s are small in
diameter they are apt to become partially choked with pieces of coke; but
an internal diameter of 1-5/8ths may be employed without inconvenience if
the draught be of medium intensity.

286. _Q._--Will you illustrate the relation between the length and diameter
of locomotive tubes by a comparison with the proportion of flues in flue
boilers?

_A._--In most locomotives the velocity of the draught is such that it would
require very long tubes to extract the heat from the products of
combustion, if the heat were transmitted through the metal of the tubes
with only the same facility as through the iron of ordinary flue boilers.
The Nile steamer, with engines of 110 nominal horses power each, and with
two boilers having two independent flues in each, of such dimensions as to
make each flue equivalent to 55 nominal horses power, works at 62 per cent.
above the nominal power, so that the actual evaporative efficacy of each
flue would be equivalent to 89 actual horses power, supposing the engines
to operate without expansion; but as the mean pressure in the cylinder is
somewhat less than the initial pressure, the evaporative efficacy of each
flue may be reckoned equivalent to 80 actual horses power. With this
evaporative power there is a calorimeter of 990 square inches, or 12.3
square inches per actual horse power; whereas in Stephenson's locomotive
with 150 tubes, if the evaporative power be taken at 200 cubic feet of
water in the hour, which is a large supposition, the engine will be equal
to 200 actual horses power. If the internal diameter of the tubes be taken
at thirteen eighths of an inch, the calorimeter per actual horse power will
only be 1.1136 square inches, or in other words the calorimeter in the
locomotive boiler will be 11.11 times less than in the flue boiler for the
same power, so that the draught in the locomotive must be 11.11 times
stronger, and the ratio of the length of the tube to its diameter 11.11
times greater than in the flue boiler, supposing the heat to be transmitted
with only the same facility. The flue of the Nile would require to be 35-
1/2 inches in diameter if made of the cylindrical form, and 47-3/4 feet
long; the tubes of a locomotive if 1-3/8ths inch diameter would only
require to be 22.19 inches long with the same velocity of draught; but as
the draught is 11.11 times faster than in a flue boiler, the tubes ought to
be 246.558 inches, or about 20-1/2 feet long according to t