285._Q._—­What is the best diameter for the tubes of locomotive boilers?

A.—­Bury’s locomotive with 14 inch cylinders contains 92 tubes of 2-1/8th inches external diameter, and 10 feet 6 inches long; whereas Stephenson’s locomotive with 15 inch cylinders contains 150 tubes of 1-5/8ths external diameter, 13 feet 6 inches long.  In Stephenson’s boiler, in order that the part of the tubes next the chimney may be of any avail for the generation of steam, the draught has to be very intense, which in its turn involves a considerable expenditure of power; and it is questionable whether the increased expenditure of power upon the blast, in Stephenson’s long tubed locomotives, is compensated by the increased generation of steam consequent upon the extension of the heating surface.  When the tubes 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