Trevithick's estimate for a new engine of the same size as the old was 2000_l._, but as the old one could be improved for 1300_l._, the latter course was adopted, the wooden main beam with its segment head was retained, a cover was added to the cylinder, and a new piston-rod and piston; a pole air-pump was used in lieu of the more usual Watt air-pump bucket; a feed-pole forced water into the boiler,--an indirect proof of increased steam pressure. The new globular boiler with internal tube weighed 8 tons; the engineer's charge for carrying out the work was 66_l._
The use of strong steam as the prime mover of the steam-engine increased more rapidly beyond than within the limits of Cornwall, for in 1802 was erected at Coalbrookdale a high-pressure steam-puffer engine, to which Trevithick attached a pump which forced water through a column of upright pipes, that the power of the engine might be accurately measured. It worked with steam of from 50 to 145 lbs. on the inch, and wholly discarded the vacuum which had been Watt's mainstay.
"The boiler is 4 feet diameter, the cylinder 7 inches diameter, 3-feet stroke. The water-piston is 10 inches in diameter, drawing and forcing 35 feet perpendicular, equal beam. I first set it off with about 50 lbs. on the inch pressure against the steam-valve, for the inspection of the engineers about this neighbourhood. The steam continued to rise the whole of the time it worked; it went from 50 to 145 lbs. to the inch.
"The engineers at this place all said that it was impossible for so small a cylinder to lift water to the top of the pumps, and degraded the principle, though at the same time they spoke highly in favour of the simple and well-contrived engine.
"After they had seen the water at the pump-head, they said that it was possible, but that the boiler would not maintain its steam at that pressure for five minutes; but after a short time they went off, with a solid countenance and a silent tongue."[60]
[Footnote 60: See Trevithick's letter, August 22nd, 1802, vol. i., p. 153.]
This high-pressure steam pumping engine in 1802 may be taken as the first pumping engine of the puffer class using such strong steam.
In the spring of the following year[61] a somewhat similar engine was erected in London. "The cylinder is 11 inches in diameter, with a 3-1/2-feet stroke. It requires the steam at a pressure of 40 to 45 lbs. to the inch to do its work well, working about twenty-six or twenty-seven strokes per minute. It is much admired by everyone that has seen it, and saves a considerable quantity of coal when compared with a Boulton and Watt. Mr. Williams, Mr. Robert Fox, Mr. Gould, and Captain William Davey were here, and much liked the engine; they gave me an order for one for Cornwall as a specimen." This particular engine was for driving machinery in a cannon manufactory. A high-pressure pumping engine was at work at Greenwich, and some were at work in Cornwall.