267. *Q.*—If to increase the perimeter
of a flue is virtually to diminish the length, then
a tubular boiler where the perimeter is in effect greatly
extended ought to have but a short length of tube?

*A.*—The flue of the Nile steamer
if reduced to the cylindrical form would be 35-1/2
inches in diameter to have the same area; but it would
then require to be made 47-3/4 feet long, to have
the same amount of heating surface, excluding the
bottom as non-effective. Supposing that with these
proportions the heat is sufficiently extracted from
the smoke, then every tube of a tubular boiler in
which the same draught existed ought to have very
nearly the same proportions.

268. *Q.*—But what are the best proportions
of the parts of tubular boilers relatively with one
another?

*A.*—The proper relative proportions
of the parts of tubular boilers may easily be ascertained
by a reference to the settled proportions of flue
boilers; for the same general principles are operative
in both cases. In the Nile steamer each boiler
of 55 horse power has about 497 square feet of flue
surface or 9 square feet per horse power, reckoning
the total surface as effective. The area of the
flue, which is rectangular is 990 square inches, therefore
the area is equal to that of a tube 35-1/2 inches in
diameter; and such a tube, to have a heating surface
of 497 square feet, must be 53.4 feet or 640.8 inches
in length. The length, therefore, of the tube,
will be about 18 times its diameter, and with the same
velocity of draught these proportions must obtain,
whatever the absolute dimensions of the tube may be.
With a calorimeter, therefore, of 18 square inches
per horse power, the length of a tube 3 inches diameter
must not exceed 4 feet 6 inches, since the heat will
be sufficiently extracted from the smoke in this length,
if the smoke only travels at the velocity due to a
calorimeter of 18 square inches per horse power.

269. *Q.*—Is this, then, the maximum
length of flue which can be used in tubular boilers
with advantage?

*A.*—By no means. The tubes of
tubular boilers are almost always more than 4 feet
6 inches long, but then the calorimeter is almost always
less than 18 square inches per horse power—generally
about two thirds of this. Indeed, tubular boilers
with a large calorimeter are not found to be so satisfactory
as where the calorimeter is small, partly from the
propensity of the smoke in such cases to pass through
a few of the tubes instead of the whole of them, and
partly from the deposit of soot which takes place
when the draught is sluggish. It is a very confusing
practice, however, to speak of nominal horse power
in connection with boilers, since that is a quantity
quite indeterminate.

270. *Q.*—The main thing after all
in boilers is their evaporative powers?