causes the barrel to revolve in a contrary direction to that in which it moved whilst winding up, and thereby gives motion to the fusee, and with it the fusee-wheel and the rest of the train. Fig. 14 represents the barrel and fusee, with the chain attached. Fig. 15 shows the balance-wheel, balance, and verge, with the hair-spring attached to it.

the lever l. When the balance is quiescent, the pin o is in the notch in the end of the lever l, and the guard-pin e in the position shown in the figure, where the tooth 1 acts on the pallet b, which causes the balance to vibrate. The guard-pin e proceeds a short distance to the right of its present position, and the lever is prevented from returning by the Fig. 17.

Fig. 14.

Fig. 13.

Fig. 15.

Details of Vertical Watch.

We shall now notice the better kinds of watches, the peculiarities of which mainly depend on the escapement.

The Duplex Escapement is shown in fig. 16. AA is the scape-wheel,

Fig. 16.

C

Duplex Escapement.

1, 2, 3, being the teeth of repose, and a, b, c the teeth of impulse, which are triangular, and stand perpendicular to the plane or surface the wheel. C D, the impulse pallet, fixed upon the arbor of the balance, and standing just above the surface of the wheel a A, receives its motion from the teeth a, b, c, &c. After the tooth a has passed the pallet c D, the tooth b comes in contact with a small roller made of ruby, and placed on the lower part of the axis of the balance, where it remains till the balance is brought back by the balance-spring to such a position that the notch, shown by the dotted line in the ruby roller, will allow the tooth 1 to enter it, and thereby pass the balancearbor, or escape, which it does by the wheel a A being constantly urged in the direction from 3 to 1. As soon as tooth 1 escapes from the notch, tooth b gives a fresh impulse to the pallet CD, and the act of escapement is thus repeated; the wheel moving forward one whole tooth, and the balance making two vibrations for each impulse given by the upright teeth.

Another effective variety, the lever escapement, is shown in fig. 17. The lever is placed on the pallets in a position at right angles to that in which it is usually placed in a watch, by which means we think the principle will be more apparent to the general reader. A A is the scapewheel moving in the direction of the arrow; bd the pallets, whose centre of motion is c. To the pallets is pinned the lever 1, in which is the guard-pin e, pointing upwards from the lever ; the roller ƒ is fixed on the axis of the balance, and stands just above the lever l, having a piece cut off from its circumference to allow the guard-pin e to pass and repass the roller, which it does when the escape takes place. o is a ruby pin fixed in the roller, and pointing downwards through the notch in the end of

Lever Escapement.

guard-pin e coming in contact with the circular edge of the roller. When an impulse is given by a tooth to the other pallet d, the lever impels the ruby pin o to the left hand, where precisely the same effects take place with regard to the guard-pin e, &c., as have been already described. If the pallets b and d were of the form shown by the dotted lines (which are supposed to be circular arcs concentric to the centre of motion c of the pallets), it is evident it would be a perfect dead beat, like the clock escapement previously described; but in order, after the escape has taken place, that the guard-pin e may be retained at a small distance from the roller, that part of each pallet on which the tooth rests when it falls on the pallet is taken off, as shown in the figure; and as the faces of the wheel-teeth are considerably undercut, the wheel advances a small distance, after having fallen on that part of either of the pallets which is within the dotted line. This further advance of the wheel draws the pallet down towards the centre of the wheel, and thereby keeps the guard-pin e at a slight distance from the edge of the roller f. As soon as the balance has performed so much of the returning vibration as to bring the ruby pin o into the notch in the lever, the momentum of the balance, acting through the medium of the ruby pin o upon the lever, moves it a short distance, and thereby lifts the pallet outwards from the centre of the wheel and unlocks it. During this unlocking the wheel retrogrades (before it can get upon the face of the pallet to give a fresh impulse) just as much as it had previously advanced after falling on the pallet. By this retrograde motion the tooth gains the inclined plane or face of the pallet, and gives a new impulse; and the same process is repeated by another tooth on the opposite pallet. pp are two pins, called banking-pins, against which the lever presses when locked, and which prevent the guard-pin e from being drawn too far away from the edge of the roller f, when the locking takes place. In fig. 18 is shown a horizontal escapement. A B C represents the Fig. 18. B

