extend it considerably towards the country. Being however regarded but as a work of secondary importance in the defence of a place, the length of its front was seldom so great as that of the sides of the polygon on which the fortifications of the enceinte were constructed, and generally did not exceed 240 yards; which, since the relief of its rampart was necessarily nearly the same as that of the enceinte, scarcely allowed the ditch before its curtain to be effectively defended. The lengths of the branches or wings were regulated by the necessity of having the ditch and covered-way in front of the salient angles of the demi-bastions within the range of a fire of musketry from the collateral works towards which the ramparts of the wings were directed; and occasionally the latter were broken, on the plan, so as to form short flanks from whence a fire might be directed towards the nearest of those salient points. That which has been found occasionally useful is too frequently, by an improper application, converted into a positive evil; this was the case with the works now being described; and at a very early period the multiplicity and injudicious disposition of them were subjects of animadversion among the best engineers. It often happened that they were constructed at great expense in situations where no end whatever was to be gained by them, and so close together that the defenders of their branches could not have avoided firing upon one another. In proportion as the means employed in the attack of places were increased the earlier fortresses became incapable of affording room for the buildings necessary to lodge the troops, and contain in security the quantity of artillery and stores which the corresponding augmentation of the means of defence demanded; and hence it was sometimes found necessary to increase the extent of the advanced works about a place. This was done, at first, not by enlarging the dimensions of the half-bastions and curtain at the head of such works, but by making that head to consist of two or more fronts of fortification, in which case they took the name of double, triple, &c., horn-works, but more generally CROWN-WORKS. At a later time however the importance of advanced works was more highly appreciated; and, both by an improved disposition of them and by giving to their fronts dimensions equal to those of the general fronts of the place, they became not only free from the defects to which the old works were subject, but also capable of making a defence equal to that of a regular fortress. The defects of the old horn-works consist in the expense of the construction being greater than is warranted by the benefit to be derived from them in the defence; in presenting to the enemy a front which, from its smallness, may be taken more easily than one of the fronts of the enceinte; in the revêtment of the latter being liable to be breached by a fire of artillery directed along the ditches of their wings from batteries formed on the glacis opposite the salient angles of the work; in the comparative security with which an enemy, after having made a lodgement in the work, might carry on his approaches in the interior in consequence of the protection afforded by the ramparts of the wings against any attempt of the besieged to impede him by sorties directed upon his flanks; and, lastly, in the large place d'armes the ditches afforded in which the assailants might assemble in large numbers unnoticed and in security for an assault. It should be observed, however, that Vauban, who constructed many such works, appears to have entertained a favourable opinion of them. He gives the preference to such as are formed immediately in front of a bastion; the wings being directed neither to that work nor to the collateral ravelins, but towards the curtains adjacent to the bastion. By this means the ditches of those wings are capable of being defended by the artillery of the curtains, while the revêtments of the latter are covered by the tenailles so as to render it impossible to breach them near the foot by a fire of artillery directed along those ditches. But his best application of a horn-work was made at Belfort, where he executed one entirely in advance of the glacis of the place; in consequence of this disposition the revêtment of the enceinte is effectually secured against being breached till after the horn-work is taken; while, at the gorge of the latter, the height of the terreplein above the ground at the foot of the glacis ensures the work itself from being taken by an assault in that direction. A nearly similar disposition was adopted by Cormontaingne iu executing the double crown-work at Metz. Beyond the glacis of that place, on one side, the ground rises with a gentle inclination, till, at some distance from thence, it forms