"Sir,--The boiler with the fire-place, cold air tubes outside the boiler but within the steam-case, fire-tubes in the boiler from the top of the fire-place to the top of the boiler, the ash-pit close, except a small door to clear out the ashes. [Illustration: FIG. 1.--PLAN SECTION.] [Illustration: FIG. 2.--ELEVATION SECTION.[174]] [Footnote 174: _a_, steam-case; _b_, boiler-case; _c_, space for condensation of steam; _d_, water and steam space; _e_, fire-tubes; _f_, fire-box; _g_, fire-door; _h_, fire-bars; _i_, ash-box; _j_, ash-box door; _k_, air-tubes in condenser; _l_, chimney; _m_, water level; a smoke-jack fan draught.] "The design is for the cold air to pass down from the top of the boiler through the air-tubes within the steam-case surrounding the boilers, becoming heated in its passage by condensing the steam in the case, and then to pass up through the fire-bars in, the hot state, nearly as hot as the steam in the case; because this air, heated to nearly 212 degrees by condensing the steam in its passage without any of its oxygen being burnt, it will not carry off so much heat from the fire as cold air would, and still have the same oxygen as cold air to consume the coal. "The cold air will be passing down the steam-case in the air-tubes, and up through the fire and fire-tubes in the boiler. I find by experiments I have made here, by placing a tin tube 2-1/2 inches in diameter, 4 feet long, inside a 4-inch tube of the same length, having boiling water and steam between the tubes, kept hot by a fire round the outer tube, with a smith's bellows blowing in at the bottom of the inside tube, having 2-2/3rds surface feet of condensing sides, measuring the inside, where the air is passing up from the bellows, heats from 60 to 134 degrees 15 square feet of cold air per minute. When you compare the effective heat of 74 degrees given to 15 cubic feet of air every minute from 2-2/3rds surface feet of tin plate, and the heat contained in 15 cubic feet of air charged with 74 degrees of effective heat, compared with steam of atmosphere strong, you will find that the condensing power of surface sides is very great, and for locomotive purposes might be carried still further, by forcing the air more quickly through the tubes. If the statements on air given in some books that I have read are correct, that there is about three times as much heat in 1 gallon of steam of atmosphere strong as there is in 1 gallon of air of 212 degrees of heat, in that case 1 surface foot of tin-plate sides of this pipe, by sending off the hot air before described, would take out the heat of 1-1/2 cubic foot of steam per minute of atmosphere strong, which in the common condensing engine would be equal to a duty of 2700 lbs. lifted 1 foot high per minute; but in the high-pressure expansive engine, the heat of 1-1/2 cubic foot of steam would give a duty of 10,800 lbs., or four times the duty of the Boulton and Watt engine.