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Bible Encyclopedias
Watch
1911 Encyclopedia Britannica
or (in 0. Eng. woecce, a keeping guard or watching, from wacian, to guard, watch, wacan, to wake), a portable timepiece. This is the most common meaning of the word in its substantival form, and is the subject of the present article. The word, by derivation, means that which keeps watchful or wakeful observation or attention over anything, and hence is used of a person or number of persons whose duty it is to protect anything by vigilance, a guard or sentry; it is thus the term for the body of persons who patrolled the streets, called the hours, and performed the duties of the modern police. The application of the term to a period of time is due to the military division of the night by the Greeks and Romans into "watches" (OvXaxai, vigiliae), marked by the change of sentries; similarly, on shipboard, time is also reckoned by "watches," and the crew is divided into two portions, the starboard and port watches, taking duty alternately.' The transference of the word to that which marks the changing hours is easy.
' In the British navy the twelve hours of the night are divided into three watches of four hours - from eight to twelve the first watch, from twelve to four the middle watch, and from four to eight the morning watch. The twelve hours of the day are divided into four watches, two of four hours - eight to midday, midday to four P.M. - and two of two hours, from four to six and six to eight. These are the "dog watches," and their purpose is to change the turn of the watches every twenty-four hours, so that the men who watch from eight to midnight on one night, shall watch from midnight till 4 AIM. on the next. The "watch bill" is the list of the men appointed to the watch, who are mustered by the officers. Time was originally kept by an hour-glass, every half-hour; the number of the half-hour The invention of portable timepieces dates from the end of the 15th century, and the earliest manufacture of them was in Germany. They were originally small clocks with mainsprings enclosed in boxes; sometimes they were of a globular form and were often called "Nuremberg eggs." Being too large for the pocket they were frequently hung from the girdle. The difficulty with these early watches was the inequality of action of the mainspring. An attempt to remedy this was provided by a contrivance called the stack-freed, which was little more than a sort of rude auxiliary spring. The problem was solved about the years 15251J40 by the invention of the fusee. By this contrivance the mainspring is made to turn a barrel on which is wound a piece of catgut, which in the latter part of the 16th century was replaced by a chain. The other end of the catgut band is wound upon a spiral drum, so contrived that as the spring runs down and becomes weaker the leverage on the axis of the spiral increases, and thus gives a stronger impulse to the works (fig.
In early watches the escapement was the same as in early clocks, namely, a crown wheel and pallets with a balance ending in small weights. Such an escapement was, of course, very imperfect, for since the angular force acting on the balance does not vary with the displacement, the time of oscillation varies with the arc, and this again varies with every variation of the driving force. An immense improvement was therefore effected when the hair-spring was added to the balance, which was replaced by a wheel. This was done about the end of the 17th century. During the 18th century a series of escapements were invented to replace the old crown wheel, ending in the chronometer escapement, and though great improvements in detail have since been made, yet the watch, even as it is to-day, may be called an 18th-century invention.
The watches of the 16th century were usually enclosed in cases ornamented with the beautiful art of that period. Sometimes the case was fashioned like a skull, and the watches were made in the form of octagonal jewels, crosses, purses, little books, dogs, sea-shells, &c., in almost every instance being finely engraved. Queen Elizabeth was very fond of receiving presents, and, as she was also fond of clocks, a number of the gifts presented to her took the form of jewelled watches.
The man to whom watch-making owes perhaps most was Thomas Tompion (1639-1713), who invented the first dead-beat escapement for watches (fig. 2). It consisted of a balance-wheel mounted on an axis of semi-cylindrical form with a notch in it, and a projecting stud. When the teeth of the scape-wheel came against the cylindrical part of the axis they were held from going forward, but when the motion of the axis was reversed, the teeth slipped past the notch and struck the projection, thus giving an impulse. This escapement was afterwards developed by George Graham (1673-1751) into the horizontal cylindrical escapement and into the well-known dead-beat escapement for clocks.
The development of escapements in the 18th century greatly is shown by striking the watch bell, hanging on a beam of the forecastle, or by the mainmast, with the clapper. One stroke is given for each half-hour. Thus 12.30 A.M. is one bell in the middle watch, and 3 A.M. is six bells. The bell was also used to indicate the course of a ship in a fog. A vessel on the starboard tack tolled the bell, a vessel on the port tack beat a drum. The watch guns were fired when setting the watch in the evening and relieving it in the morning. The gun is now only fired at sundown.
FIG. I.
improved watches. But a defect still remained, namely, the influence of temperature upon the hair-spring of the balancewheel. Many attempts were made to provide a remedy. John Harrison proposed a curb, so arranged that alterations of temperature caused unequal expansion in two pieces of metal, and thus actuated an arm which moved and mechanically altered the length of the hair-spring, thus compensating the effect of its altered elasticity. But the best solution of the problem was ultimately proposed by Pierre le Roy (1717-1785) and perfected by Thomas Earnshaw (1749-1829). This was to diminish the inertia of the balance-wheel in proportion to the increase of temperature, by means of the unequal expansion of the metals composing the rim.
Invention in watches was greatly stimulated by the need of a good timepiece for finding longitudes at sea, and many successive rewards were offered by the government for watches which would keep accurate time and yet be able to bear the rocking motion of a ship. The difficulty ended by the invention of the chronometer, which was so perfected towards the early part of the 19th century as to have even now undergone but little change of form. In fact the only great triumph of later years has been the invention of watch-making machinery, whereby the price is so lowered that an excellent watch (in a brass case) can now be purchased for about £2 and a really accurate time-keeper for about £18.
