"The area of fire-grate of the Sans Pareil was 10 feet; supposing that the area of the fire-grate of the Rocket had been the same, the consumption of the latter engine, with its power of exhaustion, would only have been 361 lbs. ; showing that the force of draught was so much greater in the Sans Famed, as to consume nearly twice the quantity of fuel in the same time.
"This will satisfactorily account for the apparent anomaly in the consumption of fuel with this engine, compared with that of the old engines, having a single tube ; otherwise, though not likely to come up to the Rocket in point of economy of fuel, we should have expected an effect considerably greater than in the old engines. The combustion of the fuel being so very rapid, and the abstracting surface so small, the heated air would pass off at a very high temperature; thus accounting for the loss of effect. The knowledge of this fact,—or rather, availing ourselves of this power for the purpose of creating a draught in the chimney,—leads us to an inquiry of great interest. By an extension in the use of these tubes of small diameter, there is little doubt of our being able (sup posing we can force the necessary quantity of air through them), to reduce the temperature of the heated air, before its exit into the chimney, nearly equal to the water in the boiler. This would be abstracting all the useful heat, and pro bably effecting all the economy of which the fuel is susceptible.
" Perhaps it would not be advisable to carry it quite so far as this ; for when the temperatures become nearly equal, the abstraction of heat would be so slow as to require a greater length of tube than it would be convenient to employ. We may, therefore, suppose that in all cases the temperature of heated air passing into the chimney will be greater than that of the water in the boiler. This heat will, however, be insufficient, in engines of this kind, to cause a suffi cient quantity of air to pass through the fire for the purpose of combustion ; and it becomes a question, whether we should allow a portion of the heat to escape for that purpose, or, by contracting the exit of the escape of the steam from the cylinders into the chimney, to effect the same object.
" Whether the last method is the most economical or not, though there is every reason to suppose it is, perhaps it is the only one with these engines that is suitable for their action upon railways, especially for quick travelling. The performance of those engines depends entirely upon the quantity of steam they can raise in a given time • and when travelling at the rate of fifteen miles an hour, or upwards, the production of steam is required to be very rapid indeed : the mode of producing a proper draught through the fire, by throwing the steam into the chimney, after its passage through the cylinders, is, perhaps, therefore the best ; as the quicker the engines travel, and when, consequently, the necessity for steam is the greatest, the then rapid and almost continuous exit of the steam into the chimney, increasing in proportion to the increased speed of the engine, produces at the same time a correspondingly greater quan tity of steam.
" the Rocket engine, this mode of increasing the draught of the chim ney was but partially used ; the steam was made tointo the chimney by two pipes. one from each cylinder, and the size aperture was not, therefore, sufficiently small to cause the steam to pass into the chimney with adequate force ; still, in that engine, we find it only required 11.7 lbs. to evapo rate a cubic foot of water,-36 per cent. less than with the old engines. We shall afterwards find, that this has been considerably more reduced in the engines lately made.
"The Novelly engine is on a different principle from those previously con sidered, the necessary supply of air to the fire being produced by a bellows. In this case a chimney becomes unnecessary, and from the way in which the Novelty is constructed, the air was forced through the fire in a very con densed or compressed state. The area of fire-grate being little more than one third of that of the Rocket, and the surface exposedto the radiant action of the fire less than one-half the temperature to which the fire was raised, must, of course, be considerably greater, to evaporate an equal quantity of water in the same time. The abstraction of heat would be probably more perfect in the Novelty, for the tube through which the flame and heated air passed in its exit to the atmo sphere was 36 feet in length in one tube; whereas in the Rocket there was the same length, though subdivided into six tubes. It is, however, extremely ques tionable, whether one tube, 36 feet long, or 6 tubes, each 6 feet long, of the same sectional area, are more preferable ; the latter would, of course, give a much greater exposure of surface. The area of exit of the heated air into the atmosphere of the Rocket, was 25 times that of the Novelty ; from which we may imagine the degree of compression necessary to force the same quantity of air through the fire ; though we do not say, that to raise an equal quantity of steam, an equal quantity of air, in that highly compressed state, is necessary.