729 



HOROLOGY. 



HOROLOGY, 



730 



external faces of the tower in which it is mounted. This apparently 

 difficult matter is accomplished in a simple and beautiful manner : by 

 placing the clock in or near the centre of an apartment either on a 

 level with the external faces, or above or below them, and causing the 

 motion of the minute-hand axis to be transmitted by bevil-gear to a 

 vertical rod, the opposite end of which carries a horizontal bevil-wheel 

 nearly on a level with, and situated centrically with reference to, the 

 four external dials. The motion of this central wheel is communicated 

 by four vertical bevil-wheels of the same size and number of teeth, 

 ranged round its circumference, to four horizontal rods, the opposite 

 ends of which, passing through the several dials, carry the four minute- 

 hands. At the back of each dial is a series of wheels and pinions, eou- 

 stituting the motion- work ; while the movement of the hands and that 

 of the striking apparatus are provided for by separate trains of wheel- 

 work, each of which is impelled by its own moving power. In a turret- 

 clock, the moving power is supplied by the descent of a weight, 

 regulated in the case of the movement, or going-train, by the 

 oscillations of a large pendulum, and in that of the striking-train by 

 the resistance of the air to the rapid revolutions of a fly or fan set in 

 motion by the wheel-work. The weights are wound up (in most cases, 

 weekly) by means of winch-handles and toothed wheels connected with 

 the massive drums round which their ropes are coiled ; and, for conve- 

 nience, they do not descend immediately from the drums or barrels, 

 but in the angles of the tower, or any convenient situation, the course 

 of their ropes being directed by guide-pulleys. Fiy. 22 may serve KB 



Striking mechanism of Turret-clocks. 



an example of the striking mechanism of turret-clocks in general, 

 although the details of course vary according to the relative situation 

 of the clock and the bell, which in some cases is the reverse of that 

 here represented. In this cut, a represents the pin-wheel, by the 

 action of the projecting pins of which upon the end of the lever 6, 

 communicated through the levers c and e, the tail of the hammer is 

 depressed, and the hammer-head is consequently raised ready for a 

 stroke. By the continued revolution of a, the end of the lever 6, after 

 being raised to a considerable height, is suddenly released, by which 

 the hammer falls upon the rim of the bell, and the connecting appa- 

 ratus resumes its original position ready for the next stroke. 



