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 boiler; and in the larger class of boilers, such as those suitable for 30 and 45 horse power engines, the capacity of the steam room does not fall below this amount, and, indeed, is nearer 6 than 5-3/4 cubic feet per horse power.  The content of water is 18-1/2 cubic feet per horse power in the two horse power boiler, and 15 cubic feet per horse power in the 20 horse power boiler.

291. Q.—­Is this the proportion Boulton and Watt allow in their marine boilers?