dulum; but in a slender cylinder, or any other uniform prism or rod suspended at the top, it is at the distance of one-third from the bottom, or two-thirds below the centre of motion.

The length of a pendulum, so measured to its centre of oscillation that it will perform each vibration in a second of time, thence called the seconds' pendulum, has, in the latitude of London, been generally taken at 39% or 39 inches; but by some very ingenious and accurate experiments, the late celebrated Mr. George Graham found the true length to be 39 inches, or 39 inches very nearly.

The length of the pendulum vibrating seconds at Paris was found by Varin, Des Hays, De Glos, and Godin, to be 440 lines; by Picard, 440 lines; and by Mairan, 440 lines.

As all woods and metals are more or less affected by changes of temperature, many ingenious contrivances have been resorted to, to counteract the effects of heat and cold, in lengthening or shortening a pendulum-rod.

The first person who observed that, by change of temperature, metals changed their length, was Godfroi Wendelinus; and he who first endeavoured to take advantage of this knowledge, to counteract the effects of heat and cold upon a pendulum, was Graham, who, in the year 1715, suggested that a combination of rods or wires of different metals would have a tendency to that effect; but being of opinion that this would not be quite adequate to the desired purpose, he did never, we believe, put it in execution. Still continuing his observations, he, a short time afterwards, conceived that mercury, from its great expansion by heat, was more adapted to the end he was pursuing, and accordingly we find, that, by the 9th of June, 1722, he had constructed a clock which had a pendulum upon this principle, and which he kept continually going, without having either the pendulum or the hands altered, for the space of three years and four months, during which he found the errors of his were but about one-eighth part of those of one of the best sort of common clocks, with which he had compared it. This pendulum, which is called the mercurial pendulum, consists of a rod of brass, branched towards the lower end, so as to embrace a cylindric glass jar 13 or 14 inches long, and about two inches diameter; which, being filled about 12 inches deep with mercury, forms the weight or ball of the pendulum. In adjusting this pendulum, if the expansion of the rod be too great for that of the mercury, more mercury must be poured into the vessel; but if the expansion of the mercury exceed that of the rod, so as to occasion the clock to go fast with heat, some of the mercury

must be taken out, to shorten the column. This pendulum, though troublesome to construct, because any filling in or taking out of the mercury from the cylinder or glass jar, to bring about the compensation, will cause a change of place in the index-point on the graduated arch or index-plate, if such a thing be used, is, notwithstanding some defect may arise from the expansion of the mercury commencing sooner than that of the rod, of much practical excellence. The mercurial pendulum has been much improved by Reid; for an account of which we must refer our readers to the article Horology," written by this gentleman, and inserted in the Edinburgh Encyclopædia.

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Mr. Harrison, of whom we have already spoken under the article Chronometers, some time previous to 1726, constructed a pendulum in which the compensation was effected by the opposite contraction of different metals. This pendulum, called the gridiron-pendulum, from, we suppose, its bearing a near resemblance to the culinary implement of that name, was made of five stee! and four brass rods, placed in alternate order, the middle rod, by which the pendulum-ball is suspended, being of steel. These rods are so connected with each other at their ends, that while the expansion of the steel rods has a tendency to lengthen the pendulum, the expansion of the brass rods, acting upwards, tends to shorten it, so that by the combined effect the pendulum is invariably preserved of the same length. This is a very ingenious and simple contrivance, and the only objections we have heard urged against this mode of compensation are, 1st. the difficulty of exactly adjusting the length of the rods; 2dly. of proportioning their thickness, so that they shall all begin to contract or expand at the same instant; 3dly. the connecting bars of a pendulum thus constructed are apt to move by starts; 4thly. this kind of pendulum is more exposed to the air's resistance than a simple pendulum.

Other modes of constructing pendulums on the principle of the opposite contraction of metals have been contrived by other ingenious artists, among whom we may notice Ellicott, Cumming, Troughton, Reid, and Ward.

In Ellicott's pendulum the ball was adjustable by levers, thence called the lever-pendulum, which can never be equal to those in which the expansion and contraction act by contact in the direct line of the pendulum-rod; the construction nevertheless evinced great ingenuity. The rod of this pendulum was composed of two bars, one of brass, and the other of steel. It had two levers, each sustaining its half of the

ball or weight, with a spring under the lower part of the ball to relieve the levers from a considerable part of its weight, and so to render their motion more smooth and easy. These levers were placed within the ball, and each had an adjusting screw to lengthen or shorten the lever, so as to render the adjustment the more perfect. See the Philos. Transact. vol. xlvii. p. 479; where Mr. Ellicott's methods of construction are described and illustrated by figures.

