oration, 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.

285._Q._--What is the best diameter for the tubes of locomotive boilers?

_A._--Bury's locomotive with 14 inch cylinders contains 92 tubes of 2-1/8th
inches external diameter, and 10 feet 6 inches long; whereas Stephenson's
locomotive with 15 inch cylinders contains 150 tubes of 1-5/8ths external
diameter, 13 feet 6 inches long. In Stephenson's boiler, in order that the
part of the tubes next the chimney may be of any avail for the generation
of steam, the draught has to be very intense, which in its turn involves a
considerable expenditure of power; and it is questionable whether the
increased expenditure of power upon the blast, in Stephenson's long tubed
locomotives, is compensated by the increased generation of steam consequent
upon the extension of the heating surface. When the tube