THE CONDENSING ENGINE.] 



APPLIED MECHANICS. 



871 



Kg. 202. 



Fig. 203. 



JL 



,,~\ F 



of the mercury in the cup would force the mercury 



up the tube to the height of 



about 30 inches, because a 



column of mercury 30 inches 



in height, presses with a force 



of 15 Ibs. on the square inch. 



But if A contained fluid press- 



ing with 2 Ibs. per square inch, 



the mercury would attain a 



height of only 26 inches ; be- 



cause the pressure of a column 



26 inches high is 13 Ibs., and 



the additional 2 Ibs. of fluid 



pressure make up 15 Ibs., the 



atmospheric pressure. 



If, then, we know the pressure 

 of steam in the cylinder (above 

 that of the atmosphere), and 

 the height of the mercurial column in the barometer, 

 we can find the effective pressure on the piston by adding 

 the steam pressure to half the height (in inches) of the 

 column. 



S A VERT'S AND NEWCOMEN'S ENGINES. It 

 is remarkable that gome of the earliest efforts made for 

 obtaining power from steam, were directed, to the con- 

 struction of apparatus in which its condensation, as well 

 as its elasticity, should afford the force required. At the 

 end of the 17th century, Captain Thomas Savery suc- 

 ceeded in constructing an engine for raising water by 



means of steam. A 

 vessel, A (Fig. 203), 

 connected by a steam- 

 pipe and cock 13, with 

 a suitable boiler, com- 

 municated with a ver- 

 tical water-pipe C, in 

 which were fitted two 

 valves, D and E, open- 

 ing upwards. Steam 

 being admitted into A, 

 forced out the air con- 

 tained in it by the up- 

 per part of the water- 

 pipe, and occupied its 

 place. The steam- 

 cock B being closed, 

 and a stream of cold 

 water made to pour 

 over A, the steam 

 within it became con- 

 densed, and formed a 

 partial vacuum; the 

 pressure of the atmo- 

 sphere acting on the 

 water at the bottom of 

 C, forced it up the 

 pipe through the valve D, and into A, so as to fill the 

 void space. The steam-cock being again opened, the 

 pressure of steam on the water in A forced it through 

 the valve E and up the pipe. The vessel A was thus 

 successively filled and emptied by the alternate closing 

 and opening of the steam-cock, and the water raised 

 through the height of the pipe C. 



A few years afterwards, Thomas Newcomen applied 

 steam to give motion to a piston in a cylinder. The 

 cylinder A (Fig. 204) communicated by a pipe and cock 

 B with a boiler generating steam at low (or nearly atmo- 

 spheric) pressure. A piston C, fitting the cylinder, was 

 connected by a chain with one end of a beam or lever D, 

 to the other end of which, K, was attached a pump-rod 

 with a heavy weight F. The ends of the beam were 

 made arcs of circles, so that while the chains were wound 

 on them or unwound from them, the piston and pump- 

 rod might move in straight lines. Steam being admitted 

 into the cylinder below the piston, so as to balance 

 the atmospheric pressure on its upper surface, the weight 

 F caused it to ascend ; and when it reached the top, the 

 steam cock was closed, and a water-cock G was opened to 

 admit a jet of water into the midst of the steam. The 



coldness of the water rapidly condensed the steam, anil 

 formed a partial vacuum below the piston, and the atino- 



Fig. 201. 



spheric pressure forced it down to the bottom, raising the 

 weight F. The water-cock being now closed, and the 

 stuam-cock opened, the ascent of the piston was re- 

 peated. 



WATT'S IMPROVEMENTS. Numerous inventors 

 contributed to the improvement of Newcomen's engine ; 

 and about the beginning of the 18th century it had be- 

 come a practically useful, but not economical apparatus 

 for pumping. The principal objection to the use of this, 

 as well as of Savery's engine, consisted in the circum- 

 stance that the alternated flow of cold water into the 

 steam-cylinder cooled it down, and greatly diminished 

 the force of the steam as it entered, a great portion of 

 its heat being ineffectively expended on the cold metal of 

 the cylinder. It was reserved for the illustrious James 

 Watt, shortly after the middle of the last century, to in- 

 troduce those improvements in the arrangement and con- 

 struction of the steam-engine, which have rendered it the 

 most useful and economical of all known sources of 

 power. His principal improvement on Newcomen's 

 engine was the use of a separate vessel for condensing 

 the steam, with an air-pump for removing the water of 

 condensation, and the air liberated from the water. But 

 his ingenuity was likewise devoted to the improvement 

 of every detail of arrangement and construction ; and, 

 having found the steam-engine in many respects rude, 

 ineffective, and costly in its working, he left it an appa- 

 ratus, as nearly perfect as any human work can be. 

 Since his time, changes have been made in the arrange- 

 ments of the parts to suit peculiar circumstances of 

 operation, and new forms have been devised for particular 

 purposes ; but in all their leading features, the steam- 

 engines of the present day are essentially the products of 

 Watt's fertile genius. 



The condensing engine, as improved by Watt, is of two 

 kinds : 



Sinyle-acting, where the steam is permitted to press on 

 one side of the piston only, so as to cause it to make a 

 single stroke ; the return stroke being effected by a 

 counterbalance weight. 



Double-acting, where the steam presses alternately on 

 each side of the piston, and its reciprocating movement 

 is converted, by a crank, into rotary motion. 



The single-acting engine is well suited for such an 

 operation as pumping, where the reciprocating movement 

 of the pump-bucket corresponds witli that of the piston. 

 The double-acting engine, again, is adapted for driving 

 machinery. In both these kinds of engine, where the 

 steam is condensed, thre are required a condenser and 

 an air-pump, which we will now describe. 



CONDENSER AND AIR-PUMP. The condenser is 

 a vessel B (Fig. 205), generally made of about th of the 

 capacity of the steam-cylinder, with which it communi- 

 cates by the pipe D. The steam, after actiag on the 



