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CONSTRUCTION OF ROBERTS’ AND DENT’S TURRET CLOCKS.

Class Xn.

ing that of most astronomical clocks. Considering thenumber of persons who make turret-clocks in this country,the smallness of the number of them in the Exhibition israther surprising: and those which deserve any favour-able notice are still fewer. It is remarkable, too, that theonly ones in the English Department which appeared tous to deserve such notice, are two clocks with cast-iron wheels; and they are both of them such as ought toconvince anybody, that the prejudice against cast-ironclock-wheels is altogether unfounded, now that they canbe made so well that the teeth do not even require touch-ing with a file after they come out of the mould. Bothof these clocks also have a remontoire apparatus in thegoing train, the effect of which is, that the escapement isdriven, not by the great clock weight, but by a smallweight, or a spring, wound up at intervals by the train,which is discharged like a striking part, or in some otherway, at every minute or half minute, by the revolutionof the scape-wheel. Most of the turret-clocks in M. Wag-ner's collection (No. 736, p. 1215), which will be noticedpresently, are also on the remontoire principle. Andthere can be no doubt that, since it can be applied at amoderate expense, all turret-clocks of high character, atleast if they have large external dials, ought to be somade, because it renders the escapement independent ofall variations in the force of the train, which are some-times very considerable, and also allows asufficientweightto be applied to the clock to drive the hands in allweathers, without making the pendulum swing too far;and moreover, as the hands of a clock of this kind do notmove an almost invisible quantity at every beat of thependulum, but by a very sensible jump at every lettingoff of the remontoire, the time can be taken from theminute-hand, at a considerable distance, as accurately asfrom the seconds-hand of a regulator; and for the samereason the striking part is also discharged more exactlyat the right time.

As regards the going part, the principal advantage ofcast-iron wheels over brass or gun-metal is their muchgreater cheapness. But in the striking part they havethe further advantage of greater strength, which, inlarge clocks, is the chief thing to be attended to, and thething in which they are most frequently deficient. InMr. Roberts’s clock the striking is done on a new plan,with a very light hammer; but the clock has only asmall hemispherical bell attached to it, and we have noproof that this very ingenious contrivance,in which neitherfly nor hammer-spring is required, would answer for alarge bell, such as that on which Mr. Dent’s clock strikes,or even a much smaller bell of the usual form. In thelatter clock the hammer is raised by broad cams cast onthe great wheel of the striking part, and of such a shape,and with the lever so arranged, that there is the smallestpossible waste of power: in fact, very little more thanone-fourth of the force of the great striking weight islost in friction, the resistance of the air to the fly, and inthe necessary interval between the fall of the hammer andthe beginning of its next rise,—a proportion which ismuch less than usual.

Both Mr. Roberts’s (No. 130, p. 422) and Mr. Dent’s(No. 55, pp. 413, 414) clocks have compensated pen-dulums. The former has a steel and brass compensation,which, Mr. Roberts says, is made in accordance with hisown experiments on the relative expansion of the twometals; though it differs from the result of all theordinary tables of expansion, the length of the brasscompensation-tubes being much less than usual. Mr.Dent’s pendulum has an iron and zinc tube compensation,made according to the usual tables, and agreeing withhis experience of the Royal Exchange clock pendulum,which has a similar one, and has now been sufficientlytested for several years.

Mr. Roberts’s escapement is a new one of his own con-trivance: the pendulum only receives its impulse on aroller at every alternate beat. The remontoire is on theendless chain plan, which is described in Reid’s book onclock-making, except that the mode of letting off isdifferent. The form of the clock-frame is pyramidal,with the pendulum hung from the top ; and it is conse-quently very steady, provided the floor it stands on be

so; though the plan of fixing turret-clocks on beams letinto the wall for the purpose, is, in most instances,steadier than any fixing to a floor can be. All thewheels are made to take out separately, as they shouldalways be, and the Jury were particularly struck withthe goodness of the casting and the form of the wheels.Mr. Roberts is well known as an eminent engine-makerat Manchester, and by the application of his machineryto the manufacturing of these clocks he is able to sellthem at a much lower price than usual. On the whole,without expressing any opinion on the supposed advan-tages of the escapement, or the mode of striking, or someother peculiarities of this clock, we consider it well de-serving of a Prize Medal.

The escapement in Mr. Dent’s clock is the ordinarypin-wheel dead escapement, but the wheel is smaller thanusual, being only four inches in diameter, and it containsforty pius. It is driven by a spiral spring, which ismerely part of a large watch-spring, and is wound up aquarter of a turn after every quarter of a turn of thescape-wheel. The pinion, or small-wlieel to which oneend of the spring is fixed, does not ride on the arbor ofthe scape-wheel, in which case there would be consider-able friction between them, but on a stud fixed into theclock-frame; so that the scape-wheel is driven by thespring without any friction whatever, except that of itsown pivots, and the very small friction which is due tothe upward pressure of the ends of the remontoire fly onthe arbor of the scape-wheel. The consequence is/thatthere cannot possibly he any variation in the force on thependulum, except that which may arise in the escape-ment itself, if the pallets are not properly oiled,—as in allother anchor or pin-wheel escapements. It may be worthwhile to state that the variation of force in the springdue to changes of temperature, if it were ten times asgreat as it is, would produce no sensible effect upon thependulum. And as this pendulum is eight feet long, andweighs above two cwt., such a clock may be reasonablyexpected to go as well as any astronomical clock ; and, infact, it has gone since the opening of the Exhibition, atleast as well as a highly-finished astronomical cloek,which was placed by the side of it, and every week com-pared with the time brought from Greenwich.*

The maintaining power for keeping it going whilewinding is of a new construction : as the common spring-going barrel is difficult to construct with both sufficientplay and sufficient power for large clocks of this kind,which take several minutes to wind up. This maintain-ing power is of the “bolt-aud-shutter ” kind, but not re-quiring any click, and so made that it is impossible tobegin Minding without raising the maintaining weighthigh enough to keep in action for seven or eight miuntes,aud yet it can be thrown out of gear as soon as the wind-ing is done. There is another point in which the arrange-ment is different from usual: the great wheel aud barrelof the going part, and of both the striking parts, are setin the great frame, and the smaller frame, which carriesall the other wheels of the goiug part, can be taken offentire, without disturbing either the great wheel or thependulum. Mr. Dent uses wire ropes, because by thatmeans he obtains the advantage of many turns of thebarrels in the eight days, without the inconvenience oflong barrels, or the baS practice of letting the rope gotwice over them.

A Council Medal has been awarded to Mr. Dent( p. 413) for this clock, on the recommendation of all themembers of this Jury, except the Chairman, who declinedto express an opinion upon it, on account of having him-self furnished the design. But as it contains hardly any-thing which had not been either previously used by Mr.Dent, or suggested in print, and therefore equally opento the adoption of any other maker, he is entitled to fullcredit for it, as well a*s for his enterprise in attempting to

* Since the close of the Exhibition, we have been in-formed that the accumulated error of the large clock, fromthe 8th August to the loth October, was -2-8 seconds; or,in other words, the pendulum lost not quite two beats inthe four millions of vibrations it performed during thatperiod.