Remontoir ; Tometimes the machinery for keeping a clock in going while it is wound up, is also called (but improperly) a remontoir,

§ Cardan's method of suspension is that of the sea-compass, called by seamen Gimbols ; it is the same with the niechanism of the rolling lamp in Defagulier's Lectures, page 57.- This contrivance seems to be the parent of the universal joint; one of the most useful inventions in mechanics. The universal joint is largely treated of by Hooke in one of his lectures at Gresham College, who represents it as his own invention, but, nevertheless, mistakes a little in giving an account of its very singular properties, Hooke's lecture is printed in 1678, but the first account of it is in Schottus's Technica Căriola, p. 664, printed in 1664. Schottus seems to have picked it up in Germany; which, abounding in metals, has ever been the nursery of the mechanic arts, especially while the Hans towns retained their independency.

ll Mr. Harrison in his three great machines used spiral Springs to break the lhocks, but applied in a manner very different from that We have now gone through the theory, and are furprised Mr. B. fhould overlook what has ever been reckoned the most important point in this whole doctrine, namely, that the force which originally gives motion to the regulator, should be very great in comparison of that which is only employed to restore so much of the motion as is lost by friction, &c. This original force in a pendulum, is that of gravity; in a balance, that of the spiral spring applied to it. The original forces are constant and invariable, and by them only would the vibrations of the regulator be preserved the same, were it not for the fmall impediments of fri&ion, &c. To repair what is thus loft, another auxiliary force must be added *, and this is the force of the weight in a clock, and of the main-spring in a watch, acting upon the regulator through the intervention of the wheel-work; the wheel-work at the same time serving to count the number of vibrations. The regulator then will owe its motion partly to the original, and partly to the auxiliary force; the former always the same, the latter varying from many causes. Therefore the less the latter is in respect of the former, the less in proportion will the fum of both, or the whole force on the regulator, be liable to vary.

And this auxiliary force may be the less, the more the impediments to the motion of the regulator can be diminished, or wholly taken away, especially those that are the moft uncertain. In a clock the force of gravity on the pendulum, is far fuperior to the force of the wheels on the pendulum, which is by no means able to raise it fenfibly from its place of reft, if the clock should stand till. Hence the great improvement made by Huygens, in applying pendulums to clocks. It is the superior power of the balance-spring upon the balance abore the power of the main spring, (through the wheels) that makes the balance in like manner a regulator of time. Hence the great improvement of Hooke, in applying a spiral spring to the balancet. This is what Mr. Harrison calls metaphorically the dominion of the balance spring, and the dominion of the

of Mr. B. Mr. H. also used cross joints in the manner of gimbals in the fufpenfion of these three machines. The lesser time-piece fcarried to the West Indies) only lay on a cushion in a square box. The box when the vessel lay down much was set horizontal by hand.

* We have here taken the liberty to borrow both the sentiments and language of a little pamphlet, intitled, “ Thoughts on the Means of improving Watches, by Thomas Mudge.”- In this hors tract, the true principles upon which all time keepers must be constructed are clearly laid down, and the means of improvement fully pointed out.

+ Mr. B. attributes the invention of the balance spring to Haygens : Hooke was undoubtedly the first inventor. See Ward's Lives of the Gresham professors, page 180.

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