of the chimney in
square inches in the smallest part. A factory chimney suitable for a 20
horse boiler is commonly made about 20 in. square inside, and 80 ft. high;
and these dimensions are those which answer to a consumption of 15 lbs. of
coal per horse power per hour, which is a very common consumption in
factory engines. If 15 lbs. of coal be consumed per horse power per hour,
the total consumption per hour in a 20 horse boiler will be 300 lbs., and
300 multiplied by 12 = 3600, and divided by 9 (the square root of the
height) = 400, which is the area of the chimney in square inches. It will
not answer well to increase the height of a chimney of this area to more
than 40 or 50 yards, without also increasing the area, nor will it be of
utility to increase the area much without also increasing the height. The
quantity of coal consumed per hour in pounds, multiplied by 5, and divided
by the square root of the height of the chimney, is the proper collective
area of the openings between the bars of the grate for the admission of air
to the fire.

289. _Q._--Is this rule applicable to the chimneys of steam vessels?

_A._--In steam vessels Boulton and Watt have heretofore been in the habit
of allowing 8-1/2 square inches of area of chimney per horse power, but
they now allow 6 square inches to 7 square inches. In some steam vessels a
steam blast like that of a locomotive, but of a smaller volume, is used in
the chimney, and many of the evils of a boiler deficient in draught may be
remedied by this expedient, but a steam blast in a low pressure engine
occasions an obvious waste of steam; it also makes an unpleasant noise, and
in steam vessels it frequently produces the inconvenience of carrying the
smaller parts of the coal up the chimney, and scattering it over the deck
among the passengers. It is advisable, therefore, to give a sufficient
calorimeter in all low pressure boilers, and a sufficient height of chimney
to enable the chimney to operate without a steam jet; but it is useful to
know that a steam jet is a resource in the case of a defective boiler, or
where the boiler has to be urged beyond its power.

STEAM ROOM AND PRIMING.

290. _Q._--What is the capacity of steam room allowed in boilers per horse
power?

_A._--The capacity of steam room allowed by Boulton and Watt in their land
wagon boilers is 8-3/4 cubic feet per horse power in the two horse power
boiler, and 5-3/4 cubic feet in the 20 horse power boile