Carriages, still in both these cases the engine has been compelled, like the traveller himself, to occupy as little room as possible, and to accommodate itself to its situation. The boiler of most boat-engines follows the shape of the vessel. In large vessels there are two boilers, placed side by side; each boiler has generally two fire-places, and the fire and flues are all within the boiler.

There are two cranks on the shaft of the paddlewheel, placed at right angles to each other, and there are two engines to work them; so that when the crank of one engine is at the "dead point” (that is, when the piston is just upon the point of returning), the crank of the other engine is at its full work; consequently no fly-wheel is wanted, and the paddle can be started in whatever position the cranks happen to stand.

manner.

Boat-engines always have short cylinders, of large diameter, and generally two beams below the cylinder, the piston-rod being furnished with a T-piece, giving motion to the beams by two connecting rods. The rods of the air-pump, &c. have also T-pieces, and are connected to beams by rods in the same The motion is communicated from the other end of the beam by a forked connecting-rod, to the crank on the shaft of the paddle-wheels. This arrangement takes up very little room, and allows the engine to be placed between decks without difficulty. The crank, connecting-rod, and part of the beam of the second engine, are indicated in Fig 54 by the dotted lines.

As stated above, two engines, working cranks at

"dead

right angles to each other, prevent any point;" but this is not the only advantage, for in case of any accident to one engine, the other may work the paddles by itself; and this has occasionally happened; and even when one of the paddle-wheels has been disabled, the voyage has been completed by the remaining wheel. A steam-boat may, therefore, be considered as furnished with a complete double apparatus, having two boilers, two engines, and two paddle-wheels, an arrangement originating in convenience, but adding a most important degree of safety to this useful invention.

In the steam-boat, the engine and its work are inseparably connected; but in general this is not the case. Our mechanical reader will not need to be told the method of connecting an engine with a machine; but the popular reader is apt to suppose that to stop the machine, we must stop the engine itself which drives it. We shall, therefore, endeavour to explain this contrivance. A long shaft, called the main shaft, or the driving shaft, is generally supported on brackets or beams, near the upper part of one of the walls of the room in which the machinery is placed. This shaft is driven by toothed wheels in gear with a wheel on the engine, and is constantly going when the engine goes. The machine to be driven has a spindle projecting about eighteen inches, on which are placed two riggers or pullies; one of these pullies is firmly fixed on the spindle, and is called the live pulley, and when turned, turns the machine; the other has a round hole in its centre, and is loose upon the round

spindle, and is called the dead pulley, and when turned, produces no effect on the machine. This construction will be readily understood from fig. 5, page 214, in which the shaded part represents the live pulley, and the outline the dead pulley. An endless strap of leather is put over one of these pullies, and over a pulley or rigger fixed on the main shaft. The middle part of each pulley is a little higher than the edges, and this prevents the strap from running off from the pulley, which is required to be turned. The strap runs between a forked piece of iron, which is easily moved by a connecting rod, called the striking-rod, terminating in a handle near to the workman, who, by shifting the handle, pushes the strap on to the live or dead pulley at pleasure; thus giving him as perfect command over the motion of the machine as if it were turned by hand.