one side of a deep valley; and along the brow are constructed, on nearly a straight line, three strong fronts of fortification. The ground is terminated on the left by an escarpment, which is crowned by a line of rampart with its coveredway and glacis; and on the right is a valley watered by a rivulet, which, being dammed, forms a lake capable of securing the works against an attack on that side. Each flank is further protected by a detached lunette, or redoubt that on the right, being surrounded by water, is nearly inaccessible; and that on the left is strengthened by a system of counter-mines. The ample capacity of the bastions and the direction of their faces, which are such as to prevent them from being enfiladed; the contraction of the ground before the works, by which the enemy would be reduced to the necessity of making his attack on a smaller extent of front than that of the defenders; and finally, the measures taken to secure the flanks, justly entitle this fortification to the character of being the most complete of its kind in Europe. HORNPIPE, a rustic musical instrument, still, we believe, known in Wales, consisting of a wooden tube, with holes, and a reed. At each end is a horn; one to collect the wind blown into it by the performer, the other to augment the sound. The Hon. Daines Barrington states, ('Archæologia, vol. iii., 1770) that "the tone, considering the materials of which the instrument is composed, is really very tolerable, and resembles an indifferent hautbois." In the Welsh language its name is pib-corn, which signifies, literally, pipe-horn. Sir John Hawkins quotes Chaucer to show that the hornpipe was a real, not an imaginary instrument; but in the Tatler,' No. 157, is a proof not only of its reality, but its actual existence so late as 1710. Hornpipe is also the name of a dance; and Barrington is of opinion, that the dance-tunes called Hornpipes were originally composed for the instrument known by the same name. Hawkins says, that the hornpipe was invented in this country. It appears,-from the 'DancingMaster,' 17th edit. 1721,-to have been in triple time, six crotchets in a bar; but the well-known tune, The College Hornpipe, is in duple measure. HOROLO'GIUM (Constellation), the Clock, a southern constellation of Lacaille. It is cut by a line passing through Canopus to the southern part of Eridanus. Its principal stars (of which it is not worth while to make a table) are a and B, 34 of Piazzi and 229 of Lacaille, or 1315 and 956 of the 'B. A. Cat.,' both of the fifth magnitude. HOROLOGY (from the Greek pa, time or hour, and λóyos, a discourse), is an explanation of the principles of the measurement of time; but in its modern sense the term is usually applied as descriptive of that art which comprehends a knowledge of the action of the various machines used for the purpose of measuring time. Sun-dials, which show apparent time, and clepsydra, which give a rude approximation to mean time, were the earliest machines used in the measurement of time. [CLEPSYDRA; SUNDIAL.] We shall in this article only treat of those pieces of mechanism, used for the measurement of time, which are kept in motion either by the constant action of gravity through the medium of a weight, or by the elastic force of a spring, and which have received names varying according to the duties they have to perform. Thus, the term timepiece is applied to any piece which is intended merely to mark the time without striking the hour; a clock is one which, in addition to showing the time, strikes every hour, on a bell or spring, a number of strokes corresponding to the hour of the day or night indicated by the hands at the time; a quarter clock is one which also strikes the quarters as the hand successively arrives at them; a watch is a pocket timepiece; and a repeater is a watch which by means of any mechanical contrivance can at pleasure be made to repeat the hour, or hour and quarters. History of Clocks and Watches.-The early history of clocks and watches is enveloped in so much obscurity, that it would be almost impossible to point out any individual who could with propriety be called the inventor. The term horologium was used very early in different parts of Europe; but this word being formerly applied indiscriminately to a sun-dial, as well as a clock, nothing decisive can be inferred from its use. Striking clocks were known in Italy as early as the latter part of the 13th or beginning of the 14th century. A fine imposed on the chief-justice of the King's Bench in 1288, was applied to the purpose of furnishing a clock for the famous clock-house near Westminster Hall. In the reign of Henry VI., the king gave the keeping of this clock to William Warby, dean of St. Stephen's, together with 6d. per day to be received at the exchequer. St. Mary's