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is too little towards the end of the arch, and requires an addition to give it that force which produces ifochronous vibrations. We will not affirm thefe are Mr. Harrison's fentiments in the cafe of pendulums, but from fome expreffions in the very concife account of his principles, we are inclined to think fo. The cafe of balances moved by a fpiral spring is very different 1.

Mr. B. recounts fome particular efcapements. He mentions fift the dead escapement of Mr. Graham, (L'é happement à repos) which has the effential condition he requires of leaving the pendulum at liberty at the end of the vibration; but he dillikes it on account of friction, and for other reafons.

The next efcapement is that of Mr. Cumming §; which Mr. B. fays, in fpight of the feeming advantages, had fufficient friction and difficulties in the execution, to deter him from making use of it.

Mr. B then mentions Mr. Harrifon's efcapement, made ufe of in his three first machines, and by which, as Mr. B. fays, the whole force of the efcapement wheel is communicated to

one.

The efcapement of Mr. H.'s laft time-piece is a very unusual The crown wheel and its teeth are of the common form, only the ftraight fide of the tooth is quite upright. As foon as one tooth has escaped, the upright face of the oppofite tooth meets the edge of the other palet. The balance at that time has paffed its point of reft, 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 hump-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 efcapements 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 fame as in the very common crown wheel efcapement.

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

the

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

The laft efcapement he mentions, and which he prefers to all others, is that in which the pendulum receives a pufh, or rather a blow in the middle of its vibration, and is then left to itfelf for the remaining part of that vibration, and alfo for the fubfequent returning vibration, fo that this impulse is given only every other vibration. Between one impulfe and the next, the escapement wheel is locked up by a detent, and is unlocked by the pendulum juft before the next impulfe. The penduJum being thus at liberty to fwing out at the end of each vibrasion, he calls in the efcapement of free vibrations. Notwithftanding a long encomium upon this, Mr. B. fays 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 fteel made perfectly hard; but laments greatly the indifpenfable neceffity of ufing oil in fuch 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 feveral centers of mo tion are placed. Upon the placing of these centers it likewife depends, whether the force of the efcapement wheel on the regulator fhall be uniform, increafing or decreasing during the time of the action of the wheel on the palets.

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

Mr. H. in his three firft machines, endeavoured to avoid the neceffity of ufing oil, by leffening as much as poffible the relative motion of thofe parts which touched each other. In the action of the balance wheel on the palets the relative motion of the tooth and palet was wholly taken away, and the palet was made of wood. In his laft time-keeper (fent to the West Indies) the ufe of oil was abfolutely neceffary, but no peculiar provifion made against its effects. Indeed all the contrivances to procure ifochronous vibrations, even though the force fhould vary, may be confidered 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 conftant going for a long while, and the nature of the efcapement fuch as occafioned a great relative motion of the tooth and palet, yet there was not in 1765 the leaft trace or mark of the palet having worn the tooth,-it was not to be expected the tooth fhould make any impreffion on the palet.

pieces.

pieces. His objections are, that a fpring is liable to break, that it changes its force by heat and cold, that it is apt to fet, and that the coils fhould 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 fpring to the escapement wheel, to give to that wheel its force. When this fpring has unrolled itself a little way, it is then to be wound up by the force of the main-fpring. This little fpring in Mr. Harrison's time piece is wound up eight times in a minute.

Our Author, has at the end of this chapter, a fhort article on the proportion the moving force of the clock ought to have to that of the regulator. We did expect he would have confidered this at the beginning of his theory. What he says in this place is loofe, and confufed; he here, and in many other places, ufes the words force and motion as fynonimous terms, and ftanding for precifely the fame idea.

The laft chapter in this part, is on the fufpenfion of marine clocks, that is, the manner of fupporting or carrying them at fea. He lays down two principles, one that the clock fhould be fo fufpended, as to keep its position always horizontal, without partaking of the motion of the ship, the other that the suf penfion fhould have (with this freedom in yielding to the motion of the fhip) great firmness and folidity.-Such a folidity that the balance may not fet in motion the cafe by which the time-keeper is defended from the external air. Our Author recommends Cardan's method of fufpenfion §; he would have the clock kept in a sort of closet to defend it from the fea air, and placed as near as conveniently may be to the center of gravity of the fhip. He alfo ufes fpiral fprings to break the force of fudden fhocks.

