THE STEAM-ENGINE. 



Before speaking of locomotive and marine en- 

 gines, we may just mention another class not yet 

 alluded to, entirely different in arrangement from 

 those already described. We refer to the class 

 called rotary engines. In these machines we still 

 have a piston and cylinder, although different in 

 construction from the parts bearing those names in 

 ordinary engines ; but the piston- and connecting- 

 rods, crank, fly-wheel, &c. are all dispensed with. 

 The main shaft runs right through the centre of 

 the cylinder, and the pistons (there are generally 

 more than one) form wings, filling up the distance 

 between the shaft and the circumference of the 

 cylinder. Steam, when admitted into the cylinder, 

 imparts either a rotary or vibrating motion to the 

 pistons, in such a way that a continuous revolving 

 motion is given to the shaft An immense amount 

 of ingenuity has been expended on these engines, 

 and the patents taken out for them are innumer- 

 able. This has arisen to a great extent from the 

 erroneous idea that in the reciprocating motion, 

 and the consequent ' dead points/ much power is 

 lost. We have not room here to shew why this is 

 not the case ; the argument will be found at length 

 in most modern works on the steam-engine. The 



friction and liability to wear in almost all rotary 

 engines hitherto constructed, as well as the diffi- 

 culty of working them expansively, has prevented 

 their coming into use except on a very small 

 scale. 



In locomotive engines, it is necessary that the 

 whole machinery should be compressed into the 

 smallest possible bulk, and this necessity is the 

 cause of their principal peculiarities. The engine 

 itself is much the same as an ordinary horizontal 

 engine ; it has two cylinders side by side placed 

 near the front of the engine, and the piston and 

 connecting-rods are arranged exactly as described 

 in fig. 13. The cylinders are sometimes placed 

 inside the main framing, which runs the whole 

 length of the engine, and sometimes outside it, 

 each plan having certain advantages. Locomotive 

 engines are always non-condensing, because of 

 the impossibility of their carrying the amount of 

 water that would be required for condensing, as 

 well as the great additional dead-weight which a 

 condenser with its necessary pumps, &c. would 

 add.* The leading feature of a locomotive is its. 

 boiler, which we will now proceed to describe 

 by the aid of fig. 15, which is a section of an 





Fig. 15. 



' inside cylinder' goods locomotive belonging to 

 the Midland Railway Company. 



At the back of the engine (the end nearest the 

 foot-plate) is the fire-box a, a rectangular box, the 

 bottom of which is formed by the grate b. Fuel 

 is introduced by the door c. The fire-box is in- 

 closed in a casing d, and the space between them, 

 forming part of the boiler, is constantly filled with 

 water. This water-space communicates with the 

 barrel of the boiler, e, which is simply a long 

 wrought-iron cylinder, made perfectly steam-tight. 

 From the back of the fire-box, numerous small 

 tubes traverse the boiler (through the water), and 

 convey the heated air and gases from the furnace 

 to the smoke-box, yj which is an iron box at the 

 front of the barrel, forming the base of the funnel 

 g. In this particular engine there are about 170 

 tubes, each 2 inches diameter, and over n feet 

 long, presenting to the water altogether a heating 

 surface of almost 1000 square feet To obtain 

 this enormous surface, which is needful for the 

 proper working of the engine, is the reason why 



this form of boiler is used. On the centre of the 

 barrel is a dome, h, and from near the top of 

 this dome the steam-pipe, k, is led away. By 

 this means the steam is obtained less mixed 

 with spray than it would have been had it 

 been taken only from the top of the barrel. 

 / is the regulator, which, by means of a handle 

 worked by the driver, governs the amount of steam 

 allowed to go to the cylinders. On the top of the 

 dome are two spring safety-valves, m, placed side 

 by side. The engine is fitted with reversing-gear 

 worked by a lever on the foot-plate, but the small 

 scale of the engraving does not permit these to be 

 shewn. The cylinders discharge their steam 

 through pipes which meet in the centre of the 

 engine, in the vertical blast-pipe, p. It is, of 



* On the Metropolitan (Underground) Railway the steam is 

 condensed for short periods at a time by discharging it into a 

 tank, but it is obvious that such a plan is only applicable to a very 

 limited extent, as the water would soon get heated so much as to 

 prevent useful condensation. The object in this case, too, is not 

 so much to get rid of back-pressure, as to avoid discharging steam 

 in the tunnels. 



439 



