The art of constructing machines for measuring time; but from the circumstance of clocks and watches having very generally superseded all other contrivances for this purpose, the term is now usually understood as referring to these latter instruments solely. In these machines a pendulum, or a spiral spring, connected to a flat wheel turning freely on its axis, (called a balance wheel,) is made to vibrate; and it being a property of these bodies, that all the vibrations, whether through large or small arcs, (within certain limits,) are made in equal times, all that is necessary to measure time' by this means, is to register the vibrations, and to prevent the vibrating body from being brought to rest by the resistance of the atmosphere, or the friction of its parts, about the centre of oscillation; and these objects are effected by means of a train of wheels put in motion by the descent of a weight, or by the action of a coiled spring, the velocity of the wheels being regulated by the vibrations of the vibrating body.

The essential difference between clocks and watches consists in the nature of the regulator employed; which in clocks is the pendulum, and in watches the balance wheel.

The pendulum requires to be suspended from steady points of support, that its vibrations may be constantly performed in a vertical plane, hence clocks cannot be made portable; but from the pendulum being acted upon by the force of gravity, which is a constant force, its motion is more equal than that of the balance spring, the force of which varies greatly, from changes of temperature, and other causes; clocks are therefore always preferred in observatories; but as the balance will perform lying in any position, watches have the advantage of being portable.

We shall now proceed to describe the construction of an ordinary eight-day clock, which we hope will be rendered sufficiently clear with the help of the accompanying engraving, which exhibits a front view of the works, the dial plate being removed to show the manner in which the hour and minute hand are made to revolve with different velocities upon the same centre. A portion of the front frame plate is likewise represented as broken away to exhibit more clearly the escapement; the striking parts, and the wheels and pinions composing the train, being hidden by the frame plate, are represented simply by dotted circles, and instead of letters of reference, they are indicated by figures, which express the number of teeth contained in them. The striking parts, as usually constructed, are so very complex as to render it extremely difficult to convey a clear idea of their operation without a very lengthened description, and several diagrams; we have therefore preferred introducing in their stead an extremely simple and ingenious arrangement for the purpose, invented by Mr. Prior, of Nessfield, in Yorkshire, and for which he received a reward from the Society of Arts.

A clock of this kind contains two independent trains of wheelwork, each with its separate first mover; one is constantly going to indicate the time by the hands on the dial plate; the other is put in motion every hour, and strikes a bell to tell the hour at a distance; the dotted circle a is the barrel of the going part; it has a catgut b wound round it, suspending the weight c which keeps the clock going; 96 is a wheel (called the first or great wheel,) of that number of teeth, upon the end of the barrel, turning a pinion of eight leaves, on an arbor which carries the minute hand. 64 is a wheel of 64 teeth, on the same arbor, (called the centre wheel,) turning the wheel 60 by a pinion of eight leaves on its arbor: this last wheel gives motion to the pinion of eight, on the arbor of the swing wheel 30, of 30 teeth; d h are the pallets of the escapement, fixed on an arbor e, going through the back plate of the clock's frame, and carrying a long lever, which has a small pin projecting from its lower end, going into an oblong hole, made in the rod f of the pendulum.

The pendulum consists of an inflexible metallic rod, suspended by a very slender piece of steel spring, from a brass bar, screwed to the frame of the clock, having a weight or bob at its lower end, in the present case 39.125 inches from the point of vertical suspension; when this pendulum is moved from the line in either direction, and suffered to fall back again, it swings nearly as much beyond the vertical on the contrary side, and then returns; this it will continue to do for some time, and each of these vibrations will be performed in one second of time, when the pendulum is of the above length. This is the measurer of the time; and the office of the clock is only to indicate the number of vibrations it has made, and give it a small impulse each time to keep it going, as the resistance of the air, and elasticity of the spring, would otherwise in a few hours cause it to stop. By the action of the weight applied to the cord b, (which is called the maintaining power,) the wheels are all turned round; and if the pallets dh were removed, the swing wheel 30 would revolve with great velocity in the direction from 30 to d, until the weight reached the ground; the teeth of these pallets are so made that one of them always engages the wheel, and prevents its turning more than half a tooth at a time.

In the drawing, the pallet d has the nearest tooth of the wheel resting on it, and the pendulum is on the side h of the perpendicular; when it returns it moves the pallet d so as to allow the tooth of the wheel to slip off; but in the mean time the pallet h has interposed its point in the way of the tooth next it, and stops the wheel till the next vibration or second; the distance between the two pallets dh is so adjusted that only half a tooth of the wheel escapes at each vibration; and as the wheel has 30 teeth, it will revolve once in 60 vibrations of one second each, or one minute; consequently a hand on the arbor of this wheel will indicate seconds on a circle on the dial plate divided into 60; the pinion of eight on its arbor is turned by a wheel of 60, which consequently will turn once in seven turns and a half of the other, or in seven minutes, 30 seconds, or one-eighth of an hour; its pinion of eight is moved by a wheel of 64, or eight times itself, which will turn in one-eighth part of the time, this will be an hour; the arbor of this wheel, therefore, carries the minute hand of the clock. The great wheel of 96 being twelve times the number of the pinion eight, will turn once in 12 hours, and the barrel a with it.