ve
boilers from 80 to 150 lbs.; but about 1 cwt. per hour is a good proportion
in locomotives, as has been already explained.

CALORIMETER AND VENT.

258. _Q._--In what manner are the proper sectional area and the proper
capacity of the flue of a boiler determined?

_A._--The proper collective area for the escape of the smoke and flame over
the furnace bridges in marine boilers is 19 square inches per nominal horse
power, according to Boulton and Watt's practice, and for the sectional area
of the flue they allow 18 square inches per horse power. The sectional area
of the flue in square inches is what is termed the _calorimeter_ of the
boiler, and the calorimeter divided by the length of the flue in feet is
what is termed the _vent_. In marine flue boilers of good construction the
vent varies between the limits of 20 and 25, according to the size of the
boiler and other circumstances--the largest boilers having generally the
largest vents; and the calorimeter divided by the vent will give the length
of the flue in feet. The flues of all flue boilers diminish in their
calorimeter as they approach the chimney, as the smoke contracts in its
volume in proportion as it parts with its heat.

259. _Q._--Is the method of determining the dimensions of a boiler flue, by
a reference to its vent and calorimeter, the method generally pursued?

_A._--It is Boulton and Watt's method; but some very satisfactory boilers
have been made by allowing a proportion of 0.6 of a square foot of fire
grate per nominal horse power, and making the sectional area of the flue at
the largest part 1/7th of the area of fire grate, and at the smallest part,
where it enters the chimney, 1/11th of the area of the fire grate. These
proportions are retained whether the boiler is flue or tubular, and from 14
to 16 square feet of tube surface is allowed per nominal horse power.

260. _Q._--Are the proportions of vent and calorimeter, taken by Boulton
and Watt for marine flue boilers, applicable also to wagon and tubular
boilers?

_A._--No. In wagon and tubular boilers very different proportions prevail,
yet the proportions of every kind of boiler are determinable on the same
general principle. In wagon boilers the proportion of the perimeter of the
flue which is effective as heating surface, is to the total perimeter as 1
to 3, or, in some cases as 1 to 2.5; and with any given area of flue,
therefore, the length of the flue must be from 3 to 2.5 times