the cylindrical form, and 47-3/4 feet long; the tubes of a locomotive if 1-3/8ths inch diameter would only require to be 22.19 inches long with the same velocity of draught; but as the draught is 11.11 times faster than in a flue boiler, the tubes ought to be 246.558 inches, or about 20-1/2 feet long according to this proportion.  In practice, however, they are one third less than this, which reduces the heating surface from 9 to 6 square feet per actual horse power, and this length even is found to be inconvenient.  It is greatly preferable therefore to increase the calorimeter, and diminish the intensity of the draught.

BOILER CHIMNEYS.

287. Q.—­By what process do you ascertain the dimensions of the chimney of a land boiler?

A.—­By a reference to the volume of air it is necessary in a given time to supply to the burning fuel, and to the velocity of motion produced by the rarefaction in the chimney; for the area of the chimney requires to be such, that with the velocity due to that rarefaction, the quantity of air requisite for the combustion of the fuel shall pass through the furnace in the specified time.  Thus if 200 cubic feet of air of the atmospheric density are required for the combustion of a pound of coal,—­though 250 lbs. is nearer the quantity generally required,—­and 10 lbs. of coal per horse power per hour are consumed by an engine, then 2000 cubic feet of air must be supplied to the furnace per horse power per hour, and the area of the chimney must be such as to deliver this quantity at the increased bulk due to the high temperature of the chimney when moving with the velocity the rarefaction within the chimney occasions, and which, in small chimneys, is usually such as to support a column of half an inch of water.  The velocity with which a denser fluid flows into a rarer one is equal to the velocity a heavy body acquires in falling through a height equal to the difference of altitude of two columns of the heavier fluid of such heights as will produce the respective pressures; and, therefore, when the difference of pressure or amount of rarefaction in the chimney is known, it is easy to tell the velocity of motion which ought to be produced by it.  In practice, however, these theoretical results are not to be trusted, until they have received such modifications as will make them representative of the practice of the most experienced constructors.

288. Q.—­What then is the rule followed by the most experienced constructors?

A.—­Boulton and Watt’s rule for the dimensions of the chimney of a land engine is as follows:—­multiply the number of pounds of coal consumed under the boiler per hour by 12, and divide the product by the square root of the height of the chimney in feet; the quotient is the area of the chimney in square inches in the smallest part.  A factory chimney suitable for a 20 horse boiler is