All About Engines

7& All About Engines 92j per cent, of the heating surface is in the tubes. The lower end of each rear header communicates with a mud drum, in which sediment is deposited. By means of firebrick baffle-plates, fixed at right angles to the tubes, the hot gases are made to pass upwards between the tubes in the front half, then downwards, and finally upwards again, before they reach the flue leading to the chimney. The tubes being of small diameter (3 or 4 inches), the water in them is heated very quickly—in fact, some of it is converted into steam. And as the mix- ture of hot water and steam is much lighter, bulk for bulk, than the cooler water in the drum, there is a constant and rapid flow of water through the in- clined tubes, up the front headers, and into the drum itself. This circulation is very effectively shown by the illustration on Plate 3. The relatively small size of the steajn drum, the strength of the “ dished ” ends, and the uniformity of tempera- ture produced by rapid circulation render stays unnecessary. In the headers, opposite the ends of each tube, is a hand-hole and cover, which admits of each tube being cleaned or replaced with a minimum of trouble, so that the boiler can be kept in good condition, and repairs effected cheaply and expeditiously. The grate area is a large proportion of the floor space, so that, compared with other types which have been considered, more water can be evaporated per pound of fuel without forcing. It is easy and cheap to transport because none of the parts are of excessive