tinder the head of "plain" boilers come all ordinary cylindrical boilers, with or with out internal furnaces, horizontal or vertical. They are the cheapest and simplest which can be made, and, if properly proportioned, possess a considerable evaporative efficiency. When it is necessary, however, to economize fuel, or space, or both, " multitubular" boilers are used. These derive their name from the fact that in them the Came and gases. of combustion are made to pasS through a great number of small tubes (surrounded' by the water) on their way to the chimney. The steam-generating power of a boiler depends greatly on the extent of surface which it presents to the flame, and it is obvious that a great number of small tubes present a much larger surface than one large tube occupying the space of them all. Thus, with the same heating surface, a multitubular boiler will occupy much less space than a plain one, and at the same time the efficiency of its surface is found to be greater. It is, however, necessarily more expensive and more liable to get out of order. Tubulous boilers differ from multi tubular boilers in not only containing tubes; but consisting of them, end having no large cylinders whatever. Their chief 'advantages are (a) their great strength, for it is easy to make a wrought-iron tube strong enough to withstand pressures far higher than any at present in use; and (b) the peculiarity, that if any accident happens, or explosion occurs, it will only be to one tube at a time, and not to an immense boiler shell (as with the common boiler), and its evil consequences will thus be greatly reduced. For this reason tubulous boilers are often called "safety" boilers. It will he readily understood that there is no distinct line of demarkation between the three classes of which we have been speaking, but that on account of the immense variety of boilers which have been designed and constructed, those of one class pass through gentle gradations into those of the next.
The commonest form of boiler for factories, etc., is that known as the Cornish, and shown in fig. 1. It consists simply of a cylindrical shell, a, a, inclosing a much smaller cylinder, f, f, called a flue. The ends of the fine are open, but the space be tween it and the shell, which contains the water, is of course closed up and made steam-tight. The fire-grate, d, is in the interior of the flue, and at the end of it is a brick bridge, c, made so us to cause the flame to impinge on the upper side of the fine. The boiler is set in brick-work; and the flame, pass ing out at the back end of the flue, is made to traverse the whole length of the boiler twice through brick flues before pasLing away to the chimney-, The Cornish boiler has often two internal, flues or tubes, which is a much' more advantageous construction than that shown in lig. 1. The Galloway boiler (called after its inventor) is a very excellent modification of the Cornish, which in outward appear ance it exactly resembles. It has two furnaces, but these join together in one chamber just behind the bridges, and the gases are made to pass through a space considerably narrowed by side pockets projecting inwards in order that they may be well mixed. From this point to the back of the boiler there is just one flue, made oval in section, and crossed by a considerable number of vertical taper tubes, which form a direct communication between the water beneath and that above the flue. These tubes (called " Galloway
tubes") both promote circulation and strengthen the flue. Multitubular boilers of many kinds are used, both for stationary engines and other purposes, but the largest number of those constructed are certainly for steamers, and a common type of marine boiler is shown in fig. 2. The shell, a, a, is cylindrical, and contains one or more cylindrical furnaces; c is the fire-grate; d, a brick bridge; e, a combustion chamber or thane-box; f, the tubes through which the flame passes back to the front of the boiler; and g, the smoke-box at the base of the funnel, h.
The line lc, k, shows the level of the water in the - hr,t•a ship it is of course an object to take up as little space as possible with boiler and machinery, and at the same time to have boilers which shall use as little coal as' possible, both because of the saving in cost, and because of the sav ing in the room taken up by coal. For all these reasons, mtr.ine boilers are invariably multitubular.
The varieties of vertical boilers are as numerous as those of horizontal. When dirty witer, or water containing 114, 111111 11 111,111U111/, .14," 11(11.7 411 1117, used (as e.g. in steam-cranes quently). they are of the simplest stniction, with nothing but an inner lire-bpx and an outer .shell (both cylindrical), the space between theta being- tilled with water all round and over the top of the fire-box.
If clean water can be had, however, and it is desired to be at all economical of fuel, some kind of multitubular vertical boiler must be used, and of these probably the best is that known as the Meld boiler, and shown in fig. 3. The peculiarity of it consists In the tubes, which are closed at the bottom, and hang down from the top of the fire-box over the grate bars, and contain inner tubes of much smaller diameter. The latter are intended to aid the circulation of the water, which passes down the inner tube and up again through the annular space around it, where, beimg most exposed to the action of time flame, it is hottest. Of the different varieties of tulailons boilers, those manufactured by Messrs. Howard of Bedford have found most favor; but so far as can be said in the absence of any extended experience as to their working, Sinclair's patent. boilers seem to be even more satisfactory. They consist of horizontal wrought-iron welded tubes placed in vertical rows, each row being connected at each end with a vertical tube; also of wrought-iron and of larger diameter. In order that the horizontal tubes may be properly fixed in the. vertical ones, a hole must be provided in the side of the latter, opposite the mouth of each of the former. That these holes may be kept tight at any pressure of steam, the ingenious device is adopted of closing them with taper plugs put in front the inside, so that the pressure of steam keeps them shut, and this higher the pressure the less possibility of leakage there is. Locomotive boilers are always multitubular, for much the saute reasons as marine boilers. The boiler of a single locomotive often contains 1300 or 1800 sq. ft. of heating surface, and occasionally as much as 2000 sq. feet.