A

Horizontal Escapement.

balance on its axis, which is a hollow cylinder c, cut away in its circumference. The teeth of the escape-wheel form a series of inclined planes, which stand on stems perpendicular to the plane of the wheel; he inclined part forming the extreme edge or acting-face of the tooth. These planes coming in contact alternately with the two edges of that

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 The detached escapement, such as is used in a modern chronometer, is shown in fig. 19. AAA is the scape-wheel, made either of brass or Fig. 19.

two teeth.

steel, the teeth 1, 2, 3, 4, &c., of which are considerably undercut on the face. The steel-roller or main-pallet BBB, 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 as a ruby, for the points of the teeth to act upon. s is a stud firmly fixed to one of the plates to which the detent-spring EE is secured by a screw c. This spring is made extremely slender and weak in the part E near the stud; and it is only 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 d, 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 9, 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 q is just coming with its face in contact with the lifting-spring p; which in the 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 5. 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 q, parts with the spring p, and the detent resumes its place on the head of the screw d. In this position the ruby-pin receives the point of tooth 6, as soon as tooth 1 has escaped from the ruby-face of the main pallet BBB. 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 vibrations 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 repeating-watch, or repeater, is applied to a watch which, in addition to showing the time on a dial, is supplied with mechanism by putting which in action the wearer is enabled to ascertain the time within certain limits. We have shown, in describing an eightday clock, how the number of blows given by the hammer to the bell is made 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 repeat 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 on the warning, and is in such a position that it cannot strike at all. This defect is remedied in repeaters. 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 goingtrain 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 regulating 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 delicate 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 through 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 considerable, and is that which must be guarded against. Before explaining more accurate and costly contrivances, it will be well to point out one recommended by Mr. Francis Baily. (Mem. Astron. Soc.,' vol. i., p. 381.) 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 lead 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 (fig. 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, aa, Bb. The zinc rods 2 and 4 are pinned below into Bb, and carry a cross-piece above, into which the steel rod 3 is pinned. Rod passes freely through a round hole in Bb (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 than double that of steel) that the effects of the expansion downwards and upwards shall have no effect on the length of the pendulum or time of oscillation. Harrison used brass instead 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

Gridiron Pendulum.

less workmanship, and only one nice fitting. One or two flat brass horizontal bands are attached to 1 and 5 to keep the zinc rods in their places.

In Graham's pendulum, a glass jar, partly filled with mercury, is supported in a sort of steel stirrup. The pendulum rod passes through the top of the stirrup, and is held by a nut and adjusting screw at D, fig. 21. The height of the mercury in the jar is about 6.7 inches; but this will vary somewhat with the diameter of the jar, the substance of the rod and frame, and perhaps the variable expansion of the steel rod. The compensation can be altered, and finally perfected, by the astronomer.

