The chief objection to two large furnaces instead of three smaller, and to three larger ones instead of four smaller, is the longer grate required to get the requisite area, and to the large amount of dead water between the furnaces at the bottom. There is also to be considered the limit placed by the rules to avoid risk of collapsing by direct crushing of the metal, which often prevents the adoption of the larger furnace with the higher pressures.

It is unusual and certainly most difficult to use plates above 14 inch thick in the construction of a boiler shell, and it is this consideration which fixes the limit of diameter. For this reason when a working pressure of 100 pounds and upwards was required, the large singleended boiler made of iron could not be employed; indeed, 80 pounds was then taken as the limit of pressure for the very large diameter boiler.

It

The Double-ended Boiler has furnaces at both ends with return tubes over them, and is generally tantamount to two single-ended boilers back to back, but with the backs removed. is made up to 16 feet diameter and as much as 20 feet long; but such very large boilers are unusual, partly owing to the want of facilities for moving such a great weight, and partly because the conditions under which such large boilers are possible are limited to very large steamers.

The double-ended boiler is lighter and cheaper in proportion to the total heating surface than a single-ended boiler, and its evaporative efficiency in practice is generally higher. On the other hand, greater care is necessary in designing and in working it. That it is lighter is obvious, and that it is cheaper may be inferred from the fact that there is less material, and less labor consequent on the reduced quantity of material.

The simplest form of this kind of boiler is one in which all the furnaces open into one common combustion chamber; this form, although at one time common enough, is now seldom adopted. The objections to it are, that the bursting of one tube will disable the whole, that the cleaning of one fire causes the efficiency to sink very low on account of the whole being affected by the inrush of cold air, and that unless special means be provided to promote proper circulation, there is a strong tendency to prime.

The next simplest form is one in which opposite furnaces have a combustion chamber in common, that is, it differs from the first by having the combustion chamber divided longitudinally by water spaces. This avoids the chief objections raised against the first form, while retaining its chief advantages, which are, simplicity of construction, by avoiding the flat back of the combustion chambers, with the necessary

stays, etc., and the greatest heating surface within the smallest limits of length. It is often urged against this form of boiler that the tubes are very liable to leakage at their back ends, arising from the rush of cold air against the tube plate when the door of the furnace opposite it is open, causing it to buckle. It sometimes happens that the tubes in this kind of boiler do show a tendency to leak, but it is then generally due to the want of expansion on the part of the first row of stays above the combustion chamber, when they are placed too close to the tubes. If these stays are at least 12 inches above the tubes, so as not to hold the front tube plates too rigidly, then when steam is being got up the expansion of the tubes simply causes the plates to spring very slightly, instead of to start their ends and cause them to leak. The leakage from springing of the tube plate from exposure to cold air can only take place when the combustion chamber is unduly short, and when there is an insufficient number of stays to the tube plates.

This particular form of boiler is very generally used; the evaporative results obtained from it are most satisfactory, and experience does not show it to be liable to more leakage than other boilers. Common care only is required in raising steam, and the opening of fire-doors to check evaporation is a reprehensible practice at all times and for all boilers. A brick semi-partition

FIG. 56.-DOUBLE-ENDED BOILER (FOX'S FURNACES).

in the middle of the combustion chamber will prevent the cold air rushing on to the opposite tube plate, and it acts also as an equalizer of temperature in the combustion chamber at all times. If, however, the combustion chamber is too small, this will only magnify the defect by causing intense local heat, and thus tending to crack the plates.

Another form of double-ended boiler (Fig. 56) has the furnaces at one end with one chamber common to them, and those at the other end with another chamber in common. The boiler is then longer than either of the other forms, and more expensive; the combustion chambers have large flat backs, requiring a very large number of stays, which prevent their being properly cleaned from scale.

The last form, which is by far the most expensive and heaviest, but is still often adopted, is one in which each furnace has an independent combustion chamber. There is little need of description, as it is to all intents and purposes as two single boilers, except that the water and steam are common to the two parts.

Oval Boilers are included under the generic term of cylindrical, as they partake of the principal features of that class. The transverse section is, however, not an ellipse, but is really formed by two semicircles with a rectangle intervening between them. The flat sides thus