Any weight attached to a rod or wire so that it can swing freely may be called a “pendulum.” But for the purpose of time-keeping, a much more accurate instrument is required; the rate of vibration or oscillation of the pendulum, does not depend upon the weight of the ball or “bob” at the lower end, but upon the distance of this from the point at which the upper end turns, nor does the rate of oscillation depend upon the distance through which the weight traverses, for every pendulum will vibrate at the exact rate (with certain restrictions) at which it is set off, until it ceases, although the distance through which it traverses, decreases at every vibration; these facts are taken advantage of in adapting the pendulum to the purposes of regulating the time a clock shall keep--the longer the pendulum the slower the vibrations. Now, as everything in nature is expanded by heat and contracted by cold, so a pendulum is constantly varying in length by every change of temperature, and, as a consequence, the rate of the clock to which it is attached will also vary. Pendulums which have an arrangement to obviate this variation, are called “compensating” pendulums; the best in use are of two kinds, one called (from its appearance) the “gridiron,” the other the “mercurial,” this last is the most accurate, and is used in nearly all good astronomical clocks. The gridiron pendulum is made of iron and brass, or zinc, and is constructed as shown in fig. 1; the rod and outer frame, A, is made of iron, the two rods inside this of zinc or brass, B B. The principle of the instrument is this--brass or zinc contract and expand much more than iron does, and the short bars of these metals will expand or contract as much as the long bar of iron forming the rod of the pendulum, so that as this expands and lets the “bob” down, the short bars expand and draw it upwards so that it keeps its place at any temperature; this requires very accurate adjustment. The mercurial pendulum is shown at fig. 2; it is on the same principle, but is easier to regulate, and more manageable, the vessel in the centre being partly filled with mercury, and forming the weight itself, and thus as the mercury expands upwards it compensates for the elongation of the rod, the same as in the gridiron pendulum. The nearer any pendulum is to the centre of the earth the more quickly does it vibrate; this has been used by scientific men, to determine by the difference of rate in one placed on a hill, and another at the bottom of a deep mine, the amount of matter which constitutes our globe; indeed by these trials the world may fairly be said to have been weighed! PLOUGHS. Ploughs are instruments used to perform more rapidly what may be effected by the spade, namely, the cutting-up and turning-over the surface of the ground so as to destroy all grass and weeds growing in it, loosen, so as to expose it to the influence of the air, and render it fit to receive the seed.