56. Furnace.-The usual dimensions of the furnace for such a boiler are for the fire bars, five feet long and four feet wide; giving an area of twenty feet, or one square foot per horse

power.

57. Depth of Water.-To find the requisite depth of water in the boiler, the following rule is used: Take half the difference of capacity between the lower and upper part, and divide it by the area of water surface, then deduct the quotient from the depth of the lower part, and the remainder is the depth of water; measuring from the seating plate of the boiler perpendicularly. Thus:

Capacity of lower part, -347.22

75.43 divided by 100-.7543 quotient ;

which, subtracted from 4.1666, the depth of lower part, gives 3.4123 feet, or 40.9476 inches. Hence the requisite depth of water was three feet five inches, nearly.

58. Flues and Heating Surface. The brick work of the side flues is gathered in two or three inches below the water level, and consequently the side surface is reduced to about 3.5 feet, and the total area for both sides is 3.5×2×20=140 square feet. The area of the ends below the tops of the flues is about 28 square feet, minus the surface covered by the brick arch over the furnace door, which about 3 square feet; and by the brick work of the back end which divides the uptake from the side flue, which equals about two square feet; hence these numbers subtracted from 28 leave 23, and 23+140 =163 square feet of surface; but about one half of this only is effective as heating surface, therefore we have only 81.5 square feet, or 9.05 square yards. But the whole area of bottom is

=

=

effective; and, measuring the curved surface, is about 94 square feet, or 10.4 square yards; consequently, the whole effective surface 10.4+9.05-19.45, or nearly 20 square yards.

=

59. From what has been shown of the twenty horse boiler, and from experiments made by scientific persons, we find that it requires one cubic foot of water to be evaporated per horse power per hour; and one cubic foot of water requires nine square feet or one square yard of heating surface, and one square foot of fire grate. The wagon boiler, as this is called, is much used in the manufacturing districts of England, and is sometimes made of enormous size. It answers only for low pres

sure steam.

60. Tubular Boilers.-Where high pressure steam is required, tubular boilers (so called) are generally made use of, chiefly on account of the shape; the cylindrical being a much stronger form for resisting pressure, either externally or internally, than the wagon shape. The diagram below shows a section of one with an internal flue and split draught; that is, the smoke passes under the boiler, and through the tube, and then divides and returns on each side to the chimney or stack.

would be a nine horse power boiler; but many who are in the habit of making flued boilers, or, as they are commonly (though erroneously) called, tubular boilers, usually consider in their calculations that the diameter of the inside fue or tube is equivalent to so much added to the width of the boiler, the result of which is nearly correct, at least in cases where the boiler is not very long in proportion.

According, therefore, to this rule, the power of the boiler is found as follows:

Horses' power, 11.7, or 11 nearly.

By observation, the evaporation of this boiler only varied from ten to eleven cubic feet of water per hour.

62. The next class of boilers are those used for locomotive engines, of the peculiar construction of which, more will be said under the head of Locomotion. These boilers, or rather boilers constructed on the same principles, are now used with great success on board many of the steam boats their extraordinary power of generating steam being so peculiarly adapted to the present system of using the steam engine (viz: using high pressure steam expansively); as they not only take up such little room, but admit of working up to a considerable pressure, when necessity may require, in so short a time.

63. These boilers were first applied by Mr. R. L. Stevens, in 1837, who has since made several different constructions, all of which answer remarkably well. The plans of several of these are shown in the volume of plates.

64. It would seem, however, that perfection is far from being attained in the art of constructing boilers; for nearly every

new boiler has some supposed improvement over the last, at least those made by the more intelligent engineers. Some manufacturers still use flues, varying in diameter from 8 to 18 inches, of which they have eight, ten, or more in each boiler. Of such construction are the boilers of the steamboat 'North America; and, from the quantity of steam supplied compared with the small quantity of coal consumed, we should pronounce these amongst the best now in use-28.28 cubic feet of steam, of the pressure of fifty pounds per square inch being generated by the combustion of one pound of coal per minute.

65. There is yet another modification of boiler, the credit of which combination is the property of Mr. R. Schuyler. In this boiler (see plate 35), the heat passes off right and left through small tubes; it then enters a long flue, through the top of which it passes into other horizontal flues, whence it makes its escape to the funnel. This boiler produces 888.864 cubic feet of steam, at a pressure of twelve pounds per square inch, with the combustion of 7.879 lbs. of coal, per minute, or 112.8 cubic feet of steam from 1 lb. of coal at the same pressure.

66. The same enterprising gentleman has constructed several boilers since the above was first put in operation, all of which have fully answered his expectations.

67. Although, in the case of the Essex,' he had a very confined room for his operations-that is to say, the height from floor to deck being so limited, as it must necessarily be in boats of such small dimensions as ferry boats usually are-nevertheless, this boiler may with safety be declared to possess the greatest known generative power of any or all the marine boilers that have ever been constructed in this country.

68. Of the boilers used on the Western waters, little can be said, as they are merely cylindrical, and similar to that already described.

69. It is well known, though notice of the fact should not be omitted, that the surface in actual contact, and in close proximity to the fire, is many times more valuable than the surface which is further off; hence, in calculating the generating sur

same.

face, care must be taken to make proper allowance for the Some authors and practical men have defined this proportion at the rate of 3 to 1, 2 to 1, and 5 to 1; the latter probably being nearest to the fact.