at Oxford was furnished with a clock in 1523, out of fines imposed on the students of the university. The middle of the 14th century seems to be the time which affords the first certain evidence of the existence of what would be now called a clock, or regulated horological machine. The first clock at Bologna was fixed up in 1356. Henry de Wyck, a German artist, placed a clock in the tower of the palace of Charles V. about the year 1364. Mention is made in Rymer's Foedera,' of protection being given by Edward III. to three Dutch horologers who were invited from Delft into England in the year 1368. Conradus Dasypodius gives an account of a clock erected at Strasburg about 1370. According to Froissart, Courtray had a clock about the same period, which was taken away by the Duke of Burgundy in 1382. Lehmann informs us that there was a clock at Spire in 1395. Nürnberg had a clock in the year 1462; Auxerre had one in 1483, and Venice in 1497. It appears, from a letter written by Ambrosius Camaldulensis (lib. xv. epis. 4) to Nicolaus of Florence, that clocks were not very uncommon in private families on the Continent about the end of the 15th century; and there is good reason for supposing that they began to become general in England about the same period. The conclusion to be drawn from the evidence here adduced is, that a regulated horological machine is neither of so ancient a date as some writers suppose, nor yet the more recent invention of the last two centuries; and that the inventor is not certainly known. Ferdinand Berthoud was of opinion that a clock, such as that of Henry de Wyck, is not the invention of one man, but a compound of successive inventions, each worthy of a separate contriver. This supposition is confirmed by analogy; for the clocks and watches of the present day have been brought to their present degree of perfection by a series of successive inventions and improvements upon what may now be called the rude clock of De Wyck, the most ancient clock of which we have a two small levers FG, called pallets, which project from and form part of an upright staff or spindle c D, on which is fixed the balance A B; and the mode of adjusting the clock to time was by shifting the two weights w w nearer to or farther from the centre. of power is derived from a coiled spring, and not from a pendulum. Chronometers are in extensive use for determining the longitude at sea, and for other purposes where an accurate measure of time is required, combined with great portability in the instrument. The general appearance of what is termed a pocket chronometer is that of a common watch; and it is generally made to go the same time with once winding up-namely, thirty hours. Those used for nautical purposes are larger, having dial-plates from three to four inches in diameter, and are usually made to go from two to eight days between the times of winding up; they have, in addition to the hour, minute, and second circles, one on which a hand denotes the time in days that the piece has been going since the last winding up. Each chronometer is well secured in a brass box, mounted on gimbals, in order that the machine may preserve one uniform position, and is inclosed in a mahogany case. A chronometer has for its moving power a main-spring, the variable force of which is equalised or rendered uniform by the introduction of the fusee, a very beautiful contrivance. This fusee is a variable lever, upon which the main-spring acts through the medium of the chain. It is a mathematical curve which has this peculiar property, that as the chain winds upon it, the distance from the centre of motion of the fusee to the semidiameter of the chain which is in contact with it continually varies. The variation is in this proportion,-namely, that the distance from the centre of motion of the fusee to the semidiameter of the chain at that point where it leaves the fusee for the barrel, multiplied by the force of the main-spring acting on the chain at that time, shall be a constant quantity; that is, shall be the same whatever point of the fusee may be taken. Thus, at any given distance from the centre of motion of the fusee, its power to turn any machinery nicates motion to all the rest, is attached to the fusee, their centres of motion coinciding with each other, it follows that the power at the teeth of the main wheel is perfectly uniform: this power is transmitted through the medium of a train of wheels and pinions till it comes to the escapement. Although this clock of De Wyck's, and indeed all those made with a balance for the regulator, without any regulation spring, must have been very imperfect machines, we find that so early as 1484 Walther, and after him the landgrave of Hesse, made use of a balance-clock for astronomical observations. Such, indeed, seems