between tube and tube was J of an inch. The number of tubes vary in dif- 

 ferent engines, some having so many as one hundred and fifty, while the num- 

 ber in some is less than ninety. The evaporating power of an engine greatly 

 depends on the proper number and magnitude of its tubes ; and the experience 

 which engineers have had on railways have led them gradually to increase the 

 number of tubes, and diminish their magnitude. In the Rocket, already men- 

 tioned as having gained the prize on the opening of the Liverpool and Man- 

 chester railway, the number of tubes was twenty-four, and their diameter three 

 inches ; but in all the engines subsequently made their number was augmented, 

 and their diameter diminished. The practical inconvenience which limits the 

 size of the tubes is their liability to become choked by cinders and ashes, 

 which get wedged in them when they are too small, and thereby obstruct the 

 draught, and diminish the evaporating power of the boiler. The tubes now in 

 use, of about an inch and a half internal diameter, not only require to be cleared 

 of the ashes and cinders, which get fastened in them after each journey, but 

 it is necessary throughout a journey of any length that the tubes should be 

 picked and cleaned by opening the fire-door at convenient intervals. 



When tubes fail, they are usually destroyed by the pressure of the water 

 crushing them inward : the water enters through the rent made in the tube, 

 and flowing upon the fire extinguishes it. When a single tube thus fails upon 

 a journey, the engine, notwithstanding the accident, may generally be made to 

 work to the end of its journey by plugging the ends of the broken tube with 

 hard wood ; the water in contact with which will prevent the fire from burning 

 it away. 



The tubes act as stays, connecting the ends of the boiler to strengthen 

 them. Besides these, there are rods of wrought iron extended from end to end 

 of the boiler above the roof of the internal fireplace. These rods are repre- 

 sented at o in their length in fig. 67, and an end view of them is seen in fig. 

 72. The smoke-box F, figs. 67, 74, containing the cylinders, steam-pipe, and 

 blast-pipe, is four feet wide, and two feet long. It is formed of wrought iron 

 plates, half an inch thick on the side next the boiler, and a quarter of an inch 

 elsewhere. The plates are riveted in the same manner as those of the fire-box 

 already described. From the top of the smoke-box, which, like the fire-box, 

 is semi-cylindrical, as seen in elevation in fig. 73, and in section in fig. 74, 

 rises the chimney G, fifteen inches diameter, and formed of \ inch iron plates, 

 riveted and bound round by hoops. It is flanged to the top of the smoke-box, 

 as represented in fig. 74. Near the bottom of the smoke-box the working 

 cylinders are placed, side by side, in a horizontal position, with the slide valves 

 upward. In the top of the external fire-box a circular aperture is formed 

 fifteen inches in diameter, and upon this aperture is placed the steam-dome T 

 figs. 67, 71, 72, two feet high, and attached around the circular aperture by a 

 flange and screw secured by nuts. This steam dome is made of brass f inch 

 thick. In stationary boilers, where magnitude is not limited, it has been 

 already explained, that the space allowed for steam is sufficiently large to 

 secure the complete separation of the vapor from the spray which is mixed 

 with it when it issues immediately from the water. In locomotive boilers 

 sufficient space cannot be allowed lor this, and the separation of the water 

 from the steam is effected by the arrangement here represented. A funnel- 

 shaped tube d', figs. 67, 72, with its wide end upward, rises into the steam- 

 dome, and reaches nearly to the top of it. This funnel bends toward the back 

 of the fire-box, and is attached by a flange and screws to the great steam-pipe 

 S, which traverses the whole length of the boiler. The steam rising from the 

 boiler fills the steam-dome T, and descends in the funnel-shaped tube d'. 

 The space it has thus to traverse enables the steam to disengage itself almost 



VOL.. II. 35 



