14

MODERN STEAM PRACTICE.

of a steam carriage for common roads having become foul withsoot, thus impeding the draught, gunpowder was wrapped up tightlyin a piece of paper and thrown on the fire, and the fire-box doorimmediately shut; a slight explosion took place, sending a cloud ofsoot up the chimney, effectually Clearing the tubes without stoppingthe machine. Of course, it would be somewhat dangerous to carryan explosive mixture about for such a purpose; but, in most cases,this means of Clearing the tubes can be cheaply and most effectu-ally carried out, and there is no danger whatever, provided toomuch gunpowder is not used at once. All vertical self-containedboilers should have air tubes ^ inch in diameter, and spaced about6 inches apart, all round the fire-box, dipping downwards, so thata current of air may mix with the flame and heated gases at aboutthe level of the top of the fuel, thus tending to the prevention ofsmoke; these tubes are screwed into the outside shell and theinside fire-box, and then rivetted over.

There is one objection common to all vertical boilers, namely, thata great portion of the heat passes directly up the chimney withoutdoing duty; to obviate this defect the flame and heated gases aredirected downwards with suitable flues; this plan must have separateflues of fire-brick, with a chimney, and is not so compact an arrange-ment as the multitubular one. The feed-water pipe passes throughthe bottom flues, thus heating the water in its passage to the boiler,and all the flues are easily reached for scaling and cleaning out.

The pot boiler derives its name from thepeculiar pot-like vessel, fitted to, and hangingfrom the top of the fire-box; this sphericalgenerator is introduced so that the lower part,made of copper, receives the full benefit of theflame; the annular space between the pot andthe fire-box is made narrow, thus the flame andheated gases impinge against the sides of thefire-box, and then pass through the small tubesinto the chimney. The ebullition of the water inthe pot is very violent, ejecting the Sediment andpreventing incrustation; the deposit finds its wayto the bottom of the boiler, and is cleaned out by

F B S FirelbS’ t ?pÖt r D A fubes' su ' ta Me sludge doors. The dry uptake can beE,Fire-door. F,Smoke- P ipe. easily removed, and the inside of the boiler in-spected through the man-hole, placed exactly over the pot, in the