THE STEAM-ENGINE. 



in most cases where space and weight are not 

 matters of vital importance. It consists simply of 

 a cylindrical shell, aa, inclosing a much smaller 

 cylinder, ff, called a flue. The ends of the flue 



Fig. 2. 



are open, but the space between it and the shell, 

 which contains the water, is of course closed up. 

 The fire-grate, d, is in the interior of the flue, and 

 at the end of it is a brick bridge, c, made so as 

 to cause the flame to impinge on the upper side 

 of the flue. The boiler is set in brickwork ; and 

 the flame, passing out at the back end of the flue, 

 is made to traverse the whole length of the boiler 

 twice, through brick flues, before passing away to 

 the chimney. Each boiler, when large enough in 

 diameter, has two furnaces instead of one, which 

 admits of a system of alternate firing, and con- 

 duces greatly to economy. In these boilers, the 

 shell requires no stays, but the flat ends have to 

 be strengthened by connecting stays, either to the 

 shell, or from end to end. Cornish boilers are 

 often strengthened, and their efficiency at the 

 same time increased, by the insertion of cross 

 tubes, called (after their inventor) ' Galloway ' 

 tubes. Mr Galloway's patent boiler is perhaps 

 the most perfect modification of the Cornish boiler 

 in use. It is similar in external appearance to a 

 Cornish boiler, and has two internal furnaces. 

 These, however, join in one just behind the 

 bridges, forming a combustion chamber, where 

 the gases are well mixed. The remainder of the 

 length of the boiler is occupied by an elliptical 

 flue fitted with Galloway tubes. The elliptical 

 form is less strong of itself than the circular, but 

 the cross tubes more than compensate for this. 



In some boilers for stationary engines the flues 

 terminate in a combustion chamber, and the gases 

 are compelled to traverse a large number of hori- 

 zontal tubes filling the space between the back of 

 the chamber and the back of the boiler, in the 

 same way as the tubes in the locomotive boiler 

 hereafter to be described. These are called 'multi- 

 tubular ' boilers ; they are very efficient steam 

 generators, and take up less room than Cornish 

 or Galloway boilers of equal power. Room, how- 

 ever, is not usually a very important object in such 

 cases, and the greater cost of these boilers has 

 prevented their coming into extended use. 



There are very many patents for ' safety ' boilers, 

 the principle of whose construction is that they 

 consist of a number of small pieces instead of one 

 large one, so that the danger from explosion is 

 reduced to a minimum, as any excessive strain 

 which might blow up a large boiler, would relieve 

 itself by simply breaking one of the pieces, which 

 would be attended by inconvenience, but not with 

 danger. The best known among these is perhaps 

 Howard's boiler, but the public set their compli- 

 cation and liability to get out of order against 

 their safety, and they do not seem to be making 



much way. A proper system of periodical inspec- 

 tion and cleaning should reduce and to a great 

 extent has reduced the risk of boiler explosions 

 to a minimum. 



The peculiar form adopted in the boilers of 

 locomotive engines will be found fully described 

 further on. 



In steam-vessels, an economical boiler is a 

 necessity, as well as one which does not take up 

 much space, and the multitubular form is there- 

 fore always adopted. 



Fig. 3 shews an 

 elevation and sec- 

 tion of a marine 

 boiler as made at 

 present. As high 

 pressures are now 

 constantly used in 

 steamers, the shell 

 aa is made cylin- 

 drical, b is the fur- 

 nace, c the fire- 

 grate, and da. brick 

 bridge ; e is a com- 

 bustion chamber 

 or flame-box ; f, /, Fig. 3. 



the tubes through 



which the flame passes back to the front of the 

 boiler ; and g, the smoke-box, communicating with 

 the funnel h. The line kk shews the ordinary 

 level of the water in the boiler. In such a boiler 

 as this, all the flat surfaces, such as the back of 

 the shell and flame-box, have to be strengthened 

 by stays. 



Boilers in the time of Newcomen and Smeaton 

 were made of cast-iron, the only material available 

 at the time, but one which was quite unsuitable 

 for the purpose of resisting pressure. The inven- 

 tions of the unfortunate Henry Cort in 1783, 

 however, coming close on those of Watt, sup- 

 plied the means of making wrought-iron boilers 

 just when it became absolutely necessary to do so. 

 Since that time, boilers have always been made 

 of wrought-iron plates fastened together with 

 rivets. 



In order to induce such a draught through the 

 furnace as to supply the fuel with sufficient air for 

 its perfect combustion, the flues of all boilers com- 

 municate with a chimney or funnel. In certain 

 cases in which it is not possible to make a chim- 

 ney high enough to give sufficient draught, an 

 artificial current has to be induced, as by the 

 steam-blast in the locomotive. 



Having mentioned the principal varieties of 

 boilers now in use, we proceed to describe their 

 principal appendages or 'mountings.' First among 

 these comes the safety-valve, the duty of which is 

 to allow the steam to escape when it reaches a 

 certain pressure, and thus to prevent undue strain 

 on the boiler-plates. Fig. 4 shews in outline the 

 principle of this valve : 00. is a dome on the top 

 of the boiler-shell ; b is a conical brass valve resting 

 on a seat, which has been smoothly bored for its 

 reception ; e is the fulcrum of a long lever ef, from 

 which at d there is a projection resting on the top 

 of the valve. The end,/ of the lever is held down 

 against the steam-pressure by the spring h. The 

 pull exerted by this spring can be adjusted by a 

 nut at/ and it is fitted with a brass case, and an 

 index so arranged as always to point to a figure 

 which indicates at what pressure per square inch 