This pendulum was much improved by Cumming, who conceived that where there were two bars only, a flexure and unequal bearing would take place, and consequently an exact compensation could not be effected. To remedy this, he constructed a pendulum of one flat bar of brass, and two bars of steel, and used three levers within the ball of the pendulum, whereas Mr. Ellicott used only two. Among many other ingenious contrivances for the more accurate adjusting of this pendulum to mean time, it is provided with a small ball and screw below the principal ball or weight, one entire revolution of which on its screw will only alter the rate of the clock's going one second per day; and its circumference is divided into 30, one of which divisions will therefore alter its rate of going one second in a month.

Troughton's tubular-pendulum, which acts on the principle of the gridiron-pendulum, is a very neat and ingenious contrivance. It is constructed of an exterior tube of brass, reaching from the bob nearly to the top, within which is another tube, and five brass wires in its belly, so disposed as to produce altogether, (like Harrison's gridiron-pendulum,) three expansions of steel downwards, and two of brass upwards, whose lengths being inversely proportioned to their dilatation, when properly combined, destroy the whole effect that either metal would have singly. The small visible part of the rod, near the top, is a brass tube, whose use is to cover the upper end of the middle wire, which is single, and otherwise unsupported. Drawings of this pendulum may be seen in Nicholson's Journal, No. 36, N.S.

Reid's pendulum is composed of a zinc tube, and three long and one short steel rods, connected by means of traverses. Two of these long rods are inserted at one end in the ball of the pendulum, and terminate at the other in the upper traverse, which keeps them exactly parallel with respect to each other. At the lower ends of these rods, not far above the ball, is another traverse, in the middle of which the short steel rod is pinned, descending thence through the centre of the ball. Another traverse is placed a little above this, on

the centre of which the zinc tube rests, extending upwards, and pressing against, or rather pressed by the upper traverse. The third or centre steel rod passes through a hole in the upper traverse, equidistant from each of the other two steel rods, thence down the zinc tube, and finally is pinned to the second traverse, or that traverse on which the zinc tube rests. By this means, the centre steel rod, when lengthened by heat, will make the lower end of the zinc tube descend with it; but the same cause which lengthens the steel rod downwards will expand the zinc tube upwards, and this will carry up the two outside steel rods with which the ball of the pendulum is connected; their expansion downwards, as well as that of the centre rod, is compensated by the upward expansion of the zinc tube. In constructing a pendulum upon this principle, it would be proper to have a few holes in the tube, for the purpose of admitting air more freely to the centre rod.

Ward's pendulum consists of two flat bars of steel, and one of zinc, connected together by three screws. The description which has been given of it in the Transactions of the Society of Arts, &c. for the year 1807, and the pamphlet which Mr. Ward published at Blandford in 1808, contain sufficient details to enable any common clock-maker to copy it.

Before we conclude this article, we shall briefly notice the sympathy or mutual action of the pendulums of clocks.

It is now nearly a century since it was known that when two clocks are set agoing on the same shelf, they will disturb each other; that the pendulum of the one will stop that of the other; and that the pendulum which was stopped will, after a while, resume its vibrations, and, in its turn, stop that of the other clock, as was observed by the late Mr. John Ellicott. When two clocks are placed near one another, in cases very slightly fixed, or when they stand on the thin boards of a floor, it has been long known that they will affect a little the motions of each other's pendulum. Mr. Ellicott observed, that two clocks resting against the same rail, which agreed to a second for several days, varied 1' 36" in twentyfour hours when separated. The slower having a longer pendulum, set the other in motion in 16 minutes, and stopped itself in 36 minutes.

BUILDING.

UNDER this general term, which implies the construction of an edifice according to the rules laid down by the different artificers employed, we purpose to treat of the respective business of the Mason, Bricklayer, Carpenter, Joiner, Plasterer, Plumber, Painter, and Glazier; previous to which it will be necessary to consider the sinking of the foundation, the due mixture of the ingredients which compose the mortar, and the art of making bricks; upon the whole of which materially depends the stability of an edifice.

As firmness of foundation is indispensable, wherever it is intended to erect a building, the earth must be pierced by an iron bar, or struck with a rammer, and if found to shake, must be bored with a well-sinker's implement, in order to ascertain whether the shake be local or general. If the soil is in general good, the loose and soft parts, if not very deep, must be excavated until the labourers arrive at a solid bed capable of sustaining the pier or piers to be built. If not very loose, it may be made good by ramming into it very large stones, packed close together, and of a breadth proportionate to the intended weight of the building; but where very bad, it must be piled and planked.

In places where the soil is loose to any great depth, and over which it is intended to place apertures, such as doors, windows, &c. while the parts on which the piers are to stand are firm, the best plan is to turn an inverted arch under each intended aperture, as then the piers in sinking will carry with them the inverted arch, and by compressing the ground compel it to act against the under sides of the arch, which, if closely jointed, so far from yielding, will, with the abutting piers, operate as one solid body; but, on the contrary, if this expedient of the inverted arch is not adopted, the part of the wall under the aperture, being of less height, and consequently of less weight than the piers, will give way to the resistance of the soil acting on its base, and not only injure the brick-work between the apertures, but fracture the window-heads and cills.