Mr. Dent has recently made many improvements in the mercurial pendulum. One consists in the use of a cast iron instead of glass cistern for the mercury; another in the attachment of the cistern directly to the pendulum-rod; a third, in the prolongation of the rod nearly to the bottom of the cistern; and a fourth, in giving impulse to the pendulum at or nearly at the centre of percussion. If an escapement could be contrived which gave its impulse to the pendulum at the middle point of its vibration, and was wholly detached from it at all other times, such an escapement would be perfect; and escapements are almost to be considered good or bad as they approach this character. Huyghens proposed a most ingenious contrivance, namely, that the upper part of the pendulum, which he made of two parallel strings, should wrap and unwrap on two cheeks, which being shaped as cycloids, caused the bob itself to describe a cycloid. Now it is a property of this curve that all arcs are described in the same time, so that Huyghens's construction was perfect in principle; it had, however, many difficulties in practice. Instead of suspending the pendulum by a perfectly flexible string, or on a knife-edge, when the motion must be in a circle, the top of the rod may be made to end in a flat spring, which has certain advantages. Mr. Frodsham has found that a spring of a particular strength rendered the oscillations of a pendulum of a certain weight isochronous, and that a considerable alteration in the length of the spring did not affect this quality of isochronism. This may perhaps be explained by supposing the lower part of the spring not to have acted when it was longest, but to have always preserved its rectilinear form. There is an investigation of the effect of atmospheric pressure on the rate of a transit clock at the Armagh observatory, in the 'Mem. Ast. Soc., vol. v., p. 125. The author, Dr. Robinson, assumes that the variations of a clock from a constant rate are expressed by the sum of two terms, one depending on the temperature, the other on the pressure of the atmosphere shown by the barometer. The isochronism of the spring is supposed, or that the effect of any change in the arc depending on the above two causes is already expressed in the terms. When the

Mercurial Pendulum.

the effect of time is generally to produce a falling off in the arc, a small addition to the clock weight might be made from time to time, so as to bring back the pendulum to its primitive arc, until the clock is cleaned, and its action restored that way. Where the clock is much exposed to variations of temperature, it should be enclosed in a second covering or closet.

To bring a clock to time, first make it nearly right by the adjusting screw D, but let it have a losing rate, which must be determined by observation after the interval of one or more days. Suppose it is losing 3 a day. Put a weight, which has been carefully ascertained, say 200 grains, upon the plate which covers the jar (Ee in the mercurial pendulum, in the gridiron pendulum anywhere near Bb), and find the fresh rate of the clock by observation. Let it now gain 10 a day. Then, as 200 grains cause a gain of 13 a day, 15'4 grains will alter it 1s per day, and, replacing the 200 grains with a weight of 46-2 grains, will bring the clock to time. The final adjustment of the compensation can be best accomplished when the clock has gone several months, and when the gain or loss in two of the warmest months in the year is compared with the gain or loss in two of the coldest.

For the mathematical and physical principles which govern the action of the pendulum, we refer to PENDULUM.

An astronomical clock, such as those used in observatories, is the best example of a pendulum clock, comprising all the most refined means of adjusting the isochronous movements of the pendulum to the measurement of small intervals of time. Turret or church clocks, however, are commercially of more importance, and of these we must now say something.

Turret-clocks differ from other machines employed for measuring time, not only in their greatly superior size, but also in the arrangement of their parts, and in the circumstance that they are usually make to strike the hours, and often the quarters also, upon large bells. They are also occasionally connected with machinery for chiming whole tunes at certain intervals upon a set of bells which, when mounted in a church tower, are so hung that, by disconnecting the hammers of the chimes and striking apparatus, they may also be rung in the ordinary manner by means of ropes. One of the peculiarities of a turret-clock consists in the circumstance that it is frequently required to indicate the time upon as many as four different dials, on the four

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 minutehands. At the back of each dial is a series of wheels and pinions, constituting the motion-work; while the movement of the hands and that of the striking apparatus are provided for by separate trains of wheelwork, each of which is impelled by its own moving power. In a turretclock, 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 convenience, 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. Fig. 22 may serve as Fig. 22.

d

C

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 b, 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 b, after being raised to a considerable height, is suddenly released, by which the hammer falls upon the rim of the bell, and the connecting apparatus 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 predetermined 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 barrel.

