in the four horse, 4.35; six horse, 3.75; eight horse, 4.33; ten
horse, 3.96; twelve horse, 3.63; eighteen horse, 3.17; thirty horse, 2.52;
and in the forty-five horse boiler, 2.05 square inches. Taking the amount
of heating surface in the 45 horse boiler at 9 square feet per horse power,
we obtain 18 square inches of sectional area of flue per horse power, which
is also Boulton and Watt's proportion of sectional area for marine boilers
with internal flues.

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