With a less area of fire-grate than the old engines, the surface exposed to the radiant heat of the fire is as 20 : 11.5, and the surface exposed to the com municative power of the heated air and flame, as 117.8 : 29.75, nearly four times as great.
"Nor is this the only difference; in the old engines, the area of the tube (of 22 inches diameter) for the passage of the flame and heated air to the chimney, was 380.13 inches ; and of this large body of flame and air passing through the tube, only an extent of surface of 69.11 inches was exposed to the water in the boiler. In the Rocket engine, the area of heated air and flame in 25 tubes, 3 inches each in diameter, was 176.7 inches, while the surface exposed was 235.6 inches.
" It is not necessary, perhaps, to pursue the comparison further. The economy of fuel which must result from the exposure of so much greater surface to the water, cannot fail to insure a more perfect abstraction of the heat, and thus not only save the fuel, but prevent great part of the previous destruction of the chimney, by the intense heat of the wasted caloric.
" The same remarks apply to the Sans Pareil of Mr. Hackworth, as to the old engine, though in a less degree. In the Rocket, the surface exposed to the radiant heat of the fire, compared with the area of fire-grate, is as : 1, while in the Sans Pareil, it is only : 1 ; the same proportion as in the old engines. In the Rocket, the surface exposed to the heated air and flame, compared with the area of fire-grating, is as 1911 : 1 ; while, in the Saw Pareil, the proportion is only 7} : I. The bulk of air passing through the tube of the latter, will, at its exit into the chimney, be 176.7 square inches, the exposed surface being 47.12, or 25 : 1, nearly; while, as before stated, the bulk of air passing through the tubes of the Rocket, is 176.7 inches, or precisely that of the Sans Pareil, while the surface exposed, is 235.6 inches, or 11 : 1. These will sufficiently account for thegyeat difference in the economy of fuel between the two engines; the Rocket requirmeonly 11.7.1bs. to convert a cubic foot of water into steam, while the Sans Pareil required 28.81bs.1 " Some explanation is, perhaps, necessary, why the Sans Pareil should, in this respect, be more extravagant than the old engines, while the extent of sur face, compared with the area of fire-grate, is much greater, and therefore should exhibit a more economical result; and this explanation is the more necessary, as, though not appearing at first sight, it involves a principle of the greatest importance in the economy of those engines; and which, if not acted upon, would render the we of the tubes, however otherwise valuable, considerably less effective.
" It will readily occur to any one, paying a little attention to the matter, that the system of tubes may be carried so far, as to reduce the temperature of the flame and heated air nearly equal to that of the water in the boiler ; in which case, when it reaches the chimney, it will be incapable, from its reduced tem perature, of producing a sufficient draught of air through the fire-grate. This would prevent all the advantages being taken of the refracting powers, which would otherwise result from the use of these tubes. It is stated in another part of this work, that on the introduction of those engines, it was necessary to resort to the application of waste steam thrown upwards into the chimney, to create a sufficient current of air through the fire ; which was afterwards laid aside, or only partially used, when only slow rates of speed were required.
" Mr. Hackworth had, it appears, in his engine, resorted to the use of this in a more forcible manner than before used, throwing it up as a jet, and which, when the engine moved at a rapid rate, and the steam thereby almost constantly issuing from the pipe, had a most powerful effect.
"This, though effecting the object for which it was intended, being carried too far, partly m consequence of the rapid speed at which the engine was made to travel, was productive of another evil, which, though operating fatally so far as regarded that particular experiment, was capable of easy remedy.
" The consequence was, that when the engine began to travel at the rate of 12 or 15 miles an hour, the draught was so great, that it actually threw the cinders out of the chimney with considerable force, producing a destruction of fuel enormously great, so much so, that the consumption was at least 692 lbs. per hour.