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 been systematised 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 unprecedented perfection, for the new Royal Exchange, under the superintendence 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 turretclock 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 completely self-contained, that the operation of fixing the clock in its destined position is one requiring but little skill, scarcely any adjustment 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 determination to secure a public clock of unexampled accuracy, the Gresham Committee required that the Exchange clock should have a compensation-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 delicately upon the rounded point of the projecting tooth of the pin-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 sufficiently 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 rods 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 downwards 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 contrivance suggested by Mr. Airy. The escapement is of the remontoire 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 remontoire-spring; which, as used in some horological machines, is a small spring employed only to set the escapement 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-power, or contrivance for maintaining the motion of the wheels during the time of winding up; it was invented a few years earlier by Mr. Airy for the clock-work of the great Northumberland telescope at the university of Cambridge. In fig. 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 7, 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: ƒ 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, is

plates at h. To the end i the end of the line k, l, is attached, after passing under a running pulley attached to the weight w. c is an internal ratchet on the first wheel a, acted upon by the long click m, which has its opposite end attached to the lever-frame near its extremity i. While the clock is going in the ordinary way, the descent of w causes that part of the line marked to turn the barrel in the direction of the arrow, carrying with it the first wheel a, the internal ratchet of which slips under, without being affected by, the click m. Under these circumstances the action of the weight w (through the line ), and the resistance of the pinion f, produce a certain pressure on the lever-frame at g; this causes the end i to assume a determinate position, in which it remains without motion so long as the weight continues to descend, and consequently to draw down the line 7. But so soon as, by the operation of winding up the clock, the pressure upon / ceases to operate, the stress of the weight upon the portion of the line marked causes the end i of the lever-frame to be depressed, and the click m, which is connected with it, to be thrust against the internal ratchet c with sufficient force to maintain the action of the first-wheel a, which turns as it were in one piece with the lever-frame round the axis h; thereby producing a pressure upon the pinion f exactly corresponding, if the axis h corresponds with the point at which the strain of the line l is applied to the rope-barrel, to the pressure which is exerted during the ordinary action of the machine.

There is one turret clock which has acquired much notoriety, on account of its size and cost, and of the extraordinary amount of controversy to which it has given rise during the long period of sixteen years; namely, the clock for the new Houses of Parliament at Westminster. In 1844 Sir Charles Barry applied to Mr. Vulliamy for plans and estimates for a clock that would strike the hours on a bell of eight to ten tons, chime the quarters upon eight smaller bells, and show the time on four dials of 30 feet diameter each. How it arose that the clock was made by Mr. Dent instead of Mr. Vulliamy: that a non-professional man, Mr. E. B. Denison, became intimately mixed up with the subject: that the plans underwent numerous modifications: and that it was not till 1859 that the clock was actually fixed in its place-it would be wearisome to tell. It cannot be said that even now (March, 1860) the clock is really finished, owing to the unfortunate failure of the great bell (for which see BELL). The chief features of the mechanism are as follow. The going part is arranged for winding-up once a week; but the striking apparatus goes for seven and a half days, to allow for slight delays; and even a delay of one whole day, though it would stop the striking, would not stop the going, the latter being arranged for eight and a half days. The ponderous weights hang down a shaft 160 feet deep. Schemes have been proposed for employing both steam power and water power to wind up the clock; but at present manual power only is employed, and a most laborious duty it is. The pendulum is 15 feet long, and weighs 680 pounds; it is formed of an exterior iron tube 4 inches in diameter, with a compensating inner tube of zinc. It is a two-seconds' pendulum, with a swing or vibrating arc of 14 inches; a small weight of only one ounce, placed on a particular part of the apparatus, will alter the rate of the clock one second per day. The hammers of all the bells [BELL] are (or rather, are intended to be) worked by their levers or handles catching against cams on the edges of wheels; the great or hour-bell being worked by eighteen cams on a wheel 37 inches in diameter. On

the four sides of the clock-tower are dial rooms, each an apartment of large size, traversed by mechanism which communicates motion from the clock to the hands. There is at Mechlin a larger clock dial than those at Westminster; but there is no second example of a clock with four dials 22 feet in diameter, and provided with minute hands as well as hour hands. The frame-work of each dial, weighing no less than 4 tons, and including the quarter-hour and minute subdivisions, is of cast-iron. The hour figures are 2 feet high and 6 feet apart; and the minute marks are 14 inches apart. There is a remontoire apparatus, to give a visible motion of the long hand every half minute, when the point of the hand makes a sudden leap of 7 inches. The hands weigh more than 2 hundredweight the pair; the minute hand being 16 feet long and the hour hand 9 feet. In order that (when all is completed) the indications of the clock may be visible at night, the framework of each dial is fitted in with opal or enamelled glass; provision is made for sixty gas jets behind the dial; and the clock itself will turn on and off the gas, according to the length of night at different seasons of the year.