Musical chimes, which form a pleasing though not very common 

 addition to the mechanism of turret-clocks, require the addition of 

 another train of mechanism, somewhat like that which constitutes 

 the striking-train ; inasmuch as it is perfectly at rest for considerable 

 periods of time, and is brought into action only at certain pre- 

 determined intervals by the action of the going-train of the clock 

 upon a detent. The mechanism of the chimes very nearly resembles, 

 on a large scale, that of a musical snuff-box ; levers connected with 

 hammers which strike upon a series of bells, being substituted for the 

 springs which in the musical snuff-box are caused to vibrate by the 

 projecting pins on the revolving barreE 



Owing to the very limited demand for turret-clocks, and their great 

 durability when well made and carefully preserved, the business of 

 making them is confined to very few establishments, and has hardly 

 ben systematise!! into a manufacture. Every clock being, in ordinary 

 cases, made individually, and with comparatively little aid from 

 machinery, turret-clocks have been very expensive, and in many cases 

 inferior in accuracy of workmanship to many far simpler, cheaper, and 

 more common machines. The late Mr. Dent, when engaged about the 

 year 1843 by the Gresham Committee to make a turret-clock, of unpre- 

 "jd perfection, for the new Royal Exchange, under the superin- 

 tendence of Mr. Airy, the astronomer-royal, determined to meet this 

 deficiency by establishing a clock-factory supplied with all the aids and 



appliances of modern ingenuity, in which the several parts of a turret- 

 clock should be produced as far as possible in the same way as the 

 component parts of a power-loom or other machine manufactured upon 

 an extensive scale. In the Exchange clock, he adopted the use of a 

 simple but strong cast-iron framing, in which every strain is so com- 

 pletely self-contained, that the operation of fixing the clock in its 

 destined position is one requiring but little skill, scarcely any adjust- 

 ment being required beyond the fixing of the frame on a firm and 

 level base. Another, and a more unusual feature, which Mr. Dent 

 (borrowing from the French) has introduced into the turret-clock 

 manufacture, although it is not adopted in the Exchange clock, is the 

 use of cast-iron wheels for the striking-train. After many experiments, 

 Mr. Dent came to the conclusion to use for the wheel-teeth (the 

 driver) the epicycloidal curve, and for the pinion (the driven) the 

 hypocycloidal curve, putting nearly the whole of the curve on the 

 wheel-teeth. He also applied this theory to the lifting of the 

 hammers, both for the striking apparatus and the chimes, by using 

 projections of an epicycloidal shape instead of the ordinary round pins 

 in the pin-wheel. Clock-wheel cutters had heretofore paid very little 

 attention to the geometry of the wheel-teeth. Among the other 

 important features of the Royal Exchange clock, which are applicable 

 to all others of similar character, whether constructed with its peculiar 

 contrivances for insuring perfect accuracy or not, We may mention 

 the use of hollow iron drums instead of wooden cylinders for the 

 driving-barrels, and the use of wire instead of hempen ropes for 

 suspending the weights. Another important arrangement is the 

 driving of the hands of the clock, and the raising of the hammers 

 of the striking apparatus, directly from the axis of the driving-barrel, 

 without the intervention of any wheels and pinions. In their deter- 

 mination to secure a public clock of unexampled accuracy, the Greshani 

 Committee required that the Exchange clock should have a compen- 

 sation-pendulum, and that it should be so constructed as not only to 

 show perfectly correct time upon the dials, but also to tell it with 

 accuracy by making the first stroke of the hour upon the bell true to a 

 second. This difficult work is provided for by an arrangement for 

 moving the lever and hammer to nearly the utmost degree required 

 before the time of striking ; and causing the end of the lever, which is 

 formed in a peculiar manner for the purpose, to remain poised deli- 

 cately upon the rounded point of the projecting tooth of the phi-wheel 

 until the moment 'of striking, when it is instantaneously released. 

 The pendulum is of a comparatively simple construction, which appears 

 well adapted for large clocks. The centre rod is of steel, and is suffi- 

 ciently long to pass completely through the bob or weight, which, 

 however, is not immediately attached to it. Upon the bottom of this 

 rod is fixed a nut, by turning which the length of the pendulum may 

 be nicely adjusted, and upon this stands a hollow or tubular column 

 of zinc, through which the steel rod passes freely. On the top of the 

 zinc column is a metal cap, from projecting portions of which descend 

 two slender steel rods ; to the lower ends of these rod* the weight, 

 which is a hollow cylinder of iron, capable of sliding freely upon the 

 zinc column, is suspended. Thus, while both the central steel rod and 

 the two smaller steel rods by which the weight is suspended, expand 

 duienmtrdu upon any increase of heat, the position of the weight in 

 reference to the point of suspension of the pendulum remains nearly 

 the same ; because the zinc column, though shorter than the central 

 steel rod, expands, owing to the different nature of the metal, to an 

 equal extent upwards, and consequently raises the weight just as 

 much as it is depressed by the lengthening of the steel rod. The 

 delicate setting or adjustment of the pendulum was effected by a con- 

 trivance suggested by Mr. Airy. The escapement is of the remuntoire 

 kind. The impulses imparted to the pendulum are not given 

 immediately from the large going-train of the clock ; seeing that 

 this impulse is exposed to variations of force and resistance. They 

 are given by a small secondary train, set in motion by the descent 

 of a ball or weight, which is itself raised at intervals of twenty seconds 

 by the mechanism of the going-train. The action is therefore very 

 similar to that of a remontiiire-tpring ; which, as used in some horo- 

 logical machines, is a small spring employed only to set the escape- 

 ment in motion, it being itself wound up at very short intervals by the 

 going-train, which receives its impulse from the prime mover. The 

 escapement is Graham's dead-beat escapement, and has the pallet 

 jewelled with large sapphires. In this clock has been introduced a 

 beautiful maintaining-jiower, or contrivance for maintaining the motion 

 of the wheels during the tune of winding up ; it was invented a few 

 years earlier by Mr. Airy for the clock-work of the great Northumber- 

 land telescope at the university of Cambridge. In )iy. 23, a represents 

 the first wheel of the clock, which is mounted, as usual, upon 

 the axis of the rope-barrel b ; with a ratchet and click so arranged 

 that the two must turn together whenever the rope-barrel is turned, 

 by the action of the weight w, through the line /, in the direction 

 indicated by the arrow. When the rope-barrel is turned in the 

 opposite direction, to wind up the weight, by the action of a windlass 

 on the axis of the wheel d, which engages the toothed wheel e on 

 the axis of the barrel, the wheel a will not turn back with the 

 barrel : / is the pinion which is turned immediately by connection 

 with the first wheel a ; and both this and the winding- wheel, or pinion, 

 d, have their axes mounted in the plates of the clock-frame. The axis 

 of -the barrel and first wheel a, instead of being thus mounted, ia 



