with bituminous coal almost instantaneously, the rate depending on the percentage of fixed carbon.

For the combustion of one pound of carbon 2.66 pounds of oxygen are necessary, and as the air contains only 23 per cent. of oxygen, it follows that 11.6 pounds of air are necessary for the combustion of one pound of carbon.

The air required for combustion in a boiler furnace has to pass through the spaces between the grate bars, and the layers of fuel on them, the rapidity with which it passes through depending on the intensity of the draft and condition of the fire.

When the fuel is supplied in too great a quantity, or the supply of air is insufficient, the carbonic acid, formed in the lower layers of the fuel, takes up another portion of carbon in the upper layers, and forms carbonic oxide or carbon monoxide, which passes through the boiler unconsumed, and frequently re-ignites at the top of the funnel, where it comes into contact with sufficient air to enable its combustion to be completed. Thus, flaming at the top of the funnel or in the flues beyond the boiler, is generally a sure sign of unsatisfactory conditions of combustion.

Anthracite coal, and coke, may be called comparatively slow combustion fuels, and to provide that a certain quantity shall be consumed for a given size of boiler, either the grate surface must be increased, as compared with bituminous coal, or the intensity of the draft-in other words, the velocity of the air supply-must be increased. From this arises the fact that when burning anthracite coal in a boiler furnace proportioned for bituminous coal, either an extra high funnel is required, or an artificial method of intensifying the draft commonly called "forced draft," must be used.

Anthracite and semi-anthracite are the coals for which it is easiest to design a suitable furnace, and experience has shown that with all types of boilers, for these fuels the plain level grate is the most practical; it is the cheapest in up-keep, and it requires the least skill on the part of the fireman.

Naturally, the size of lump, the percentage of ash, the rate of combustion required and the strength of draft, determine such details as width of bar, extent of grate surface, form of bar, and size of air opening.

With semi-bituminous coal, owing to its larger percentage of volatile matter and the rapidity with which this inflammable gas is distilled off, more space must be provided in the furnace and care taken to prevent the burning gases coming in contact with the boiler heating surface and being cooled before combustion is complete.

These points are still further accentuated in relation to bituminous coal and lignite, and neglect to observe their importance leads to great loss in the use of these fuels.

The best methods of handling semi-bituminous coal and the bitumin

ous coals having the larger percentage of fixed carbon, is to fire it on the front end of the grate, where it is "coked," the volatile gases passing back over the incandescent fuel and burning completely before touching the heating surface. The coke left on the front is then pushed back and a fresh charge of coal fired.

With the very volatile bituminous coals and lignite, it is impossible to handle the fuel in this way, as it does not coke and has a tendency to form bad and troublesome clinker when worked with the fire tools. This fuel should be spread in very light charges evenly from the front to the back, covering each half of the grate alternately.

Semi-bituminous and the coking variety of bituminous coal may also

be fired in this way with no loss in economy if the firing is skillful.

The method of firing and the design of the furInace have a material effect on the production of smoke; but it may be mentioned that while smoke is an indication that the conditions of combustion are susceptible of improvement, an absence of smoke is not by any means a sure sign of proper combustion, for it may be brought about by too much air being supplied, and consequent dilution of the gases; nor is the production of smoke by any means an indication that much waste takes place, for the quantity of unconsumed carbon sufficient to color the escaping gases from a boiler is an exceedingly small percentage of the total amount of fuel.

The Babcock & Wilcox Marine Boiler, here illustrated, is the best. of all water-tube boilers, so far designed, for obtaining a high efficiency with bituminous coals.

It will be seen that the gases evolved from the fuel, pass under the roof located over the front portion of the lowest row of tubes, to a high combustion chamber at the rear, and are thoroughly mixed and burned before entering the bank of tubes forming the heating surface.

Generally speaking, with this boiler and with careful firing and favorable conditions, from 70 to 75 per cent. of the heat units which

a coal is found to contain theoretically, can be transferred to the water and steam. Claims have been made that more than this can be obtainedup to 80 per cent.-with certain classes of boilers; we do not wish to dispute the possibility of obtaining this, but certainly it is only obtainable under conditions which are so carefully studied as to be impracticable or impossible to maintain in ordinary practice. The remaining 30 to 25 per cent. is lost in radiation, in the heat carried away in the waste gases, and in imperfect combustion, due either to unavoidable excess of air in the furnace, or to a lack of sufficient air, depending upon the furnace conditions. A greater proportion of the heat can usually be saved and utilized when anthracite and semi-bituminous coals are employed. And as the volatile matter in the fuel increases, the greater becomes the probable loss from incomplete combustion.

Higher evaporative efficiencies can generally be obtained from watertube boilers than from shell boilers, for the reason, principally, that in the former there are furnaces which are capacious, and in which combustion takes place more quickly than in the furnaces of shell boilers, where not only is the space for combustion confined, but the fuel surrounded by cool boiler surface.

STEAM WHALER MARY D. HUME" IN THE ARCTIC
OWNERS: PACIFIC STEAM WHALING Co. (FROM "THE FROZEN NORTHLAND" BY
WINFIELD SCOTT MASON, BY COURTESY OF THE AUTHOR.) BABCOCK &
WILCOX BOILER, 400 I. H. P.

66,180

Feed water per hour, from and at 212° F. per lb. carbon value of fuel*

9.62

10.10

9.34

10.42

9.94