A modern watch consists of a case and framework containing the four essential parts of every timepiece, namely, a mainspring and apparatus for winding it up, a train of wheels with hands and a face, an escapement and a balance-wheel and hair-spring. We shall describe these in order.
The Mainspring
As has been said, the mainspring of an oldfashioned watch was provided with a drum and fusee so as to equalize its action on the train. An arrangement was provided to prevent overwinding, consisting of a hook which when the chain was nearly wound up was pushed aside so as to engage a pin, and thus prevent further winding (see fig. I). Another arrangement for watches without a fusee, called a Geneva stop, consists of a wheel with one tooth affixed to the barrel arbour, working into another with only four or five teeth. This allows the barrel arbour only to be turned round four or five times.
The "going-barrel," which is fitted to most modern watches, contains no fusee, but the spring is delicately made to diminish in size from one end to the other, and it is wound up for only a few turns, so that the force derived from it does not vary very substantially. The unevenness of drive is in modern watches sought to be counteracted by the construction of the escapement and balance-wheel.
Watches used formerly to be wound with a separate key. They are now wound by a key permanently fixed to the case. The depression of a small knob gears the winding key with the hands so as to enable them to be set. With this contrivance watches are well protected against the entry of dust and damp.
Watch Escapements
The escapements that have come into practical use are - (I) the old vertical escapement, now disused; (2) the lever, very much the most common in English watches; (3) the horizontal or cylinder, which is equally common in foreign watches, though it was of English invention; (4) the duplex, which used to be more in fashion for first-rate watches than it is now; and (5) the detached or chronometer escapement, so called because it is always used in marine chronometers.
The vertical escapement is simply the original clock escapement adapted to the position of the wheels in a watch and the balance, in the manner exhibited in fig. 3. As it requires considerable thickness in the watch, is inferior in going to all the others and is no cheaper than the level escapement can now be made, it has gone out of use.
The lever escapement, as it is now universally made, was brought into use late in the 18th century by Thomas Mudge. Fig. 4 shows its action. The position of the lever with reference to the pallets is immaterial in principle, and is only a question of convenience in the arrangement; but it is generally such as we have given it. The principle is the same as in the dead-beat clock escapement, with the advantage that there is no friction on the dead faces of the pallets beyond what is necessary for locking. The reason why this friction cannot be avoided with a pendulum is that its arc of vibration is so small that the requisite depth of intersection cannot be got between the two circles described by the end S of the lever and any pin in the pendulum which would work into it; whereas, in a watch, the pin P, which is set in a cylinder on the verge of the balance, does not generally slip out of the nick in the end of the lever until the balance has got 15° past its middle position. The pallets are undercut a little, as it is called, i.e. the dead faces are so sloped as to give a little recoil the wrong way, or slightly to resist the unlocking, because otherwise there would be a risk that a shake of the watch would let a tooth escape while the pin is disengaged from the lever. There is also a further provision added for safety. In the cylinder which carries the impulse pin P there is a notch just in front of P, into which the other pin S on the lever fits as they pass; but when the notch has got past the cylinder it would prevent the lever from returning, because the safety-pin S cannot pass except through the notch, which is only in the position for letting it pass at the same time that the impulse-pin is engaged in the lever. The pallets in a lever escapement (except bad and cheap ones) are always jewelled, and the scape-wheel is of brass. FIG. 4. The staff of the lever also has jewelled pivot holes in expensive watches, and the scape-wheel has in all good ones. The holes for the balance-pivots are now always jewelled. The scape-wheel in this and most of the watch escapements generally beats five times in a second, in large chronometers four times; and the wheel next to the scape-wheel carries the seconds-hand.
Fig. 5 is a plan of the horizontal or cylinder escapement, cutting through the cylinder, which is on the verge of the balance, at the level of the tops of the teeth of the escape-wheel; for the triangular pieces A, B are not flat projections in the same plane as the teeth, but are raised on short stems above the plane of the wheel; and still more of the cylinder than the portion shown at ACD is cut away where the wheel itself has to pass. The author of this escapement was G. Graham, and it resembles his dead escapements in clocks in principle more than the lever escapement does, though much less in appearance, because in this escapement there is the dead friction of the teeth against the cylinder, first on the outside, as here represented, and then on the inside, as shown by the dotted lines, during the whole vibration of the balance, except that portion which belongs to the impulse. The impulse is given by the oblique outside edges Aa, Bb of the teeth against the edges A, D of the cylinder alternately. The portion of the cylinder which is cut away at the point of action is about 30° less than the semicircle. The cylinder itself is made either of steel or ruby, and, from the small quantity of it which is left at the level of the wheel, it is very delicate; and, probably this has been the main reason why, although it is an English invention, it has been most entirely abandoned by the English watchmakers in favour of the lever, which was originally a French invention, though very much improved by Mudge, for before his invention the lever had a rack or portion of a toothed wheel on its end, working into a pinion on the balance verge, and consequently it was affected by the dead friction, and that of this wheel and pinion besides. This used to be called the rack lever, and Mudge's the detached lever; but, the rack lever being now quite obsolete, the word "detached" has become confined to the chronometer, to which it is more appropriate, as will be seen presently. The Swiss watches have almost universally the horizontal escapement. It is found that - for some reason which is apparently unknown, as the rule certainly does not hold in cases seemingly analogous - a steel scape-wheel acts better in this escapement than a brass one, although in some other cases steel upon steel, or even upon a ruby, very soon throws off a film of rust, unless they are kept well oiled, while brass and steel, or stone, will act with scarcely any oil at all, and in some cases with none.