We

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

§ Cardan's method of fufpenfion is that of the fea-compafs, called by feamen Gimbols; it is the fame with the mechanism of the rolling lamp in Defagulier's Lectures, page 57.-This contrivance feems to be the parent of the univerfal joint; one of the most useful inventions in mechanics. The univerfal joint is largely treated of by Hooke in one of his lectures at Gresham College, who reprefents it as his own invention, but, nevertheless, miftakes a little in giving an account of its very fingular properties. Hooke's lecture is printed in 1678, but the first account of it is in Schottus's Technica Curiofa, p. 664, printed in 1664. Schottus feems to have picked it up in Germany; which, abounding in metals, has ever been the nurfery of the mechanic arts, especially while the Hans towns retained their independency.

Mr. Harrison in his three great machines ufed fpiral fprings to break the fhocks, 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, fhould be very great in comparison of that which is only employed to restore so much of the motion as is loft by friction, &c. This original force in a pendulum, is that of gravity; in a balance, that of the spiral fpring applied to it. The original forces are conftant and invariable, and by them only would the vibrations of the regulator be preferved the fame, were it not for the fmall impediments of friction, &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-fpring in a watch, acting upon the regulator through the intervention of the wheel-work; the wheel-work at the fame time ferving 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 fame, the latter varying from many causes. Therefore the lefs the latter is in refpect 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 lefs, the more the impediments to the motion of the regulator can be diminished, or wholly taken away, especially thofe that are the most 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 fhould ftand till. Hence the great improvement made by Huygens, in applying pendulums to clocks. It is the fuperior power of the balance-fpring upon the balance above 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 fpiral fpring to the balance t. This is what Mr. Harrifon calls metaphorically the dominion of the balance spring, and the dominion of the

of Mr. B. Mr. H. alfo ufed crofs joints in the manner of gimbals in the fufpenfion of these three machines. The leffer time-piece (carried to the Weft Indies) only lay on a cushion in a fquare box. The box when the vessel lay down much was fet horizontal by hand.

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

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

wheels

wheels over the balance. If a clock be ftopped, the pendulum must be raised from reft through a certain arch, before the fwing wheel can efcape, and the clock go again t. In like manner if a watch be ftopped, the balance must be drawn round from its place of reft to a certain degree, before the watch can go again. If the preffure of the main fpring (through the wheels) upon the balance, can fufficiently force the balance from its place of reft (against the effort of the balance spring to bring it back), the main fpring will then be always able to fet the watch a-going if ftopped. But to do this, it is evident the power of the main fpring on the balance, muft equal the power of the balance fpring, at the place of escapement. It follows therefore, if a watch be fo conftructed as to fet itself a-going when stopped, it cannot poffibly be a good time-keeper, because the wheels muft then have fo great a dominion over the balance §. The principles Mr. H. proceeded upon therefore, instead of serving to improve pocket-watches, as fome expected before the difcovery was made; ferve only to fhow that fuch watches are incapable of great improvement. Improvement is incompatible with a very neceffary property in a pocket-watch, that of fetting itself a going when stopped. Nor will Mr. H. allow to fuch a bauble as a pocket-watch, the refpectable appellation of a TIME-KEEPER .

[To be concluded in our Review for August.]

The arch through which the pendulum or balance must be raised from its lowest point, or from the place of reft, that the tooth of the wheel may escape from under the palet, is called the arch of efcapement. by the French arc de levée.

It is faid Mr. H. boafted in the H. of C. that if his time keeper was stopped, it would not fet itfelf a-going again,-to the amaze. ment of the members, who could by no means comprehend the benefit of fuch a property.

"In common watches the wheels have about one-third the dominion over the balance that the balance fpring has, but in this my timekeeper the wheels have only about one-eightieth part of the power over the balance that the balance fpring has; and it must be allowed the lefs the wheels have to do with the balance the better."-Principles of Mr. Harrison's Time-Keeper, page 20.

ART. IX.

Recherches fur les Modifications, &c. Inquiries into the different Modifications of the Atmosphere, &c. By J. A. De Luc. Concluded.

IN

[From the Appendix to our last Volume, Page 579.]

N confequence of the large extracts which we have already made from this elaborate performance, we fhall content ourfelves, in this our concluding account of it, with giving the fubftance of a few mifcellaneous obfervations felected from

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