APPARATUS FOR REVERSING ENGINES.

439

cylinders, one on each side of the engine, each having a valve-chest and slide-valve, by means of which steam is admitted alternately before and behind the pistons. The steam escapes from the cylinder, through the blast-pipe v, up the chimney, and thus increases the draught of the fire. a is one of the pistons, b the piston-rod, cc' the connecting-rod, which is jointed to the crank d on the axle of the driving-wheel m. The eranks of the two driving-wheels, one on each side of the engine, are set at right angles to each other, so that, when one is at a dead point, the other is in the most advantageous position. w is a spring safety-valve, and J the steam-whistle.

390. Apparatus for Reversing: Link-motion.-The method usually employed for reversing engines is known as Stephenson's link

motion, having been first employed in locomotives constructed by Robert Stephenson, son of the maker of the "Rocket." The merit of the invention belongs to one or both of two workmen in his employ—Williams, a draughtsman, who first designed it, and Howe, a pattern-maker, who, being employed by Williams to construct a model of his invention, introduced some important improvements. The link-motion, which is represented in Fig. 330, serves two purposes; first, to make the engine travel forwards or backwards at pleasure; and, secondly, to regulate the amount of expansion which

shall take place in the cylinder. Two oppositely placed eccentrics, A and A', have their connecting-rods jointed to the two extremities of the link BB', which is a curved bar, having a slit, of uniform width, extending along nearly its whole length. In this slit travels a stud or button C, forming part of a lever, which turns about a fixed point E. The end D of the lever DE is jointed to the connectingrod DN, which moves the rod P of the slide-valve. The link itself is connected with an arrangement of rods LIKH,1 which enables the engine-driver to raise or lower it at pleasure by means of the handle GHF. When the link is lowered to the fullest extent, the end B of the connecting-rod, driven by the eccentric A, is very near the runner C which governs the movement of the slide-valve; this valve, accordingly, which can only move in a straight line, obeys the eccentric A almost exclusively. When the link is raised as much as possible, the slide-valve obeys the other eccentric A', and this change reverses the engine. When the link is exactly midway between the two extreme positions, the slide-valve is influenced by both eccentrics equally, and consequently remains nearly stationary in its middle position, so that no steam is admitted to the cylinder, and the engine stops. By keeping the link near the middle position, steam is admitted during only a small part of the stroke, and consequently undergoes large expansion. By moving it nearer to one of its extreme positions, the travel of the slide-valve is increased, the ports are opened wider and kept open longer, and the engine will accordingly be driven faster, but with less expansion of the steam. As a means of regulating expansion, the link-motion is far from perfect, but its general advantages are such that it has come into very extensive use, not only for locomotives but for all engines which need reversal.

393. Gas-engines. This name includes engines in which work is obtained by the expansion of a mixture of coal-gas and air, on ignition or explosion. In Lenoir's engine a piston is driven alternately in opposite directions by successive ignitions of such a mixture on opposite sides of it, the proportions of gas and air being such as not to yield a true explosion.

In the engine of Otto and Langen (Fig. 331), a true explosive mixture is introduced beneath the piston, and is exploded by means

1I is a fixed centre of motion, and the rods KI, ML are rigidly connected at right angles to each other. M is a heavy piece, serving to counterpoise the link and eccentric rods.

of a lighted jet, which is brought into contact with the mixture by means of a hole in a movable plate of metal, driven by an eccentric. The upward movement of the piston thus produced is too violent to admit of being directly communicated to machinery. The piston-rod is a rack, working with a pinion, which is so mounted that it can slip round on the shaft when the piston ascends, but carries the shaft with it when it turns in the opposite direction during the descent of the piston, this descent being produced by the pressure of the atmosphere, as the steam resulting from the explosion condenses, and the unexploded gases cool. The vessel shown on the right contains cold water, which is employed to cool the cylinder by circulating round the lower part of it. The quantity of water required for this purpose is much smaller, and the consumption of gas is also much less, than in Lenoir's engine.

CHAPTER XXXIV.

TERRESTRIAL TEMPERATURES.

394. Temperature of the Air.-By the temperature of a place meteorologists commonly understand the temperature of the air at a moderate distance (5 or 10 feet) from the ground. This element is easily determined when there is much wind; but in calm weather, and especially when the sun is shining powerfully, it is often difficult to avoid the disturbing effect of radiation. Thermometers for observing the temperature of the air must be sheltered from rain and sunshine, but exposed to a free circulation of air.

395. Mean Temperature of a Place. The mean temperature of a day is obtained by making numerous observations at equal intervals of time throughout the day (24 hours), and dividing the sum of the observed temperatures by their number. The accuracy of the determination is increased by increasing the number of observations; as the mean temperature, properly speaking, is the mean of an infinite number of temperatures observed at infinitely short intervals.

If the curve of temperature for the day is given, temperature being represented by height of the curve above a horizontal datum line, the mean temperature is the height of a horizontal line which gives and takes equal areas; or is the height of the middle point of any straight line (terminated by the extreme ordinates of the curve) which gives and takes equal areas.

Attempts have been made to lay down rules for computing the mean temperature of a day from two, three, or four observations at stated hours; but such rules are of very limited application, owing to the different character of the diurnal variation at different places; and at best they cannot pretend to give the temperature of an individual day, but merely results which are correct in the long run. Observations at 9 A.M. and 9 PM. are very usual in this country; and the half-sum of the temperatures at these hours is in general a good