7T 



STEAM AND STEAM-ENGINE. 



STEAM AND STKAM EN T OINE. 



Watt's improvements. Thin can be accomplished by fottr-punagt 

 raft, originally invented by Leupolil in 1720, and since improved by 

 Brmmnh and other*. The principle of a four-way cock will t>e under- 

 stood by the annexed figure (jig. 5) of the plan of one employed by 

 MSMTS. Mandrfay and Field in their small engine*. 



I. 



A is a portion of the cylinder ; B, the steam-pipe ; c, D, r, three 

 passages, one communicating with the top, another with the bottom of 

 the cylinder, and the third with the condenser ; r in the four-passage 

 cock, which, by turning alternately to the right and left, establishes a 

 communication between one of the former with the latter passage. 

 fig. 6 represents the conical valve with its side apertures, and that 

 ;it the top, by which the steam enters. 



Watt employed flat conical valves for the purpose under considera- 

 tion, which were raised or depressed by cranks acting on a guide-rod at 

 right angles to the plane of the valve, which therefore did not turn on 

 a hinge like the common elarlr-ralrc of a pump. In some of his 

 engines the valves were raised or depressed by toothed sectors acting 

 on a rack in the guide-rod, so that the valve might rise from its seat 

 without altering the parallelism of its plane. Two such valves were 

 mounted in one box, one above the other, the guide-rod of the lowest 

 passing through that of the upper. 



fig. 7 shows the valves of this construction, of the large engine 

 erected by Messrs. Maudslay for the Chelsea Water- Works, i-, part 

 of the cylinder ; p, the piston ; T, the " plug-tree ; " u, the gearing 

 handles, which are struck by the tappets on the plug-tree, and thun 

 open and close the valves V ; s, the steam-passage to the upper and 

 lower parts of the cylinder ; D, the passage to the condenser. 



Fig.?. 



In mott engine! of the present day, however, the slide-mire, OH it is 

 termed, has mperaeded the use of the others, excepting in large 

 pumping engines : a perfectly flat surface slides on another, termi- 

 nating the orifice* which are to be opened and shut; such is the 

 general principle, but the forms and arrangements are too numerous to 

 be mentioned. Fi,. 8 shows a part of the cylinder of an engine with 

 box-iKde valve*, now much used. 



8, the orifice of the steam-pipe ; the (team passes to the upper part 

 of the cylinder at D, the lower passage R being shut off in the position 

 of the valve shown and shaded in the figure ; the slide is moved by 

 the rod R, and it is shown in it* second position in dotted linen, iii 

 which position it will be seen that the steam can then enter l H ,,,,iil, 

 the piston, while the passage p to the condenser is in turn in com- 



munication with the upper part of the cylinder by means of the tube 

 T of the slide. 



Fi*-. 8. 



The characteristic and most valuable part of this principle is this, of 

 making part of the slide act as a pipe to connect the two parts of the 

 cylinder alternately with the condenser. The steam, by pressing on 

 the slide in the common form of slide, enormously increases the friction 

 with the surface against which it acts, and also produces rapid 

 the parts ; this defect is remedied in the box-slide and all others which 

 possess this peculiarity. 



Slide-valves were proposed by Murray, in 1799, but were abandoned, 

 till improved workmanship allowed of their being more perfectly made ; 

 they have been successively improved in principle by Murdoch, Bramah, 

 Millington, Maudslay, and Seward, the slides of the last-named being 

 now much used in marine engines. 



It has been mentioned that the alternate action of the valves in the 

 atmospheric and Watt's engines was produced by pins, or lii/,p<-lf, ad- 

 justed on a rod called tli , suspended from the beam ; as the 

 plug-tree moved up and down with the beam, the tappets stnick the 

 ends of bent levers or cranks, which raised or depressed the valves in 

 proper succession : some of these levers were so formed that the tappet 

 by pressing against them might keep the valve closed (hiring the 

 greater part of the stroke * of the piston, and others required an inter- 

 mediate shorter lever, or claw, to act on the valve-rod ; BO that the 

 whole arrangement was inevitably complicated and cumbrous. But 

 when the Blide-valve superseded Watt's double conical valves, and the 

 steam passages could be opened and closed by the motion of one rod 

 only, connected with the slide, this motion could be readily produced 

 by what is termed an cxcenlric, which for this purpose usually consists 

 of a circular plate of metal, keyed r.ro ni,-imlhi on the shaft of the fly- 

 wheel, and working within a ring attached to the end of a frame in- 

 tended to move a crank directly connected with the slide-rod at its 

 other extremity. As the shaft revolves, the excentric plate imparts 

 an alternating motion to the frame, which, transmitted by the crank, 

 alternately raises and depresses the slide-rod. The principle of the 

 excentric is one of the most valuable of those mechanical contrivances 

 by means of which a continuous circular can be converted into an alter- 

 nating rectilinear motion. 



The "beam" so frequently alluded to, was obviously the readiest 

 mode of connecting the alternating motion of the piston with the 

 pump to be worked, in the atmospheric engine; and owing to the 

 facilities it offers of working the plug-tree aim the three pumps neces- 

 sary in Watt's condensing engines, continued to form a part of the 

 arrangement whether the engine were intended to pump a mine or to 

 drive machinery. The beams of the first engines were made of two 

 or more trees, bolted together to obtain the requisite rigidity, and 

 further strengthened by a kind of truss, as is seen in tin- diagram of 

 Newcomen's engine. But when the art of making heavy iron-castings 

 was perfected, that metal was substituted for wood, to the mm 

 improvement of the engine in every respect. Watt also removed the 

 cumbrous arched heads, which had been previously employed for the 

 purpose of causing the piston-rod to move up and down in the same 

 right line, though connected with the end of the beam, which neces- 

 sarily described an are of a circle, as turning on a fixed centre ; this 

 arrangement implied the use of a flexible chain, to suspend the piston, 

 which might wind round, and unwind from, the arch, but a chain 



* The term stroke i technically mod by engineers to express the whole 

 motion of the pUton from the top to the bottom of the cylinder and back again. 



