"Sir,—The engine has been worked. The result is ten strokes per minute, 6-feet stroke, with half a bushel of coal per hour, lifting six thousand pounds weight. This was done with water in the cistern round the condenser, which water came up to 180 degrees of heat, and remained so. The water sides of the condenser covered with this hot water was 50 surface feet. I tried it to work with the cold air sides, but I found that the cold air sides of 120 feet would only work it four strokes per minute. I should have worked the steam much higher than 50 lbs. to the inch, but being an old boiler I thought it a risk. I am now placing an old boiler of 350 feet of cold sides more to the condenser, to give a fair trial to condensing with cold sides alone. The steam below the piston was about 6 or 7 lbs. to the inch above the atmosphere. The force-pump to the boiler was about one-fifth part of the content of the cylinder, and the valve close to the boiler lifted when the force-piston was down about two-thirds of its stroke, at which time the returned steam entered the boiler again. I have no doubt of doing near ten times the duty that is now done on board ships, without using salt water in the boiler, as at present. Our boiler has been working three days and the water has not sunk 1 inch per day. I am quite satisfied the trial will be a great success.
"Mr. Praed and Sir John St. Aubyn are anxious to get a high bank carried out from Chapel Angel to 15 feet below low-water mark on the bar, to make Hayle a floating harbour.
"I have proposed to make a sand-lifting engine. When I built that engine for deepening Woolwich Harbour, we lifted 300 tons per hour through 36 feet of water, and 20 feet above water, 56 feet above the bottom. This was done with two bushels of coal per hour, therefore it will not cost above one penny per square fathom to lift the sand over this embankment. It is intended to get down Mr. Telford to give his opinion on it. Your remarks on it would be of service.
"I remain, Sir,
"Your humble servant,
"Richard. Trevithick.
The writer having worked at these experiments, knows that their object was to employ high-pressure steam in the boiler, using it very expansively in the cylinder, and by cold surface sides reducing its bulk either to low-pressure steam or boiling water, and then force it again into the boiler.
"Hayle, November 14th, 1829.
""Mr. Gilbert,
"Sir,—I have both of your letters and sketches, which shall be put in hand. I understand it perfectly well. Since I wrote to you last I have made several satisfactory trials of the engine, and think it unnecessary to make any further experiments. The statement below may be depended on for a future data. The load of the engine was 6280 lbs., being 20 lbs. to the inch for a 20-inch cylinder with a 6-feet stroke, 12 strokes per minute, with three-quarters of a bushel of coals per hour, giving a duty of 361,728,000 for 1 bushel of coal, a duty far beyond anything done in the county by so small an engine. The cold water sides round the condenser was 60 feet, and the water at 112 degrees temperature, not having a sufficient stream of cold water to supply the cistern. Each foot of cold water sides did 7536 lbs. per minute, about three times the work done in the county per foot of hot boiler sides; therefore the condenser need not be more than one-third of the boiler sides. By making the condenser of 4-inch copper tubes and of an inch thick, it would stand in one-twentieth part of the space of the boiler.
"I put a boiler naked to try cold air sides; it was very rusty, and did not condense as fast as I expected. The engine worked exceedingly well, but slow. The duty performed for each foot of cold air sides was 565 lbs. per minute, about one-thirteenth part of the condensing of cold water sides. We never wanted to get the steam above 60 lbs. to the inch. I have no doubt but that copper pipes of 1/32nd of an inch thick, clean and small, would do considerably more, because the hot water that came out of the boiler from the condensed steam was but 170 decrees, and the external sides the same heat when the steam was 15 lbs. above the atmosphere in the condensing boiler. This boiler was 4 feet 6 inches diameter, and I think that towards the external sides of the boiler there was a colder atmosphere, if I may call it so, than what it was in the middle of this large condensing boiler, because I found by trying a small tin tube, that it would condense 1500 lbs. for each foot of cold air sides.
"However, as it is, it will do exceedingly well for portable purposes.