to have been the conparative utility of the clock thus early for astronomical purposes, that Gemma Frisius proposed the use of a portable one for ascertaining the longitude at sea about the year 1530. In 1560 Tycho Brahé possessed four clocks, which indicated hours, minutes, and seconds; the largest had but three wheels, the diameter of one of them being 3 feet, and containing 1200 teeth, a proof of the imperfect state of clock-is uniformly the same; and as the great or main wheel, which commuwork at that period. Tycho also observed irregularities in his clocks dependent upon changes in the atmosphere, but does not appear to have been aware how they were produced. In 1577 Moestlin had a clock which made 2528 beats in an hour, and by counting the number of beats made during the time of the sun's passage over a meridian, the sun's diameter was determined to be 34' 13". At what time the size of clocks was reduced to a state of portability is uncertain, but it must have been prior to 1544; for in that year the corporation of master clock-makers at Paris obtained from Francis I. a statute in their favour, forbidding any one who was not an admitted master to make clocks, watches, or alarms, large or small. Before portable clocks could be made, the substitution of the main-spring for a weight, as the moving power, must have taken place; and this may be considered a second era in horology, from which may be dated the application of the fusee; for these inventions completely altered the form and principles of horological machines. Such was the state of clock-work when Galileo observed that heavy bodies, suspended by strings of the same length, made their vibrations, whether in long or short arcs, in very nearly, if not exactly, the same spaces of time. Although he never applied the pendulum as a regulator to supersede the balance in clocks, yet his discovery was the prelude to a third era in clock-work, namely, the origin of the pendulum clock. The honour of first applying the pendulum to a clock has been a matter of much contention. Huyghens, whether the inventor or not, undoubtedly applied it in the more masterly and scientific manner, and hence has generally been considered the inventor; but it is now known that a London artist, named Richard Harris, invented and made a long-pendulum clock in 1641. Very soon after the application of the pendulum to clocks, the idea of Gemma Frisius was attempted to be realised by the ingenious Huyghens in the construction of a marine clock. He also discovered that its pendulum vibrated slower as it approached the equator, which has led the way to a subsequent discovery that the earth is not a globe, but an oblate spheroid. In 1676, Barlow, a London clockmaker, invented the repeating mechanism | by which the hour last struck may be known by pulling a string. Several artists followed in the same line, particularly Quare, Julien le Roy, Collier, Larçay, and Thiout. Clocks were soon after this made to show not only mean but apparent time. The principal artists employed in this more curious than useful part of horology were Sully, Father Alexander, Le Bon, Le Roy, Kriegseissen, Enderlin, L'Admiraud, Passement, Rivar, and Graham. The anchor escapement was the work of Clement, a London clockmaker, in 1680. This change in the escapement introduced the practice of suspending the pendulum by a thin and flexible spring. The seconds' pendulum, with this escapement, was called the royal pendulum. Another era in the history of clock-work commenced with the beginning of the 18th century. In 1715 George Graham sought for a means of rectifying the errors of the pendulum, caused by the contraction and expansion of metals under changes of temperature; and this means he found in the celebrated mercurial pendulum. John Harrison improved on Graham's arrangement of the pendulum; and Graham himself afterwards introduced his dead-beat escapement, as an improvement on the anchor or recoil escapement previously in use. From the days of Graham and Harrison successive improvements have been introduced in every part of the art. Such of those as are of primary importance will be noticed as we proceed. Chronometers. A chronometer, an eight-day spring clock, a time. piece, and a pocket watch have this in common,-that their source A chronometer differs from a common watch in the escapement, and in having a compensation for heat and cold. The peculiar mode of effecting this compensation consists in having what is technically termed an expansion balance. The figs. 2 and 3 represent each a Fig. 2. Fig. 3. C balance, some being made with weights, ww, as in fig. 2; others being found, it is not desirable to alter it in bringing the machine to time. To effect an adjustment, the two screws c c have been introduced, the drawing out of which from