greater than
would be necessary if the total surface were effective, else the requisite
quantity of heating surface will not be obtained. If, then, the vent be the
calorimeter, divided by the length, and the length be made 3 or 2.5 times
greater, the vent must become 3 or 2.5 times less; and in wagon boilers
accordingly, the vent varies from 8 to 11 instead of from 21 to 25, as in
the case of marine flue boilers. In tubular marine boilers the calorimeter
is usually made only about half the amount allowed by Boulton and Watt for
marine flue boilers, or, in other words, the collective sectional area of
the tubes, for the transmission of the smoke, is from 8 to 9 square inches
per nominal horse power. It is better, however, to make the sectional area
larger than this, and to work the boiler with the damper sufficiently
closed to prevent the smoke and flame from rushing exclusively through a
few of the tubes.

261. _Q._--What are the ordinary dimensions of the flue in wagon boilers?

_A._--In Boulton and Watt's 45 horse wagon boiler the area of flue is 18
square inches per horse power, but the area per horse power increases very
rapidly as the size of the boiler becomes less, and amounts to about 80
square inches per horse power in a boiler of 2 horse power. Some such
increase is obviously inevitable, if a similar form of flue be retained in
the larger and smaller powers, and at the same time the elongation of the
flue in the same proportion as the increase of any other dimension is
prevented; but in the smaller class of wagon boilers the consideration of
facility of cleaning the flues is also operative in inducing a large
proportion of sectional area. Boulton and Watt's 2 horse power wagon boiler
has 30 square feet of surface, and the flue is 18 inches high above the
level of the boiler bottom, by 9 inches wide; while their 12 horse wagon
boiler has 118 square feet of heating surface, and the dimensions of the
flue similarly measured are 36 inches by 13 inches. The width of the
smaller flue, if similarly proportioned to the larger one, would be 6-1/2
inches, instead of 9 inches, and, by assuming this dimension, we should
have the same proportion of sectional area per square foot of heating
surface in both boilers. The length of flue in the 2 horse boiler is 19.5
ft., and in the 12 horse boiler 39 ft., so that the length and height of
the flue are increased in the same proportion.

262. _Q._--Will you give an example of t