THE STEAM-ENGINE. 



253 



Pig. 212. beneath the piston. The pressure of 



the atmosphere then acting upon the 

 other side of the piston, ■will drive 

 it do^-n. The position of the valves 

 in R and B being reversed, the piston 

 may be raised ane'w- by the admis- 

 sion of more steam, to be condensed 

 in its turn, and in this manner the 

 alternate motion may be continued 

 indefinitely. The alternating, or re- 

 ciprocating motion of the piston, is 

 converted, by means of a lever and crank attached to the top of the pis- 

 ton-rod, into a rotary motion, suitable for driving-wheels, shafts, and other 

 machinery. 



Such an arrangement as described constituted the first practical steam- 

 engine. It received the name of the atmospheric engine, from the fact that 

 the pressure of the atmosphere was employed to press down the piston after 

 it had been elevated by the steam. 



577. In modern engines, the pressure of the atmosphere is 

 not employed to drive the piston down. The steam is ad- 

 mitted into the cylinder above the piston, at the same time 

 that it is condensed or withdrawn from below, and thus 

 exerts its expansive force in the returning as well as in the 

 ascending stroke. 



This results in a gr^cat increase of power. By the condensation or with- 

 draw.al of the steam, a vacuum is created below the piston, and the steam 

 admitted into the cylinder above the piston, forces it through the vacuum 

 with an ease and rapidity far greater than would be possible if atmospheric 

 or other resistance were to be overcome.* 



The withdrawal or condensation of the steam, in order to produce a vacuum 

 either above or below the piston, is accomplished by opening at the proper 

 time a communication between the cylinder and a strong vessel situated at a 

 distance from it, called the condenser. Into this vessel a jet of cold water is 

 thro■^^^l, which instantly condenses the steam, escaping from the bottom of the 

 cylinder, into water. 



Wliat is the 

 construction 

 and operation 

 of a condens- 

 ing steam-en- 

 gine ? 



• "A proof of the extraordinary power obtained in this uray, through the combnstion 

 of fuel, is presented in the following cnlculations : — One cubic inch of water is converti- 

 ble into steam, of one atmospheric pressure, by 15} grains of coal, and this expansion of 

 the water into steam is capable of raising a weight of one ton the height of a foot. Tha 

 one c\ibic inch of water becomes very nearly one cubic foot of steam, or 1,7'2S cubic inches. 

 When a vacuum is produced by the condensation of this ste;im, a piston of one squaro 

 inch surface, that may have been lifted 1,7-8 inches, or 144 feet, will fall with a velocity of a 

 heavy body rushing by gravity down a perpendicular height of ir",5i10 feet. This would 

 give the falling body a velocity, at the termination of its descent, equal to 1,300 feet per 

 gecond, great'^r than that of the transmission of sound. From this we can form some 

 estimate of the strength of the terhpest which alternately blows the piston in its cylinder, 

 when elastic steam of high-pressure is employed." — Prof. U. D, Rogers, 



