STEAM. EN' 



..37 



Tin-: MT:A.YI-KN<;INE, II. 



HER SCPABATK CONDENSER BINOLB-ACTINO KNOINE. 



WK inuMt now pass on to the consideration of tho mean* by 



whirli 1)1.- steam is made- t. produce a mechanical ffln-i. An 

 iron ti, diameter, called tho cylinder, U employed for 



accompli.- 1 u n -.r tin.-,. Tin- inside of this is turned so aa to be on 

 true an possible, and a solid plug or piston is made to fit it 

 BO tightly aa to prevent the steam pawing, but at 

 .mi' time .it, freedom to allow of its 



moving up and down in the tube without a wasteful 

 expenditure of force. Both ends of tho cylinder :m< 

 oloaod by dines. Let A (Fig. 6) represent tho piston, 

 and r the cylinder in which it moves. Also let i> 

 and E be two apertures by which tho steam can bo 

 admitted to tho uppor and lower ends of tho cylin- 

 der r. -;.. -lively. If the piston be at tho bottom, 

 and tin- steam be admitted at , while n remains 

 open to tho air, the piston will be raised to the top 

 of tho cylinder, tho air above A escaping at D. Now 

 let tho steam be turned off from E, and this aper- 

 ture l>o left open ; tho weight of A will then cause it 

 to fall, driving out the steam before it. If, instead 

 of this, the steam be made to act through D on 

 tho uppor side of the piston, it will bo driven down 

 with much greater force. 



It is clear, then, that if we have some way of allowing the 

 steam to act first on one side of the piston and then on the 

 other, we can make it travel alternately from end to end of 

 tho cylinder with a greater degree of force. 



Instead of this wo may drive the piston up to the top of the 

 cylinder by tho steam, and then, closing the aperture E alto- 

 gether, condense the steam which fills the cylinder by applying 

 cold to it. A vacuum will thus 

 be produced, and the piston 

 will be forcibly driven down. 



As yet, however, wo have 

 no mode of imparting the mo- 

 tion of the piston to any me- 

 chanism outside the cylinder. 

 This, however, is not a great 

 difficulty. A piston-rod (B) is 

 firmly attached to the piston, 

 BO as to be at right angles to 

 it. An aperture is then made 

 in the upper cap of the cylin- 

 der, through which this rud 

 may pass ; and to guard 

 against the waste of steam by 

 escape through this aperture, 

 a "stuffing-box," c, is placed 

 there. This is packed with 

 hemp, or some similar mate- 

 rial, saturated with oil or 

 tallow, and thus a steam-tight 

 contact is produced. 



A ring is sometimes turned 

 in the edge of the piston, which 

 is packed in a similar way 

 with hemp, but what is known 

 as a metallic packing is more 

 commonly employed. In this 

 the piston consists of metallic 

 ring* placed one above tho 

 other, and divided into seg- 

 ments, the joints in one disc being midway between those above 

 and below. Those segments are pressed outwards by spiral 

 springs, and thus wear so as exactly to fit the cylinder. Very 

 great perfection is now attained in tho manufacture of these. 

 pistons. 



Wo then see that there are two ways in which the piston 

 may be moved up and down in the cylinder. We may either 

 admit tho steam at one end only, and thon condense it so as to 

 produce a vacuum, into which the pressure of the air will drive 

 the piston, or we may admit the steam alternately at each end, 

 and thus drive the piston in each direction. These two classes 

 are respectively distinguished aa single-acting and double-actinj 

 wnjrines. 



152 >'.E. 



Fig. 7. 



The former wen first employed the engine* onmfaHiif of 

 cylinder and piston, and an arrangement by which steam oonld 

 be admitted below the piston. When this was driven to th 

 t .].. tho steam was shut off and the cylinder cooled by tho 

 .'ion of cold water to it* external snrfftee. This con- 

 : the steam, and thus created a vacuum, and the prs 

 Huro of tho air on the upper side of the piston forced it down. 

 Those engine* wore, however, Tory imperfect, an it took son* 

 little time to condense tho steam, and at the rmmt 

 time there was a great waste of heat. The cylinder 

 I had each time to be cooled below the boiling-point, 



or otherwise much of tho steam remained onoon- 

 densed, so that the piston could nut reach the 

 bottom. 



One day, in repairing an engine of this clasn, th* 

 piston was observed to descend much more rapidly 

 than usual, and on inquiry it was found that, owing 

 to a leak, cold water wac allowed to enter the cylin- 

 der, instead of being merely applied externally. 

 This answered so much better, that arrangement* 

 were at once made for injecting cold water below 

 the piston, in order to condense the steam, and suit- 

 able valves were provided for the escape of tbs 

 water and the air which always accompanied it ii. 

 email portions. There was still, however, a great 

 loss by the cooling of the cylinder at every stroke. 

 At length, however, just about a hundred years ago, James Watt, 

 the celebrated engineer, directed his attention to the subject, 

 and endeavoured to find some modo of preventing this waste of 

 heat. At length the idea occurred to him, that if he opened a 

 communication between the cylinder and another vessel quite 

 void of air or vapour, tho steam would immediately diffuse 

 itself through the two ; and if this second vessel were kept at a 



low temperature, it might all 

 be fully condensed without 

 lowering the temperature of 

 the cylinder. 



Experiment soon convinced 

 him of the benefits of this idea, 

 and the introduction in this 

 way of a separate condenser 

 was one of the greatest im- 

 provements ever effected in 

 the steam-engine. This was 

 patented by Watt, and from it 

 he received a large income for 

 some time. Many other im- 

 provements, which added in no 

 small degree to the efficiency 

 of the engine, were likewise 

 effected by him. To some of 

 these we shall have occasion 

 to refer. 



We shall now be able to 

 understand the construction 

 of the single-acting engine, a 

 view of which we give in Fig. 

 7. For most purposes this 

 form has now been superseded, 

 but it is still frequently em- 

 ployed for pumping the water 

 from mines. 



p is the piston, which works 

 in the cylinder A, and T is 

 the steam-pipe by which com* 



uuuiicaLion is made with the boiler. A suitable valve is pro- 

 vided in this, so that the steam may bo shut off at pleasure 

 when tho engine is to be stopped. B B is the beam which oscil- 

 lates on bearings fixed in the masonry ; at its ends are large 

 wooden arcs with chains fixed to them, the object of which is to 

 allow the piston-rod to work vertically. Were it jointed directly 

 to the end of the beam it would bo swayed from side to side, 

 and the packing of the piston and stuffing-box would be de- 

 stroyed. To the further end of the beam a counterpoise, Q, is 

 placed; this is sufficiently heavy to draw the piston to tho 

 upper end of the cylinder after it has been forced down by 

 the pressure of the steam on its upper surface. The pump- rod* 

 are attached below Q. 