the centre causes the machine to lose, and the screwing them in to gain. Many important matters relating to chronometers apply in a smaller degree to spring clocks and to pocket-watches, and will receive notice as we go on. But it is necessary here to say a few words concerning the competitive trials of chronometers, which have partly resulted from, and have partly suggested the beautiful inventions perfected within the last few years by Eiffe, Frodsham, Loseby, Dent, and others. Between the years 1822 and 1835, the Admiralty gave rewards as prizes for the good performance of individual chronometers: two or three sums being given annually to the makers of those which showed the smallest amount of error. Nothing was given for any new principle; but the general construction was improved by this competition. The relative order of merit was arrived at by taking the number of seconds in the greatest difference between one week's indications and the next, and adding to it twice the greatest difference between one week's rate and the next; the sum was the trial number; and that chronometer gained the prize which exhibited the smallest trial number. The trials were conducted at Greenwich Observatory, under the Astronomer Royal. After the year 1835 the prizes were withdrawn; but the makers were equally ready to compete, on the score of reputation and commercial advantage. The test was rendered more severe; for every chronometer was exposed in the open air to the fiercest cold of winter; and at other times to the air of a chamber heated to 100° Fahr. In successive years different makers obtained the post of honour; for instance Poole in 1845, Hutton in 1846, Frodsham in 1847, Hewett in 1848, Eiffe in 1849. In the five years here indicated, 219 chronometers were tested at Greenwich; of which 79 were bought for public use, at a maximum price of 62. During the course of these refined experiments, a fact was observed which amounts almost to a new discovery. However perfect may be the compensation of a chronometer for certain temperatures, it was found that it did not remain constant for all temperatures. If a chronometer be planned to resist great cold and great heat, it gains at medium temperatures; but if specially adapted for the latter, it errs slightly in very cold and very hot weather. The late Mr. Dent explained this fact by supposing that the inertia of a spring balance, as usually constructed, cannot be made to vary uniformly according to temperature, but will vary more rapidly in cold weather than in hot. Chronometer-makers at once set about devising a remedy; and this remedy, however produced, is called the secondary compensation. Loseby, Dent, Eiffe, Molyneux, and other makers, have invented highly-curious arrangements of spring balance, all of which have been severely tested at Greenwich, and some of them rewarded by government grants. Loseby’s arrangement, which received high commendation from the Astronomer Royal, may be thus briefly described. Attached to the balance is a curved tube containing mercury. The mercury, on expanding with an increase of temperature, arrives at certain parts of the tube inclined in different degrees to the radii of the balance; and therefore its successive expansions produce successive effects of different magnitudes on the momentum of the balance. By giving different forms to the tube, the law of the successive alteration of the momentum may be made to adapt itself to the law of alteration of the elasticity of the spring, whatever that law may be. Spring Clocks.-In describing spring-clocks, we shall at the same time treat of many parts of the mechanism which are applicable likewise to chronometers, but not described in the foregoing paragraphs. In fig. 4, A B C D may be taken as representing the front plate of an eight-day spring clock (which is supposed to be transparent), and is attached to another plate of similar form by five strong pillars between which the wheels here shown are placed. EE are two barrels containing springs; the one on the right gives motion to the train of wheels called the going or watch train; the other to the striking train of wheels e, f, g, h, and fly i. In producing the series of movements, a is the main wheel of 96 teeth, acting in the centre wheel-pinion k of eight leaves, to which is attached the centre wheel b; this revolves in an hour, and acts in the third wheel-pinion 1, on which is fixed the third wheel c, acting in the swing or scape wheel-pinion m (not seen in the cut); to this pinion is fixed the swing-wheel d, whose teeth act alternately on the two pallets n o, and thereby give motion to the pendulum by means of a piece attached to the arbor of the pallets, one end of which enters a slit made in the pendulum for its reception. FF are the two fusees, the use of