281. Q.—­Is the exhaustion produced by the blast as great in the fire box as in the smoke box?

A.—­Experiments have been made to determine this, and in few cases has it been found to be more than about half as great as ordinary speeds; but much depends on the amount of contraction in the tubes.  In an experiment made with an engine having 147 tubes of 1-3/4 inches external diameter, and 13 feet 10 inches long, and with a fire grate having an area of 9-1/2 square feet, the exhaustion at all speeds was found to be three times greater in the smoke box than in the fire box.  The exhaustion in the smoke box was generally equivalent to 12 inches of water, while in the fire box it was equivalent to only 4 inches of water; showing that 4 inches were required to draw the air through the grate and 8 inches through the tubes.

282. Q.—­What will be the increase of evaporation in a locomotive from a given increase of exhaustion?

A.—­The rate of evaporation in a locomotive or any other boiler will vary as the quantity of air passing through the fire, and the quantity of air passing through the fire will vary nearly as the square root of the exhaustion.  With four times the exhaustion, therefore, there will be about twice the evaporation, and experiment shows that this theoretical law holds with tolerable accuracy in practice.

283. Q.—­But the same exhaustion will not be produced by a given strength of blast in all engines?

A.—­No; engines with contracted fire grates and an inadequate sectional area of tubes, will require a stronger blast than engines of better proportions; but in any given engine the relations between the blast exhaustion and evaporation, hold which have been already defined.

284. Q.—­Is the intensity of the draught under easy regulation?

A.—­The intensity of the draught may easily be diminished by partially closing the damper in the chimney, and it may be increased by contracting the orifice of the blast.  A variable blast pipe, the orifice of which may be enlarged or contracted at pleasure, has been much used.  There are various devices for this purpose, but the best appears to be that adopted in Stephenson’s engine, where a conical nozzle is moved up or down within the blast pipe, which is made somewhat larger in diameter than the base of the cone, but with a ring projecting internally, against which the base of the cone abuts when the nozzle is pushed up.  When the nozzle stands at the top of the pipe the whole of the steam has to pass through it, and the intensity of the blast is increased by the increased velocity thus given to the steam; whereas when the nozzle is moved downward the steam escapes through the annular opening left between the nozzle and the pipe, as well as through the nozzle itself, and the intensity of the blast is diminished by the enlargement of the opening for the escape of the steam thus made available.