sudden jerk which would result from the starting off of a train quite inelastic in its length. The engines used are of that class called high-pressure or non-condensing, and there are two cylinders and pistons, which have a stroke of about eighteen inches. The boiler is so contrived that a large quantity of steam shall be rapidly produced; for this purpose tubes of brass are made to pass side by side from the fireplace through the boiler, and through these tubes the flame and hot air must go before reaching the funnel, giving out in its course a great amount of heat to the water and converting it rapidly into steam. The steam from each cylinder passes at each stroke of the piston into the funnel, assisting to form a draught which draws the flame from the fire through the tubes and increases the fierceness of the combustion. The necessity for two cylinders and pistons is owing to the impossibility of having a fly-wheel, and as the driving wheels of the engine have to be turned at an equal rate, the axle has two cranks so placed that the greatest power of one piston is exerted where the other exerts the least (see “Steam-engine”). Illustration: FIG. 11. (‡ SIGNAL TOWER.) Illustration: FIG. 12. (‡ SIGNAL TOWER.) Illustration: FIG. 13. (‡ SIGNAL LAMPS.) As the trains when going at a considerable speed cannot be suddenly stopped, it is necessary to have signals placed in certain conspicuous positions, that the engine driver may begin to stop the train (when necessary) in time; this he effects by what is called a “break,” a contrivance by which two pieces of wood are made (by turning a screw) to grasp firmly each of a pair of wheels, and so prevent them turning round, this produces so much friction against the trams that (after the steam is turned off) the onward motion of the train is soon stopped. The signals are of three kinds generally, a red flag to indicate danger, a green one to caution, and a white one to show that the way is clear; these are (on most occasions) held by a man and waved to and fro to attract attention, but there are however a great many occasions for fixed signals, as at stations and bends in the line where the engine driver can only see a short distance ahead; these fixed signals consist of tall posts placed where they can be seen at a considerable distance. These posts have an arrangement at the top consisting of a lamp with a “bull’s-eye” or lens at each side pointing up and down the line, and a pair of arms capable of being let down into the post, raised at right angles with it, or into a position midway between the post and a right angle (as shown in figs. 11 and 12). One side of each arm is painted red, the other white, one arm serving for a signal up the line and the other down; attached to the joint of each arm, close to the post, are two iron frames each holding a piece of colored glass, one red the other green, and so arranged that when the arm is at right angles to the