which has already been described in connection with chronometers. The method in which the fusees are attached to their respective wheels a and e is shown in fig. 6, where is the main wheel of 96 teeth hollowed out to receive the click b and its spring c which are attached to the wheel, the ratchet d being attached to the under side of the fusee by two screws. In fig. 4, e is the striking main wheel, having 84 teeth; this drives the pinion p of eight leaves, on which is the pin-wheel ƒ of 64 teeth, into the rim of which are put eight pins to lift the hammer s, by acting upon its tail t. The pin-wheel ƒ drives the pallet-pinion q of eight leaves, on which is fixed the pallet-wheel g of 56 teeth; this pallet wheel acts in the warning wheel-pinion of seven leaves, on which is the warning-wheel h of 48 or 50 teeth, acting in the fly-pinion i. When in action a pin in the pin-wheel catches the tail of the hammer t, and raising it, the hammer-head s recedes from through the front plate about one and a half inches, is placed the minute wheel a (fig. 5), which revolves with the centre wheel in an hour, and carries the minute-hand of the clock. This wheel has a pipe nearly as long as the centre-wheel arbor, the upper end of which is squared to receive the minute hand; and by means of a small spring beneath the wheel, which rests upon a shoulder just above the upper surface of the front plate, and acts against the upper surface of the wheel, the wheel, together with the hand, is forced against a pin over Fig. 8. A Fig. 9. Forcing-spring. nism of the clock, or rather that part of it which is called the going or watch train; for the minute-wheel a gives motion to another minutewheel b, which, as it must revolve also in an hour, has the same number A pinion in the centre of the wheel b has six leaves, of teeth as a. and acts in the hour-wheel c of 72 teeth, which is placed over the minute-wheel a, and consequently revolves once in 12 hours, and has screwed to its socket, at the upper end, the hour-hand. To the socket of the hour-wheel, about one-eighth of an inch above the wheel, is fixed a piece in which are 12 steps, each of which includes an angle of 30°, or a twelfth part of a circle; this piece is called the snail, and is represented by d. e is a rack whose centre of motion is a stud or pin f, on which it acts by means of a pipe about half an inch long, and on to the upper end of which is riveted the rack-tail g; in this rack-tail is a short pin h, pointing perpendicularly downwards to the front plate of the clock. The rack lies about the tenth of an inch above the front plate; but the pipe which acts on the stud is long enough to carry the rack-tail just clear of the snail when the rack is forced back by the spring i; whilst the pin h is long enough to strike against the steps in the snail, and yet so short as to be perfectly free of the hour-wheel c. Near this is k, the rack-hook moving freely on a stud; m the lifting piece, also moving freely on a stud p; n the tail of the lifting-piece firmly pinned on to the upper part, and moving with it; o the gathering pallet, which has a square hole through it, and is fixed upon the square end of the arbor q of the wheel g (fig. 4) which revolves once for every blow given by the hammer. A pin in the warning-wheel h always stands in the same position when the striking part is at rest, which is the position represented in fig. 4. On the end of the lifting-piece is a small piece q (fig. 5), which passes through a slit in the front plate, and resting on the bottom of the slit, keeps the lifting piece in its proper position. The gathering pallet o rests on a pin r in the rack, and thereby prevents any motion in the internal wheel-work of the striking train. Such being the mechanism, the mode of action will be understood from a study of the figures, without a detailed description. A small piece x is called the pull-piece, by pulling a string at the end of which the lifting-piece is raised, and the clock is made to repeat the hour last struck at any required time. y is a spring to force the pull-piece x against the pin z fixed in the plate of the clock; s is another pin to limit the motion given to the pull-piece when the string t is pulled. that of A; and in its progress in passing a tooth of the wheel the small Endless Cord. A, and also in slight spring s. When clocks and watches had acquired a certain Fig. 10. 