416. _Q._--Is any inconvenience experienced from the intense heat in a locomotive furnace?
_A._--The fire bars in locomotives have always been a source of trouble, as from the intensity of the heat in the furnace they become so hot as to throw off a scale, and to bend under the weight of the fuel. The best alleviation of these evils lies in making the bars deep and thin: 4 or 5 inches deep by five eighths of an inch thick on the upper side, and three eighths of an inch on the under side, are found in practice to be good dimensions. In some locomotives a frame carrying a number of fire bars is made so that it may be dropped suddenly by loosening a catch; but it is found that any such mechanism can rarely be long kept in working order, as the molten clinker by running down between the frame and the boiler will generally glue the frame into its place. It is therefore found preferable to fix the frame, and to lift up the bars by the dart used by the stoker, when any cause requires the fire to be withdrawn. The furnace bars of locomotives are always made of malleable iron, and indeed for every species of boiler malleable iron bars are to be preferred to bars of cast iron, as they are more durable, and may if thin be set closer together, whereby the small coal or coke is saved that would otherwise fall into the ash pit. The ash box of locomotives is made of plate iron, a quarter thick: it should not be less than 10 in. deep, and its bottom should be about 9 in. above the level of the rails. The chimney of a locomotive is made of plate iron one eighth of an inch thick: it is usually of the same diameter as the cylinder, but is better smaller, and must not stand more than 14 ft. high above the level of the rails.
417. _Q._--Are locomotive boilers provided with a steam chest?
_A._--The upper portion of the external fire box is usually formed into a steam chest, which is sometimes dome shaped, sometimes semicircular, and sometimes of a pyramidical form, and from this steam chest the steam is conducted away by an internal pipe to the cylinders; but in other cases an independent steam chest is set upon the barrel of the boiler, consisting of a plate iron cylinder, 20 inches in diameter, 2 feet high, and three eighths of an inch thick, with a dome shaped top, and with the seam welded and the edge turned over to form a flange of attachment to the boiler. The pyramidical dome, of the form employed in Stephenson's locomotives, presents a considerable extent of flat surface to the pressure of the steam, and this flat surface requires to be very strongly stayed with angle irons and tension rods; whereas the semiglobular dome of the kind employed in Bury's engines requires no staying whatever. Latterly, however, these domes over the fire box have been either much reduced in size or abandoned altogether.