FIG. 3.
The barometer is designed to indicate the weight or pressure of the air on any surface, at any particular time or place; for the air, although invisible, is still of considerable weight, as there are many miles of it pressing from above downwards on all parts of everything upon the earth, and the barometer is for the purpose of ascertaining how much this pressure amounts to. It is formed as follows: a piece of glass tubing, about three feet long, is first closed at one end, then turned up at the other and expanded ([fig. 1]); when this tube is filled with mercury and held with the bulb downwards, the mercury sinks in the stalk to a certain height (say twenty-nine inches), and that height shows the weight or pressure of the air. The reason of this will be understood by supposing a piece of straight glass tubing, three feet long, to be closed at one end and then filled with mercury; if the finger be placed on the end not closed, and that end turned downwards and put into a basin of mercury ([fig. 2]) before the finger is withdrawn, the fluid, if the air exerted no pressure, would all sink down from the inside of the tube into that in the basin, leaving a “vacuum” or empty space in the hollow of the tube, but it is evident if the air exerted any pressure on the surface of the mercury in the basin, this pressure would force the mercury up the tube (for there is no opposing pressure in an empty space), and that the mercury would rise higher and higher the greater the the pressure. Well, then, the air really exerts this pressure, and to such an extent as to raise the mercury somewhere about thirty inches in height, and the pressure necessary to do this is found by calculation to be about fifteen pounds upon every square inch of surface. The barometer tube is divided into a scale of inches and fractions of inches. What are called weather glasses, are barometers having the lower part brought up by a curve, and a small weight resting on the mercury in it, which being attached to a corresponding weight by means of a cord running over a little wheel or pulley fixed to hands moving round a sort of dial, turns them as the mercury rises or sinks ([fig. 3]), for as the mercury falls in the stalk it must of course rise in the short stalk of the curve; the hands by these means are turned round, and the rise or fall of the fluid will cause them to point to “fair,” “rain,” &c., as the case may be, for these names are marked where a corresponding change of the weather may so influence the weight of the air, as to raise or depress the mercury, and so bring the hands in a position to point to them.
PENDULUMS.
FIG. 1.
FIG. 2.
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!