Eight-Day Clocks.—In an eight-day clock there are two barrels containing springs, one to give motion to the train of wheels called the going train, and the other to the striking train. The first of these springs gives motion to a wheel called the main wheel, which in its turn acts upon several other wheels, the time of rotation of which corresponds with that of the minute and hour hand. The last wheel of this system acts upon two little levers or pallets, which give an alternating motion to the pendulum of the clock. A fusee, which is a kind of spiral system of grooves, is fitted to the main wheel, and enables the main wheel to maintain an equable motion during the varying pulling force of the spring. All the above-named wheels helongto the going train. There is a somewhat similar train belonging to the striking action ; but the teeth of the wheels, instead of being so cut as to facilitate the movement of two index bands, act upon certain pins and levers which move the ham mer belonging to the bell ; this part of the mechanism is exceedingly beautiful, especially ' when the clock strikes the quarters.
Once in eight days the clock is wound up,' which consists in coiling up the going spring very tightly in its barrel. In its efforts to un coil itself, the spring forcibly pulls round the main wheel with which it is placed in connec tion ; and the fusee causes this, motion to go on pretty steadily and equably. As the main wheel cannot rotate without moving the train of wheels to which it is connected, all these latter also rotate, with a velocity depending on the number of teeth which act on each other. In two of the wheels these numbers are so regulated that the wheels revolve in one hour and twelve hours respectively ; and the axes of these wheels serve as axes for the minute and hour hands. Meanwhile the se cond spring gives motion to the second train of wheels, which are so connected with the going wheels as to enable the bell-hammer to be moved at the proper time. One of the many kinds of pendulum, concealed behind or within the clock, is set in motion by one of the wheels, and, by its isochronous or equal timed' Thrations, tends to give regularity to the movements of the wheels generally, Large Pendulum Clocks.—Tun.et clocks dif fer from other machines employed for measur ing time, not only in their greatly superior size, but because such a clock is frequently required to indicate the time upon as many as four different dials, on the four external faces of the tower in which it is mounted. This is accomplished by placing the clock near the 3entre of an apartment, and causing the mo ion of the axis which under ordinary circular. stances would carry the minute hand (which !evolves once in an hour), to be transmitted )y bevil-gear to a vertical rod, the opposite and of which carries a horizontal bevil-wheel nearly on a level with, and situated centrically with reference to, the four external dials. The
notion of this central wheel is cominnuicated )y four vertical bevil-wheels of the same size ;nd number of teeth, ranged round its circum erence, to four horizontal rods, the opposite Inds of which, passing through the several lials, carry the four minute-hands. At the ack of each dial is a series of wheels and ;Miens, by which motion is imparted to the tour-hand, which revolves once in twelve lours. In a turret-clock, the moving power 3 supplied by the descent of a weight, regula ed in the case of the movement, or going rain, by the oscillations of a large pendulum, nd in that of the striking train by the resist nee of the air to the rapid revolutions of a fly or fan set in motion c.;,i',"theelwork. Owing to the necessity for using a very heavy ham mer to strike the hours in a church clock, the power required for working the striking-train considerably exceeds that of the going train.
Musical Chimes.—These require the addition of another train of mechanism, somewhat like that which constitutes the striking train. The mechanism of the chimes very nearly resem bles, on a large scale, that of a musical snuff box : levers, connected with hammers which strike upon a series of bells, being substituted for the springs which in the musical snuff-box are caused to vibrate by the projecting pins on the revolving barrel.
In the new Royal Exchange clock, made by Mr. Dent, many improvements have been in troduced. There is a simple but strong cast iron framing, which enables the several parts of the clock to be put together with less strain than usual. The wheels for the striking train are made of cast iron ; more durable and less costly than the usual gun-metal wheels. The wheels of the going train, smaller and requir ing to be more nicely adjusted than those of the striking train, are made of hammered brass. Hollow iron drums are used instead of wooden cylinders for the driving barrels, and wire instead of hemp or ropes for suspend ing the weights. It was required, by the terms of contract, that this most admirable clock should have a compensation-pendulum, and that it should be so constructed as not only to show perfectly correct time upon the dials, but also to tell it with accuracy by making the first stroke of the hour upon the bell true to a second. This object is attained by a beautiful arrangement of mechanism. In this clock has also been introduced a beautiful contrivance for maintaining the motion of the wheels dtt ring the time of winding up, which was in vented a few years since by Professor Airy for the clock-work of the great Northumberland telescope at the University of Cambridge.