Electrical and Illuminated Clocks.-So numerous have been the novelties in the clock and watch manufacture within the last few years, that the bare enumeration of the names of inventors would occupy a considerable space. We need only, however, notice electrical and

illuminated clocks.

The hopes once entertained concerning electrical clocks have scarcely been realised. These ingenious machines are not yet largely employed in our great centres of business. There are two kinds, which may be distinguished as electrical dials and electrical clocks proper. An electrical dial is a clock-dial, without any body belonging to it. There is a standard clock at some other place, such as the Observatory at Greenwich; there is an electric wire connecting the standard clock with the index hands of the dial; and there is apparatus for sending a galvanic current through the wire at certain equidistant intervals of time. The result of this arrangement is, that the dial-hands make a leap over a small portion of their circular course, whenever a current passes through the wire; and the figures marked on the dial give a time-value to this movement. The interval of the shocks may be a minute, half-minute, or any other amount chosen. It is quite possible to make the hands mark seconds' movements, to correspond with the pendulum-beats of a standard-clock; but in practice it is found better that the long hand should only make half-minute jumps. An electric clock, in the proper sense of the term, is one that carries its source of power with it, independent of any conducting wire from another building. Various modes of effecting this have been devised by Shepherd, Dent, Airy, and others. In the first attempts, electricity was employed to impel the pendulum itself; but it was afterwards found better to apply the power in raising a small weight, which may then work the pendulum after the manner of a gravity escapement. In Shepherd's Magnetic Striking Clock, the vibrations of the pendulum are caused by the repeated impulses of a fine spring: the attraction of an electro-magnet being employed solely to relieve the pendulum from the action of the spring during the return or reflex vibration. The hands are moved by separate electromagnets, the circuit of the wires being completed and broken by the pendulum as it swings. The number to be struck is regulated by a locking-plate divided in the usual way, the hammer being moved by the direct action of an electro-magnet. Electrical clocks or electrical dials are used in astronomical observatories, and by telegraph companies, but very little in other quarters.

Intimately allied with this subject is the action of Electric Time-balls. These are contrivances for showing exact time once a day only, and in a manner to be visible throughout a whole district. The interval may be more or less than one whole day; but this is selected as being most convenient. A large ball is seen to fall at one o'clock in the day; and this fall may safely be taken, by mariners and others, as a guide for correcting chronometers, watches, and clocks. The time-ball at Greenwich Observatory, and that of the Electric Telegraph station in the Strand, may be taken as familiar examples. The Greenwich ball is a basket of wicker-work, covered on the exterior; when it descends, a piston plunges into a tube, compresses the air, and thus, makes a kind of soft cushion which enables the ball to drop without concussion—a small hole in the bottom being left for the air to escape gradually. At a few minutes before one o'clock, the ball is wound up by hand to the top of the staff. At one o'clock, to a single second, the standard clock in the observatory, by means of delicate mechanism, sends a current of electricity which loosens a trigger and lets the ball sink. The timeball in the Strand, of later construction than that at Greenwich, exhibits many improvements in detail. Several others have since been established at the outports.

The rendering of clocks visible at night is one of the many improvements introduced in recent years. The illumination is effected in many ways. At the Horse Guards in Whitehall, light is thrown on the face of a dial from a gas-flame hidden behind a parapet. The more usual method is to make the dial either wholly or chiefly of semi-opaque glass, and to manage the lighting in the way just noticed concerning the (prospective) arrangements for the Westminster clock. Such dials, however, are not so conspicuous and convenient during the day as those of ordinary construction; and it is difficult to maintain the hands in a well-adjusted condition. A suggestion was