2.92
sq. ft.
Area though ferules 4 sq. ft. 1.64 sqft 2 sq. ft. 2.04
sq. ft.
Area of chimney 1.77 sqft 1.23 sqft .921 sq. ft. 1.31
sq. ft.
Area of blast orifice 23.76 sqin 16.8 sqin 14.18 sq. in. 17.7
sq. in.
Heating surface of tubes 1627 sqft 668.7 sqft 823 sq. ft. 864
sq. ft.

THE BLAST IN LOCOMOTIVES.

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 asce