68. Apparently the area of a chimney may be found by dividing the volume of the chimney gases, in cubic feet per minute, by the theoretical velocity due to their temperature. But no reliable rule can be given for this purpose, because a large proportion of the draft pressure is expended in forcing the air through the fire, and in overcoming friction, etc. in the flues leading to the chimney. The velocity of the gases in the chimney is thereby reduced to 50, or even 25, per cent. of the theoretical velocity. It is necessary, therefore, to depend for information upon the data secured by tests and actual service.

The rate of combustion of anthracite coal, per square foot of grate per hour, which may be attained in practice with various heights of chimneys, is shown in the following table. The area of the chimney required, per pound of coal thus burned, is also given for each height. This is based upon the proportion of 1 square foot of chimney area to 8 square feet of grate surface.

Table L0. Size Of Chimneys

The size of a chimney should be adapted to the maximum work that it may ever be called upon to do. Chimneys for domestic heating apparatus, etc., which are built into the walls of a house, should be made of generous dimensions, so as to avoid all possible overheating, and the consequent danger from fire.