1 2 5 3 Harrison's Maintaining Power-Going Fusee. Fusee auxiliary ratchet, and fusee-wheel attached; 2, fusee-wheel and auxiliary spring, separate; 3, 4, upper and under sides of the fusee, separate; 5, auxiliary ratchet q, with clicks a a, and springs e e, attached. the wheel by a pin at about one-fourth of its circumference from the end a, namely, at b. The wheel has a short notch cut through it, near the other end of the spring. The spring passes over this notch; and by means of a pin c, fixed firmly in the spring and projecting through the notch in the wheel, a motion is allowed to the spring, which in extent is equal to the difference between the length of It is the reaction of this spring through the short the notch in the wheel and the thickness of the pin which passes through it. distance already mentioned which maintains the motion in the watch during the time of winding up. Instead of any click and spring being attached to this fusee-wheel, as in an ordinary eight-days' clock, there is a circular disc of steel, rather larger than the bottom of the fusee, and smaller than the fusee-wheel, having very fine ratchet-teeth cut which the ratchet fixed on the under side of the fusee, and called in its edge, and two clicks a a and springs e e on its upper surface, in the fusee-ratchet, acts. The steel-ratchet is called the auxiliary-ratchet, and its teeth stand in a direction opposed to those of the fusee-ratchet. We will now suppose the auxiliary ratchet to be laid on to the fuseewheel over the spring a b c; a hole in its centre passing over a short pipe in the centre of the fusee-wheel retaining it in its situation; the pin c, which we have described as projecting through the notch in the fusee-wheel, also projecting upwards just equal to the thickness of the auxiliary-ratchet, through which it likewise passes; and the pin exactly fitting the hole in the ratchet. In this situation the wheel and ratchet are ready to receive the fusee with its ratchet; but it must be borne in mind, that though the pin c fits exactly in the hole in the auxiliaryratchet, and thereby prevents it from turning round, it does not prevent its having as much motion as the spring itself has in the notch in the fusee-wheel; the spring must also be conceived to have been forced into its place with the pin pressing strongly against the end of the notch o. The fusee is now attached to the wheel by passing its arbor through the hole in the centre of the wheel, and is secured in its place by a pin and collet on the opposite side, which prevent their separa tion, at the same time allowing the fusee to turn with a moderate degree of force. In this state the fusee, &c., must be considered as placed within the frames of the clock or watch in connection with the other part of the train of wheels, &c. A click, or, as it is sometimes called, a detent, is also placed between the frames, and by means of a slight spring is made to act in the teeth of the auxiliary-ratchet. The action of the whole apparatus will be understood by comparing the relative positions of the several parts. The space through which the spring a b c acts in the notch op with sufficient force to maintain the motion of the watch, is about equal to two teeth of the fusee-wheel; and the time in which the fusee-wheel goes through a distance equal to two teeth varies in different watches from 10 to 12 minutes, a time more than sufficient for the operation of winding. We must next describe the escapement, so important in horology. This term is applied to a combination of parts in a clock or watch, which has for its object the conversion of the circular motion of the wheels into a vibratory motion, as exhibited in the pendulum. In the description we are about to give, the term will be made to include the scape-wheel, the pallets with their arbor or axis, and a bent lever attached thereto, called the crutch, which last piece maintains the motion of the pendulum. In a watch this combination consists of the scape-wheel, together with all those parts lying between it and the balance, and which are concerned in converting the circular motion of the wheels into the alternating one of the balance. In Graham's deadbeat escapement the distance between the centre of motion p of the pallets and the centre of the scape-wheel is equal to one diameter of the scape-wheel. In fig. 11, the tooth i has just given impulse to the pallet P and escaped from it; the tooth o has in consequence fallen upon that part of the pallet Q called its arc of rest, which, in both pallets, is formed by a circle struck from the centre of motion p of the pallets. The impulse given by i causes the pendulum, and with it the pallets, to vibrate some distance after i has left P and o has fallen on Q; but the arc of rest being concentric with the centre of motion of the pallets, the wheel ceases to rotate, or remains dead, until the pendulum by its returning vibration lifts the pallet Q so high as to allow the tooth o to get upon the face or inclined plane of the pallet, upon which it then acts. The tooth drives up the pallet, and with it the pendulum, until Fig. 11. Dead-Beat Escapement. the tooth o escapes from the pallet 9; when another tooth