CHAMBERS'S INFORMATION FOR THE PEOPLE. 



We must now proceed to describe the principal 

 varieties of steam-engines at present in use. Of 

 these, the one most resembling Watt's engine is 

 called the ' Cornish ' engine, from the fact that it 

 is principally used in the Cornish mines. It is 

 used as a pumping-engine exclusively, like New- 

 comen's, and has no rotary motion. Unlike his, 

 however, the steam, and not the atmospheric 

 pressure, is made to do the work. It has a ver- 

 tical cylinder connected with a beam, like that in 

 Watf s engine ; but the other end of the beam is 

 simply attached direct to the upper end of the 

 pump-rod which descends the shaft. It has a 

 condenser and air-pump like Watt's double-acting 

 engine. Its mode of action is as follows : When 

 the pump-plunger is at the bottom of its stroke, 

 ready for a lift, the piston is at the top of the 

 cylinder. Steam is admitted above the piston of 

 a pressure very much higher than that required 

 to raise the weight of the water and pump-rod. 

 This sets the beam in motion, lifting the water, 

 pump-rods, &c. The steam is very soon ' cut off,' 

 and allowed to expand, and the pressure in the 

 cylinder therefore falls rapidly, as before explained. 

 Some time before the stroke is finished, the force 

 acting on the piston has come to be less than the 

 resistance at the other end of the beam, but the mo- 

 mentum acquired by the latter is sufficient to keep 

 it from stopping suddenly ; and the steam-pressure 

 and momentum are so arranged that the piston is 

 brought gradually to rest just before it touches 

 the bottom of the cylinder. The work of the 



Fig- 13- 



stroke has now been done, and valves are opened 

 which establish free communication between the 

 two ends of the cylinder. The steam being thus 

 able to flow from the upper to the under side of 

 the piston, and offering no resistance to its 

 motion, the weights at the pump end of the beam 

 pull it up to the top of the cylinder again, ready 

 for another stroke. A small volume of steam is 

 always left above the piston to act as a cushion, 

 and prevent the piston striking the cylinder cover. 

 During the descent of the piston, the space 

 beneath it is opened to the condenser in the usual 

 way. 



It is difficult to say why Cornish engines have 

 remained so long in their original form. They 

 are economical of fuel, owing to the great expan- 

 sion used, but the same expansion could be used 

 with many other forms of engine. They are very 

 costly, and extremely heavy and unwieldy, and it 

 seems probable that it is only prejudice which 

 stands in the way of the substitution for them of 

 small engines running at very high speeds, which 



428 



would do the same work with a very much smaller 

 first cost. 



In all the engines we have hitherto mentioned, 

 there has been some part or parts intermediate 

 between the piston-rod and the connecting-rod. In 

 modern engines, however, there is more and more 

 a tendency to discard such intermediate parts, 

 and to make the engine ' direct-acting.' The com- 

 monest type of direct-acting engine is the Hori- 

 zontal, shewn in fig. 13. For all ordinary pur- 

 poses, this type of engine is rapidly superseding 

 every other form of stationary engine. It pos- 

 sesses the merits of having great simplicity and 

 few working parts, and of all these parts being 

 easily accessible to the engine-driver ; and at the 

 same time any required degree of economical 

 working can be attained as well by it as by any 

 form. It will be seen from the engraving that 

 the cylinder and other fixed parts of the engine 

 are bolted down to a 'bedplate' which extends 

 along its whole length. The connecting-rod is 

 attached direct to the piston - rod - head, and 

 through the latter is fixed a piece called a cross- 

 head. The ends of this cross-head are compelled 

 to move in straight lines by working between 

 rigid guide-bars. The other end of the connect- 

 ing-rod is attached, as before, to the crank-pin. 

 The feed-pump is worked from one end of the 

 cross-head, as shewn in the figure. This form of 

 engine was for a long time only used as a non- 

 condensing, or, as it was then called, high-pressure 

 engine, but it is now frequently made with a con- 

 denser. In this case, the bedplate is 

 prolonged behind the cylinder, and on 

 this prolongation a casting is fixed 

 which contains condenser, hot well, 

 and air-pump. The last is horizontal, 

 and worked by a prolongation of 

 the piston-rod through the cylinder 

 cover. 



Two other forms of direct-acting 

 engines have been much used in their 

 time, but are now being rapidly aban- 

 doned, except under special circum- 

 stances. These are called respectively 

 the ' oscillating ' and the ' trunk ' en- 

 gine. The former has rarely been used 

 except for marine engines. In it the 

 crank-shaft is above the cylinder, the 

 piston - rod - head is attached to the crank-pin, 

 and the connecting-rod is dispensed with by 

 allowing the cylinder to vibrate or oscillate on 

 large hollow centres, called trunnions, and so to 

 adapt itself to the various positions of the crank- 

 pin. The steam passes to and from the cylinder 

 through the hollow trunnions. The principle of the 

 trunk-engine is illustrated in fig. 14. 

 In this engine, instead of shortening 

 the distance between the cylinder 

 and shaft by dispensing with the 

 connecting-rod, the same object is 

 attained by omitting the piston-rod. 

 In the figure, aa is the cylinder, b the 

 piston, and cc a hollow cylinder or 

 trunk attached to it, and taking the 

 place of the piston-rod. The con- 

 necting-rod, d, is attached to the 



Fig. 14. 



piston at one end, and to the crank-pin at the 

 other, the trunk, c, being of sufficient diameter 

 to allow for its vibration. This form of engine, like 

 the oscillating, was chiefly used in marine work. 



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