left between the semi-cylinders require staying, the first rows being at the commencement of the flat. There are both single- and doubleended oval boilers, which for pressures under 80 lbs. may be made both simply and economically to very large sizes, as the thickness of shell plate depends on the diameter of the cylindrical part. Two very large furnaces may be thus fitted into a cylindrical part of comparatively small diameter, sufficient heating surface being obtained by giving the requisite height. This form is most convenient when the boilers have to be stowed fore and aft, when the diameter is limited by the breadth of the ship between the stringers.

Dimensions of a Boiler.-The amount of grate area is the consideration which chiefly affects the choice of dimensions of boiler, and to a very large extent the number and form of the boilers also are governed by it. The rectangular boiler can be made of any breadth without in any way affecting its length or height, so that the number of furnaces can be settled arbitrarily, and any addition to the number only means some additional breadth. In small ships with the boiler athwartships and fore and aft stoking, the breadth of the ship does place a limit to the breadth of boiler, even when rectangular, but it seldom operates so as to seriously interfere with the boiler arrangement. The cylindrical

boiler is not so elastic in the hands of the designer; to increase the number of furnaces in it the diameter must be increased, which means that both breadth and height are affected. If two furnaces of 40 inches diameter be the limit for a boiler 10 feet in diameter, that there may be adequate heating surface, and 14 feet is the suitable diameter for three furnaces of 40 inches diameter, the grate bars being of the same length in both cases, the increase in boiler capacity is 96 per cent. for an increase of 50 per cent. of grate. The smallest diameter of shell into which three 40-inch furnaces can be fitted so as to give adequate heating surface, is 13 feet 6 inches, which is an increase in capacity of 82 per cent. over the boiler 10 feet in diameter. Four 40-inch furnaces require a shell of at least 16 feet diameter, which means an increase of 156 per cent. to obtain 100 per cent. increase of grate. To arrange four 40-inch furnaces so as to be convenient for stoking, a shell of 17 feet diameter is required, which means an increase of 189 per cent. over the shell of 10 feet diameter; if, instead of increasing the number of furnaces by increasing the diameter of shell, the number of shells be increased, the space occupied is considerably in excess of the direct ratio of grate areas.

It is true that to some extent increase of grate area may be obtained by increasing the length

of furnace, but the efficiency of a grate in practice is nearly inversely as its length; for a long grate cannot be nearly so well attended to as a short one, nor is the air supply either under or over the bars so good with a long furnace; since the area of section at the mouth, with the same diameter of furnace, is the same whether the bars be short or long.

Area of Fire Grate.-The area of fire grate required for the evaporation of a certain weight of steam depends on the quantity and quality of the fuel burned on it; the quantity of coal is generally dependent to a large extent on the quality. It may be assumed that one pound of good steam coal will evaporate 10 pounds of water in the ordinary marine boiler, 7 pounds in a locomotive boiler, as fitted to torpedo boats, when not being forced, and 6 pounds when forced to the utmost; also that in the mercantile marine, where the coal is only of average quality, 8 to 9 pounds is a fair result, and 6 to 8 pounds only can be obtained with the coal supplied in some foreign ports. The quantity of coal burnt on a square foot of grate per hour with natural draught is about 20 pounds, under favorable circumstances; with good stoking and very good draught as much as 25 pounds may be consumed; but under ordinary circumstances only 15 pounds should be supplied to obtain complete combustion and economical results.

From this it will be seen, (1) that the greatest weight of steam evaporated per square foot of grate per hour, under the most favorable circumstances, is 10x25, or 250 pounds; (2) that with bad fuel and economical stoking it may be only 6x15, or 90 pounds; (3) that with fairly good fuel and favorable circumstances it may be 9x20, or 180 pounds, and (4) that with fairly good coal and careful stoking about 150 pounds may be expected. In practice, therefore, for trial trips with choice coal and picked stokers, calculations may be based on an evaporation of 250 pounds; for mail steamships using good coal, calculations should be based on an evaporation of 150 pounds; and if a ship is going to trade in the East or localities where inferior coal is to be used, the boilers should be designed on the assumption of an evaporation of only 100 pounds of water per square foot of grate.

If the weight of steam required per hour for a given engine be calculated, and divided by one of these numbers, the result will be the number of square feet required.

If the draught be increased by artificial means, the quantity of fuel consumed per square foot of grate may be as high as 100 pounds per hour, with an air pressure of 6 inches in the stokehole; and 50 pounds with only 2 inches, the corresponding evaporations being 570 pounds and 350 pounds per square foot of grate.

The consumption of fuel per I. H. P. per hour for engines working at full power is 4 pounds, with surface-condensing expansive engines, using steam of 30 pounds pressure above the atmosphere; 34 to 31⁄2 pounds with similar engines of best make and large size; 234 pounds with compound naval engines when forced, and 24 to 21⁄2 pounds when of moderate size and working at two-thirds power; 24 pounds with compound engines of moderate size and as generally fitted in the mercantile marine when working at full speed; 2 pounds with mercantile compound engines well designed and carefully worked at sea full speed; 134 pounds with large compound engines as fitted in modern mail steamers when working at sea full speed under favorable circumstances; 11⁄2 pounds with good triple expansion engines using coal of good quality, and 13 pounds when ordinary steam coal is used; the consumption of water with these engines being about 121⁄2 lbs.; the consumption in torpedo boats is 31⁄2 to 4 pounds when working nearly full speed.

Assuming the consumption of coal to be 12 pounds per I. H. P. per hour, and the grate to burn 15 pounds per square foot, there should be O. I square foot of grate per I. H. P. If the sea full speed I. H. P. of a merchant ship be multiplied by 0.1, the result is the grate area required for that power.