the steam chest V, and thence through the passage N into the cylinder C, where its pressure forces the piston P to the left. It will be seen from the figure that as the driving rod R moves toward the left the so-called eccentric rod R', which controls the valve V, moves toward the right. Hence, when the piston has reached the left end of its stroke, the passage

FIG. 181. Ideal diagram of a steam engine

N will have been closed, while the passage M will have been opened, thus throwing the pressure from the right to the left side of the piston, and at the same time putting the right end of the cylinder, which is full of spent steam, into connection with the exhaust pipe E. This operation goes on continually, the rod R' opening and closing the passages M and N at just the proper moments to keep the piston moving back and forth throughout the length of the cylinder. The shaft carries a heavy flywheel W, the great inertia of which insures constancy in speed. The motion of the shaft is communicated to any

The land battleships called tanks were invented to combat the deadly machine guns. They
rolled along on two endless steel belts running lengthwise of the machine. Some of the tanks
were armed with 3-inch guns and could travel over the roughest ground, down into great shell
holes and out again, over trenches, and through masses of barbed wire, and could even break
down and run over trees a foot in diameter, as shown in the picture. The smaller tanks were
armed with machine guns and could travel as fast as the enemy infantry could run

This 400-horse-power motor, one of America's important contributions to the World War, was developed for use on the larger types of bombing airplanes. It makes 1700 revolutions per minute and has twelve cylinders, which are water-cooled. It weighs 806 pounds, or about 2 pounds per horse power. The NC-4, which made the first transatlantic flight, was equipped with three of these motors

desired machinery by means of a belt which passes over the pulley W'. Within the boiler the steam is at high pressure and high temperature (§ 226). The steam falls in temperature within the cylinder while doing the work of pushing the piston. A steam engine is a mechanical device which accomplishes useful work by transforming heat energy into mechanical energy.

234. Condensing and noncondensing engines. In most stationary engines the exhaust E leads to a condenser which consists of a chamber Q, into which plays a jet of cold water T, and in which a partial vacuum is maintained by means of an air pump. In the best engines the pressure within is not more than from 3 to 5 centimeters of mercury, that is, not more than a pound to the square inch. Hence the condenser reduces the back pressure against that end of the piston which is open to the atmosphere from 15 pounds down to 1 pound, and thus increases the effective pressure which the steam on the other side of the piston can exert.

R

R

M

N

235. The eccentric. In practice the valve rod R' is not attached as in the ideal engine indicated in Fig. 181, but motion is communicated to it by a so-called eccentric. This consists of a circular disk K (Fig. 182) rigidly attached to the axle but so set that its center does not coincide with the center of the axle A. The disk K rotates inside the collar C and thus communicates to the eccentric rod R' a back-and-forth motion which operates the valve V in such a way as to admit steam alternately through M and N at the proper time. 236. The boiler. When an engine is at work, steam is being removed very rapidly from the boiler; for example, a railway locomotive consumes from 3 to 6 tons of water per hour. It is therefore necessary to have

FIG. 182. The eccentric

the fire in contact with as large a surface as possible. In the tubular boiler this end is accomplished by causing the flames to pass through a large number of metal tubes immersed in water. The arrangement

of the furnace and the boiler may be seen from the diagram of a locomotive shown in Fig. 183. (See early and modern types opposite p. 123.)

237. The draft. In order to force the flames through the tubes B of the boiler a powerful draft is required. In locomotives this is obtained by running the exhaust steam from the cylinder C (Fig. 183) into the smokestack E through the blower F. The strong current through F draws with it a portion of the air from the smoke box D, thus producing within Da partial vacuum into which a powerful draft rushes from the furnace through the tubes B. The coal consumption of an ordinary locomotive is from one-fourth ton to one ton per hour.

B

FIG. 184. The

governor

In stationary engines a draft is obtained by making the smokestack very high. Since in this case the pressure which is forcing the air through the furnace is equal to the difference in the weights of columns of air of unit cross section inside and outside the chimney, it is evident that this pressure will be greater the greater the height of the smokestack. This is the reason for the immense heights given to chimneys in large power plants.

238. The governor. Fig. 184 shows an ingenious device of Watt's, called a governor, for automatically regulating the speed with which a stationary engine runs. If it runs too fast, the heavy rotating balls B