h, on the opposite side of the wheel, falls on the arc of rest of the pallet P, which arc is in this pallet on the outside, and on which the tooth rests until by the return of the pendulum the pallet p is lifted so high as to allow h to get on the inclined plane or face of the pallet P, upon which it acts. The tooth raises the pallet and with it the pendulum, till it escapes and gains the position i, when the same process is repeated: the wheel alternately giving impulse to one pallet and resting on the circular part of the other, which we have denominated the arc of rest. When the pendulum is in a state of rest, some one tooth is always resting on one of the circular arcs; the pendulum being put in motion brings a pallet into a position to receive an impulse from the wheeltooth, when the process already described commences. Most of the great improvements in pocket watches have been made in the horizontal or flat forms; but the common vertical watch must be first described. The annexed cut (fig. 12) represents such a watch as it would appear G M if the dial (which is here omitted) were turned downwards. A is the barrel; B, the fusee; b, the chain by which motion is communicated from the barrel to the fusee, on which is the great or fusee wheel acting on the centre-wheel pinion D, on which is riveted the centrewheel: the arbor of the pinion D being prolonged through the plate of the watch as far as l. The centre-wheel and its pinion revolve in an hour. Upon that part of the arbor D which is on the outside of the plate or frame is placed the cannon-pinion, which has a hole quite through it for the reception of the centre-wheel arbor, on which it turns spring-tight. The cannon-pinion is secured in its place by a small pin through the end of the centre-wheel arbor 1, the end of the pinion being squared to receive the minute-hand h. The cannon-pinion has 12 leaves acting in the minute-wheel, of 48 teeth, causing the latter to revolve once in four hours. Concentric with the minute-wheel, and attached to it, is a pinion, having a hole through their common centre, through which passes a stud fixed on the plate. This pinion, having 14 leaves, drives the hour-wheel m, of 42 teeth, once round in 12 hours; this wheel is placed over the cannon-pinion by a socket, which has a hole through it for the cannon-pinion to pass through; on this socket is fixed the hour-hand k. It will be perceived that by this arrangement the cannon-pinion, minute-wheel, pinion, and hour-wheel, together with the hands, can all be turned backward or forward without affecting the interior mechanism of the watch, simply by the application of a key to the squared end of the cannon-pinion. The assemblage of wheels, &c., thus put in motion is called the motionwork of the watch; that between the plates, the movement, which we shall next describe. The centre-wheel gives motion to the third ARTS AND SCI. DIV. VOL. IV. h I L wheel-pinion, to which is attached the third wheel a, acting upon the contrate-wheel pinion. On this is placed the contrate-wheel I, acting in the pinion of the balance-wheel L, which is also called the scape-wheel. We have already explained the mode by which the balancewheel teeth act upon the pallets, so as to cause an alternating motion in the balance M. One end of the balance-wheel arbor works in a piece called the dovetail, which is inserted in a piece p, called the potence, firmly attached by a screw to one of the plates of the watch; the other end works in a piece called the follower, which is inserted in another piece riveted into the plate called the counter-potence, (left out of the figure to prevent confusion). Another part of the potence, called the foot, receives one end of the balance arbor or spindle, called the verge (on which are the pallets); the other end works in a hole in the pin o, which passes through the centre of the cock qq, which is secured to the upper plate of the watch; the pendulum-spring (also called the regulating-spring and hair-spring) has one end, immediately below the balance, secured to a stud fixed in the plate, and the other pinned fast to a small collet, which goes spring-tight unto the axis of the verge, and is seen just under the balance. Figs. 13, 14, and 15, represent some of the parts separately. Fig. 13 shows the main-spring in a relaxed state as it would appear out of the barrel; to which, when in, one end of it is attached, the other being held by a hook in the arbor of the barrel, which comes through the plate, as shown in fig. 12, and is kept from turning by a ratchet and click. The spring is wound up by the chain acting on the barrel and pulling it round, which operation is performed by turning a key placed on the squared end of the fuseearbor. The effort of the spring to unbend itself after being wound up 3 A |