STEAM-ENGINE 



701 



for the first time also the condensation of the steam 

 was made an instantaneous process, instead of a 

 low and gradual one. Newcomen's engine was 

 chiefly used, like all former steam-engines, in raising 

 water. To one end of a beam moving on an axis, 1, 

 was attached the rod, N, of the pump to be worked ; 

 to the other the rod, M, of a piston, P, moving in a 

 cylinder, C, below. The cylinder was placed over a 

 boiler, B, and was connected with it by a pipe pro- 

 vided with a stopcock, V, to cut off or admit the 

 sfiiiu. Suppose the pump-rod depressed, and the 

 pUton raised to the top of the cylinder which was 

 effected by weights suspended at the pump-end of 

 the beam the steam-cock was then turned to cut off 

 the steam, and a dash of cold water was thrown into 

 the cylinder by turning a cock, R, on a water-pipe, 

 A, connected with a cistern, C'. This condensed the 

 steam in the cylinder, and caused a vacuum below 

 the piston, which was then forced down by the 

 pressure of the atmosphere, bringing with it the 

 end of the beam to which it was attached, and 

 raising the other along with the pump-rod. The 

 cock was then turned to admit fresh steam below 

 the piston, which was raised by the counterpoise ; 

 And thus the motion began anew. The opening 

 and shutting of the cocks was at first performed 

 by an attendant, but subsequently a boy named 

 Humphrey Potter (to save, it is said, the trouble 

 of personal superintendence) devised a system of 

 strings and levers by which the engine was made 

 to work its own valves. In 1717 Henry Heighten, 

 an F. R.S. , invented a simpler and more scientific 

 system of ' hand -gear,' which rendered the engine 

 completely self-acting. During the latter part of 

 the time that elapsed before Watt's discoveries 

 changed everything Smeaton brought Newcomen's 

 engine to a very high degree of perfection. As the 

 result of study and experiment he made many 

 improvements in it, in the form of the boiler, the 

 proportions of the cylinder, &c. It was he, too, 

 who invented the cataract, a very ingenious self- 

 acting valve arrangement, which is still used in 

 Cornish engines. In 1725 Leupold invented an 

 engine in which steam of a higher pressure than 

 that of the atmosphere was employed in the 

 cylinder, but hU engine possessed defects that pre- 

 vented its practical use. 



The next essential improvements on the steam- 

 engine were those of Watt, which began a new 

 era in the history of steam-power. The first and 

 most important improvement made by Watt was 

 the separate condenser, patented in 1769. He had 

 observed that the jet of cold water thrown into the 

 cvlinder to condense the steam necessarily reduced 

 the temperature of the cylinder so much that a 

 great deal of the steam flowing in at each upward 

 stroke of the piston was condensed before the 

 cylinder got back the heat abstracted from it by 

 the spurt of cold water used for condensing the 

 steam in the cylinder. The loss of steam arising 

 from this was so great that only about one-fourth 

 of what was admitted into the cylinder was actu- 

 ally available as motive-power. Watt therefore 

 provided a separate vessel in which to condense 

 the steam, and which could be kept constantly in 

 a state of vacuum, without the loss which arose 

 when the cylinder itself was used as a condenser. 

 This device, which now looks simple enough, was 

 the greatest of Watt's inventions, and forms the 

 foundation of his fame. His genius was such that 

 in a few years he changed the steam-engine from 

 a clumsy, wasteful, almost impracticable machine 

 into a nfachine practically the same as that which 

 we now have. The principal improvements since 

 his time have been either in matters relating to 

 the l)oiler ; in details of construction consequent 

 on our increased facilities, improving machinery, 

 and greater knowledge of the strength of materials; 



in the enlarged application of his principle of 

 expansive working ; or in the application of the 

 steam-engine to the propulsion of carriages and 

 vessels. His principal inventions were : ( 1 ) The 

 condensation of steam in a vessel separate from 

 the cylinder, so as to avoid the cooling of the 

 latter; (2) the use of a pump, called an 'air- 

 pump,' to withdraw the condensed water and 

 mixed steam and air from the condenser; (3) the 

 surrounding of the cylinder with a steam-jacket, 

 in order to prevent loss of heat from condensa- 

 tion (these three, with others, were included in 

 the specification of 1769); (4) the use of the 

 steam expansively in the way explained further on 

 in this article (this was invented before 1769, but 

 not published till 1782) ; and (5) the now univers- 

 ally used double-acting engine, and the conversion 

 of the reciprocating motion of the beam into a 

 rotary motion by means of a crank (both these 

 were invented before 1778, the engine being 

 patented in 1782, but the crank having before that 

 date been pirated and patented by another). In 

 1784 Watt also patented and published his parallel 

 motion, throttle- valve, governor, and indicator; 

 all four of which are in substance still used. 



The common mode of employing steam in an 

 engine is by causing it to press alternately on the 

 two surfaces of a movable diaphragm or piston 

 enclosed in a fixed, steam-tight, cylindrical box. 

 In fig. 2 A is the piston and B a section of the box. 

 The piston, by means of 

 a rod, E, passing through 

 the end of the box, is 

 made to communicate 

 motion to the rest of 

 the machinery. The 

 steam is first admitted 

 to one end of the cylinder 

 through an opening or 

 ' port, D, and forces the 

 piston along to the other 



end. The current of steam from the boiler is then 

 allowed to pass into the other end of the cylinder 

 through the opening C, and forces the piston back 

 again to its original position, and so on. But it is 

 obvious that while this return-motion is going on 

 the steam previously admitted at D must be 

 allowed some exit, or the piston could not be forced 

 back. The manner of this exit constitutes the 

 difference between the two principal classes of 

 engines, according as the steam is allowed simply 

 to rush out into the atmosphere or is conducted 

 into a separate vessel, and there 'condensed.' 



The simplest way in which steam can be used in 

 a cylinder is at the same time the most wasteful. 

 It consists in filling each end of the cylinder alter- 

 nately full of steam direct from the l>oiler, and 

 having the full boiler pressure, and thus forcing 

 the piston along in exactly the same way as that 

 in which it would have to be forced were water the 

 fluid used instead of steam. If we imagine the 

 cylinder to have a capacity of 7 cubic feet, then, 

 if it be filled entirely with steam from the boiler 

 at 60 11. absolute pressure, it will contain (about) 

 one pound-weight of steam. The total heat in this 

 pound of steam (al>ove 32 F. ), as given in the 

 table, is equivalent to 1171 thermal units in excess 

 of that possessed by a pound of water at 32 F. 

 When the piston, A, has reached the end of its 

 stroke, the steam contained in the cylinder is thus 

 in itself a great storehouse of work, for each of these 

 thermal units is equivalent to 772 'foot-pounds' of 

 mechanical energy, so that the total represents 

 about 904,000 foot-pounds, of which we shall see 

 later on only about ^ has been utilised during 

 the stroke, leaving $$ untouched. Instead of 

 making any attempt to utilise this huge balance, 

 at the moment when the cylinder is full of steam 



Fig. 2. 



