396 WATCH Fig. 4. We shall give a short description of the principal classes of inven tions for this purpose. The first that was disclosed was Eiffe s (sometimes called Moly- neux s), communicated to the astronomer-royal in 1835. In one of several methods proposed by him a compensation curb was used ; and, though, for the reasons given before, this will not answer for the primary compensation, it may for the secondary, where the motion required is very much smaller. In another the primary compensation bar, or a screw in it, was made to reach a spring set within it with a small weight attached at some mean temperature, and, as it bent farther in, it carried this secondary compensation weight along with it. The obvious objection to this is that it is discontinuous ; but the whole motion is so small, not more than the thickness of a piece of paper, that this and other compensations 011 the same principle appear to have been on some occasions quite successful. Molyneux took a patent for a secondary compensation exactly the same as this of Eiffe s, then before the astronomer-royal. Another large class of balances, all more or less alike, may be represented by E. J. Dent s, which came next in order of time. He described several forms of his invention ; the following descrip tion applies to the one he thought the best. In fig. 4 the flat crossbar rr is itself a compensation bar which bends upwards under increased heat ; so that, if the weights v, v were merely set upon up right stems rising from the ends of the cross bar, they would approach the axis when that bar bends upwards. But, instead of the stems rising from the crossbar, they rise from the two secondary compensation pieces stu, in the form of staples, which are set on the crossbar ; and, as these secondary pieces themselves also bend upwards, they make the weights approach the axis more rapidly as the heat increases ; and by a proper adjustment of the height of the weights on the stems the moment of inertia of the balance can be made to vary in the proper ratio to the variation of the intensity of the spring. The cylindrical spring stands above the crossbar and between the staples. Fig. 5 represents Loseby s mercurial compensation balance. Besides the weights D, D, set near the end of the primary compen sation bars B, B, there are small bent tubes FE, FE with mercury in them, like a thermometer, the bulbs being at F, F. As the heat increases, not only do the primary weights D, D and the bulbs F, F approach the centre of the bal ance, but some of the mercury is driven along the tube, thus carry ing some more of the weight towards the centre, at a ratio in creasing more rapidly than the temperature. The tubes are sealed at the thin end, with a little air included. The action is here equally continuous with Dent s, and the adjustments for primary and secondary compensation are apparently more independent of each other; and this modification of Le Roy s use of mercury for compensated balances (which does not appear to have answered) is certainly very elegant and ingenious. Nevertheless an analysis of the Greenwich lists for seven years of Mr Loseby s trials proved that the advantage of this method over the others was more theo retical than practical ; Dent s compensation was the most successful of all in three years out of the seven, and Loseby s in only one. Loseby s method has never been adopted by any other chronometer- maker, whereas the principles both of Eiffe s and of Dent s methods have been adopted by several other makers. A few chronometers have been made with glass balance-springs, which have the advantage of requiring very little primary and no secondary compensation, on account of the very small variation in their elasticity, compared with springs of steel or any other metal. Dent also invented a very different method of effecting the primary and secondary compensation at once, and without any additional appendage to the balance or addition to the cost. He called it the prismatic balance, from the shape of the steel rim, of which the section is shown in fig. 6, BG being the brass, and the dark triangle Avithin it the section of the steel part of the rim. A prism of cast-steel will bend more easily from the edge than the other way, and consequently the motion is greater when it is being curved by heat than when it is pulled straighter by cold, which is exactly what is wanted. The difference is not quite so great as it ought to be for complete secondary com pensation for a very wide range of temperature, but it is enough to give the requisite compensation for all ordinary variations of tem- perature, and chronometers so compensated were found to be also more than usually steady in their rate, for even in the best chrono meters there appear every now and then quite capricious variations. Several other forms of secondary compensation have been invented and found successful, all on the same principle of so arranging the compound bars that the weights move more under any given change of temperature in hot weather, or when they are nearest to the axis of the balance, than in cold when they are farthest off. Notices of these may be seen in various volumes of the Horological Journal. The best chronometers, with all these improvements, cannot be relied on to keep a rate equal to that of a good astronomical clock of the usual kind, though they occasionally do so for a short time. Waicli Escapements. The escapements in practical use are (1) the old vertical escapement, now almost disused ; (2) the lover, 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 detacJuid or chronometer escapement, so called because it is always used in marine chronometers. The vertical escapement is simply the original clock escapement (see CLOCKS, fig. 3) adapted to the position of the wheels in a watch and the balance, in the manner exhibited in fig. 7. As it requires considerable thickness in the watch, it is inferior in going to all the others, and no cheaper than the lever escapement can now be made ; and for these reasons it has gone out of use. The lever escapement, as it is now uni versally made, was invented late in the last century by Thomas Mudge. Fig. 8 shows its action. The position of the lever with reference to the pallets is immaterial in prin ciple, 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 escapement clock (see CLOCKS, fig. 5), 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 under cut 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 xinlock- ing, 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 pre vent 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. 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, if nothing else is. 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. Macdowall s single-pin, escapement was adapted to watches exactly as the dead escapement of clocks is turned into the lever escapement of watches. Fig. 9 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 trian gular 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 Graham, and it resembles his dead escapement 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, B&