metals and a little ingenuity the uncouth appearance may be completely overcome and the compensation be made not only efficient in action, but also presentable, or even beautiful in appearance. Graham's mercurial compensation. This is a combination of steel and mercury. Mercury, at ordinary temperatures, is a melted metal having a high index of expansion, and being nearly twice as heavy as an equal bulk of steel. If it were a light metal it would not, on account of its liquidity, be very manageable as a compensating material. But on account of its high density the metal is used for its weight as well as for its expansion index, and the compensation becomes very simple. Of course the mercury must be confined, like any other liquid, and cannot be used in the form of a rod. CC is a hollow steel cylinder, shown in section, about 10 inches long inside and of any convenient diameter, depending upon how heavy the bob is to be. For a seconds pendulum the cylinder may have a diameter of from 2 to 3 inches. The pendulum rod, R, passes through the threaded box, B, on the cap of the cylinder, and the clock is regulated by screwing the bob upwards or downwards as may be required. At f is an opening through which mercury may be put in or taken out, and which is closed by a plug. Steel Steel R B с Hg The depth of mercury required to compensate a steel pendulum rod swinging seconds, is from 8 to 9 inches, the exact quantity being found by trial. If the clock gains with a rise of temperature there is too much mercury in the bob, and some must be removed. If, on the other hand, the clock loses with a rise of temperature, some additional mercury must be added. This is probably the simplest and best of all compensations, but on account of the high cost of mercury it is too expensive for common use, and, as a consequence, is mostly confined to costly and superior clocks. modified. But the statement is practically true between all limits of temperature to which a clock would reasonably be exposed. The compensation arrangements that have been proposed are numerous, and to describe them all would be tedious; besides many of them have never come into common use because having nothing in particular to recommend them. We shall content ourselves then with describing a few characteristic ones. The Julien Le Roy compensation. This compensation, although probably nowhere in use at the present day, is well adapted to illustrate principles, and for that purpose it is here chosen. KK is the cock of the pendulum, having in it a slot through which the spring can slide with a close fit. B is a brass tube standing on the cock, and supporting the pendulum by a cap C on the top. The pendulum spring is connected with the rod R and another steel rod reaching up to C. Then from C to O is a length of steel expanding downwards, and from K to C is a length of brass expanding upwards. And we must have CO 87 units and KC 53 units, while PO is 39.14 inches. B -steel -brass KAK R Β' inches, while the And it is easily We find by a little calculation that KC, or the length of the brass tube, is about 61 whole length from C to O is 100.14 inches. shown that the expansion of CO downwards is equal to that of KC upwards, so that PO remains invariable. This form of compensation was used by Cassini in his observatory in 1748, and he spoke highly of its performance. The arrangement is rather uncouth in form, having a principal part of the compensation standing over five feet above the cock of the pendulum, or what would naturally be the top of the clock; but it might not be very objectionable in a fixed observatory. However, the principle remains the same whatever may be the disposition of the parts, and by the use of different metals and a little ingenuity the uncouth appearance may be completely overcome and the compensation be made not only efficient in action, but also presentable, or even beautiful in appearance. Graham's mercurial compensation. This is a combination of steel and mercury. Mercury, at ordinary temperatures, is a melted metal having a high index of expansion, and being nearly twice as heavy as an equal bulk of steel. If it were a light metal it would not, on account of its liquidity, be very manageable as a compensating material. But on account of its high density the metal is used for its weight as well as for its expansion index, and the compensation becomes very simple. Of course the mercury must be confined, like any other liquid, and cannot be used in the form of a rod. CC is a hollow steel cylinder, shown in section, about 10 inches long inside and of any convenient diameter, depending upon how heavy the bob is to be. For a seconds pendulum the cylinder may have a diameter of from 2 to 3 inches. The pendulum rod, R, passes through the threaded box, B, on the cap of the cylinder, and the clock is regulated by screwing the bob upwards or downwards as may be required. At f is an opening through which mercury may be put in or taken out, and which is closed by a plug. Steel с Steel R B f с Hg The depth of mercury required to compensate a steel pendulum rod swinging seconds, is from 8 to 9 inches, the exact quantity being found by trial. If the clock gains with a rise of temperature there is too much mercury in the bob, and some must be removed. If, on the other hand, the clock loses with a rise of temperature, some additional mercury must be added. This is probably the simplest and best of all compensations, but on account of the high cost of mercury it is too expensive for common use, and, as a consequence, is mostly confined to costly and superior clocks. A cylinder 2 inches in diameter would require upwards of 13 pounds of mercury; a cylinder of 2 inches, upwards of 20 pounds, and a cylinder of 3 inches upwards of 30 pounds, in order to compensate a steel seconds pendulum rod. Other compensations. Of the many compensations that have been invented, whether ever put into practical use or not, we may describe a few of those in most common use. Wood and lead compensation. This is a very cheap compensation as employing only cheap materials, but it requires care in the choice and preparation of the wood which forms the pendulum rod. Wood is prone to absorb moisture and thus get heavier, which would interfere with the running of the clock. For making a pendulum rod the wood should be sound and uniform straight grained pine, and after being worked into form it should receive several coats of shellac, followed by a good coat of copal varnish. The wood rod, W, is firmly fastened to the spring chock at the top, as shown at C, and has at the bottom a washer A and an adjusting nut for regulating. On the washer rests the leaden cylinder L, fitting loosely around the rod, but close enough to prevent shaking or rattling. As the rod expands downwards the leaden bob expands upwards relatively to the washer A, and if matters be properly adjusted the effective length of the pendulum remains constant. с Wood Lead Wood On the same scale as given for steel and brass, the expansion indexes of wood and lead are 19 and 133 respectively. And from these it results that the bob should be about 13 inches long. This is on the supposition that the rod is all wood. When we include the steel in the spring and the steel pin at the bottom of the rod, the length of the leaden bob must be about 14 inches, and the whole length of the pendulum from the cock to the washer will be about 46.25 inches. Harrison's Gridiron Compensation. This is so named because it is made of a number (9) of rods, alternately steel and brass, lying side by side. In the illustration diagram 1, the heavy lines denote steel rods and the lighter ones brass rods, and they are joined at their ends as represented. Then, concerned in the downwards expansion there is an equivalent of a steel rod from K to O, together with the two shorter steel rods of length equal to that from I to L, each. And in the upward expansion there are the two brass rods each equal in length to the distance from I to L. And if these be properly proportioned the distance KO will be unaffected by temperature variations. N JI K 2 By a little calculation it is found that the side rods should be about 30 inches each. And, of course, for convenience in construction, one of the side rods of brass or steel may be made shorter, so that the sum of their lengths may be 60 inches. In practice, and for the sake of symmetry the short rods are repeated on the other side of the middle, one, and guiding cross bars enclose the upper and lower ends of the arrangement so as to render the whole strong and stable, without interfering with the free movements due to expansion. When all the rods are highly polished the arrangement has a very ornamental and imposing appearance. For this latter reason an apparent gridiron compensation is often put upon the pendulums of cheap clocks, with the intention, not of correcting any error, but of making the clock more saleable to people who are pleased with what is showy, whether it be of any particular use or not. Invar. A few years ago Mr. Charles E. Guillaume, of the International office of weights and measures at Sevres, near Paris, discovered an alloy of steel and nickel which has an index of expansion only one-eleventh that of steel itself. This, on account of its low expansion index has been named invar from the first two syllables of the word invariable. The alloy has properties of the nature of those of steel, is not very expensive, and is easily compensated by any of the other metals. And it is just possible that among the innumerable alloys that may be made, some one may yet be found |