nd 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.

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 evap