277. Q.—­What is the amount of draught produced in locomotive boilers in comparison with that existing in other boilers?

A.—­A good chimney of a land engine will produce a degree of exhaustion equal to from 1-1/2 to 2-1/2 inches of water.  In locomotive boilers the exhaustion is in some cases equal to 12 or 13 inches of water, but from 3 to 6 inches is a more common proportion.

278. Q.—­And what force of blast is necessary to produce this exhaustion?

A.—­The amount varies in different engines, depending on the sectional area of the tubes and other circumstances.  But on the average, it may be asserted that such a pressure of blast as will support an inch of mercury, will maintain sufficient exhaustion in the smoke box to support an inch of water; and this ratio holds whether the exhaustion is little or great.  To produce an exhaustion in the smoke box, therefore, of 6 inches of water, the waste steam would require to be of sufficient pressure to support a column of 6 inches of mercury, which is equivalent to a pressure of 3 lbs. on the square inch.

279. Q.—­How is the force of the blast determined?

A.—­By the amount of contraction given to the mouth of the blast pipe, which is a pipe which conducts the waste steam from the cylinders and debouches at the foot of the chimney.  If a strong blast be required, the mouth of this pipe requires to be correspondingly contracted, but such contraction throws a back pressure on the piston, and it is desirable to obtain the necessary draught with as little contraction of the blast pipe as possible.  The blast pipe is generally a breeches pipe of which the legs join just before reaching the chimney; but it is better to join the two cylinders below, and to let a single pipe ascend to within 12 or 18 inches of the foot of the chimney.  If made with too short a piece of pipe above the joining, the steam will be projected against each side of the chimney alternately, and the draught will be damaged and the chimney worn.  The blast pipe should not be regularly tapered, but should be large in the body and gathered in at the mouth.

280. Q.—­Is a large and high chimney conducive to strength of draught in locomotives?

A.—­It has not been found to be so.  A chimney of three or four times its own diameter in height appears to answer fully as well as a longer one; and it was found that when in an engine with 17 inch cylinders a chimney of 15-1/4 inches was substituted for a chimney of 17-1/2 inches, a superior performance was the result.  The chimney of a locomotive should have half the area of the tubes at the ferules, which is the most contracted part, and the blast orifice should have 1/10th of the area of the chimney.  The sectional area of the tubes through the ferules should be as large as possible.  Tubes without ferules it is found pass one fourth more air, and tubes with ferules only at the smoke box end pass one tenth more air than when there are ferules at both ends.