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HOROLOGY. 



HOROLOGY. 



720 



part of the cylinder which has the least portion of its circumference 

 taken away, when a tooth is in the cylinder, the point rubs against 

 the internal surface until the balance by its vibration gets into such a 

 situation that the inclined plane can act upon its edge. It then 

 impels the cylinder in the direction from D to A ; until the highest 

 part of the plane escapes from the inside of the cylinder, and the next 

 tooth falls upon the outside. This tooth continues to rub until the 

 balance completes its vibration and has returned so far as to permit 

 the point of the tooth, which has been rubbing on the outside of the 

 cylinder, to get upon its edge, where it gives impulse to the cylinder ; 

 and when its heel escapes, the point falls on the inside of the cylinder, 

 and the former process is repeated. 1, 2, 3, &c., are teeth of the 

 horizontal or scape-wheel, one of which is seen inside the cylinder ; 

 the dotted lines represent the face or inclined plane of the tooth, 

 which is just coming in contact with the edge of the cylinder. 

 The direction of the motion of the wheel is from 1 to 3 ; the 

 proportion of the cylinder to the wheel is such, that a tooth of the 

 wheel, when in the cylinder, may just have sensible shake; and 

 the outside diameter must be sensibly less than the distance between 

 two teeth. 



The detached aeapement, such as is used in a modern chronometer, 

 is shown in, fig. 19. A A A is the scape- wheel, made either of brass or 



Fig. 19. 



*| 



Detached Escapement. 



steel, the teeth 1, 2, 3, 4, &c., of which are considerably undercut on 

 the face. The steel-roller or main-pallet BBS, which is fixed on the 

 arbor of the balance, has an opening in it, the face of which is also 

 much undercut as shown near B, and has set in it a piece of hard stone, 

 such ait a ruby, for the points of the teeth to act upon. 8 is a stud 

 firmly fixed to one of the plates to which the detent-spring E E is 

 secured by a screw c. This spring is made extremely slender and weak 

 in the part E near the stud ; and it is mily by the yielding of this thin 

 part of the detent-spring that any motion can be given to the detent 

 for the purpose of unlocking the wheel ; so that some part of this 

 spring may be considered as the centre of motion of the detent. D 

 is a stud also fixed to the plate of the watch, into which is inserted a 

 screw </, against the head of which the detent rests, o is a ruby pin 

 inserted in the detent, pointing downwards from the detent ; so that 

 one of the teeth of the wheel which is supposed to pass under 

 the detent may rest on the pin ; and in this state the wheel is said 

 to be locked. To the inner side of the detent is attached a very 

 delicate spring, called the lifting-spring, which rests upon and extends 

 a little beyond the end of the detent. Concentric with the main 

 pallet, and just above it, is a small lifting-pallet q, which should 

 be flat on its face or lifting-side, and rounded off on the other 

 side. In the position given in the figure, the lifting-pallet 7 is just 

 coming with its face in contact with the lifting-spring p; which 

 in t>e course of vibration it lifts, and with it the detent (on whose 

 point the lifting-spring presses), so as to raise the pin o clear of the 

 wheel-tooth fi. By the time the wheel is free from the ruby-pin, 

 the main-pallet has advanced so far as to be ready to receive an impulse 

 from the tooth 1 ; and before the tooth escapes the lifting-pallet </, 

 parts with the spring />, and the detent resumes its place on the head 

 of the screw d. In this position the ruby-pin receives the point of 

 tooth G, as soon as tooth 1 has escaped from the ruby -face of the main- 

 pallet B B B. The balance, having performed this vibration by the 

 impulse given to the main-pallet, returns by the force of the balance- 



spring, and with it the lifting-pallet q. The rounded side of the latter 

 pressing against the lifting-spring p, raises it from the detent, and 

 passes without disturbing the detent, which is not again lifted till the 

 balance has completed the present vibration, and returning for the 

 next. In so doing it again brings the face of the lifting-pallet in 

 contact with the lifting-spring, which (with the detent) it raises, and 

 the act of escaping again takes place ; the balance making two vibra- 

 tions for every impulse, as in the duplex. This escapement, which 

 was invented by Earnshaw, is one of the best for simplicity and for 

 performance. 



The name repeatbig-imtch, or repeater, is applied to a watch which, 

 in addition to showing the time on a dial, is supplied with mechan- 

 ism by putting which in action the wearer is enabled to ascertain the 

 tune within certain limits. We have shown, in describing an eight- 

 day clock, how the number of blows given by the hammer to the 

 bell is mode to correspond with the hour denoted by the hands of the 

 dial ; and also that, by pulling a string, the clock will at any time re- 

 peat the hour last struck. But .this will not be the case where the 

 minute hand has approached within about ten minutes of twelve o'clock, 

 for from that time till the hand comes to twelve the clock is OH the 

 warning, and is in such a position that it cannot strike at all. This 

 defect is remedied in repeater*. Most repeaters are watches which are 

 capable of striking on a bell or spring the hours and quarters ; but 

 there are others which also strike the minutes, and these by way of 

 distinction are called minute repeaters. In a repeater, besides the going- 

 train and the motion-work, there is an additional train of wheels 

 between the frame-plates, called the runners or little wheel-work, or 

 sometimes the repeating-train. This train serves the purpose of regu- 

 lating the rapidity with which the successive blows shall be given to 

 the bell, and consists generally of five wheels and five pinions. The 

 last pinion in the train, performing the office of a fly-wheel, is generally 

 called the fly-pinion ; and, when the striking is regulated to its ordinary 

 rate, makes about two hundred revolutions to every blow of the 

 hammer. The chief use of these intricate pieces of mechanism is to 

 furnish the means of knowing the hour of the night in the dark. 



All the more delicaU pivots of chronometers, and of the better 

 kind of watches, work in jewelled holes, which will be found described 

 under JEWELLING. 



Pendulum Clocks. We now arrive at the consideration of those 

 horological machines which receive their regulating adjustment by 

 means of the pendulum. 



The sensible equality of the oscillations of a weight suspended by a 

 string or wire was first applied as a regulator to a clock by Huyghens 

 about 1657. The successive improvements in the escapement, which 

 sustains the motion of the pendulum and records its vibrations, and 

 those in the pendulum itself, which secure a perfect equality in the 

 duration of each oscillation, have finally produced the astronomical 

 clock, the most accurate machine which man has hitherto constructed, 

 and one of the most essential instruments in a modern observatory. 

 We shall suppose that the dead-beat, or Graham's escapement, is that 

 adopted. The pallets PQ, fig. 11, have motion on an arbor which 

 passes thi ough p, and has its pivots resting in holes in the clock-frame. 

 A slender bar or wire, called the crutch, is attached to this arbor, and 

 a notched piece projecting outwards and backwards from the crutch 

 clasps the rod of the pendulum. The pendulum is hung from a cock 

 at the back of the frame, and moves with the crutch. In a well-made 

 clock, the error arising from expansion from temperature is the most con- 

 siderable, and is that which must be guarded against. Before explain- 

 ing more accurate and costly contrivances, it will be well to point out 

 one recommended by Mr. Francis Baily. (' Mem. Astron. Soc.,' vol. i., 

 p. 881.) Take a cylinder of lead about 14 inches long, and pierced 

 through its axis, as a bead, with a hole large enough to admit freely 

 the rod of a wooden pendulum. This hollow cylinder rests on a nut, 

 which works on a screw in the continuation of the rod below. The 

 rod itself, from the centre of motion to the nut, will be about 46'0 

 inches. As it is easier to cut the cylinder shorter than to lengthen it, 

 and as the expansion of the spring is not allowed for, and that of the 

 wood is somewhat uncertain, it will be better to make the leaden 

 cylinder an inch longer for a first trial ; but even if the pendulum 

 should turn out to be under compensated, an additional ring of load 

 may be added, above or below, of the thickness required. 



To the best clocks it is usual to apply either the gridiron pendulum 

 of Harrison (which was once chiefly used in England, and is still in 

 repute abroad), or the mercurial pendulum of Graham. The annexed 

 figure (Jig. 20) is not exactly the pendulum as arranged by Harrison , 

 but accords with his principle. The steel rods 1 and 5 are pinned into 

 two brass cross-pieces, \a, 06. The zinc rods 2 and 4 are pinned below 

 into B6, and carry a cross-piece above, into which the steel rod 3 is 

 pinned. Rod 3 passes freely through a round hole in B6 (this is shown 

 by dotted lines), and is tapped into a screw below ; the bob rests upon 

 the nut, which works on the screw. The steel rods 1 and 5 expand 

 downwards, the zinc rods 2 and 4 expand upwards, and the steel rod 3 

 downwards ; and it is possible so to adjust their lengths (the expansion 

 of zinc being more thin double that of steel) that the effects of the 

 expansion downwards ami upwards shall have no eft'eet on the length 

 of the pendulum or time of oscillation. Harrison used brass instr.id 

 of zinc for the upward expansion ; and in order to produce a perfect 

 compensation, was forced to use four more rods, a second pair of brass 



