In p. 431, it was that Mr. Evans over rated the economy in fuel from the use of his pressure engines. His deductions were the result of ex periments recorded in the seventeenth vol. of the Ency. Britannica. But later experiments by the French Academicians, Arago, Dulong, Sze., give a different result. According to those, an increase of 30° of Fahrenheit does not double the pressure of steam. Even by the experiments of Dalton, made long since, 40° of lithe. are required to produce this pressure. The French committee prove that the doubling of the pressure at high temperature, requires a greater aug mentation of heat than at a lower temperature. Thus, while an augmentation of 21° of the Centi grade therm.=38°.5 of Fahr. increases the pressure from one to two atmospheres, it requires an addition of 31A° Fahr. of temperature to aug ment the pressure from 8 to 16 atmospheres: and to raise the pressure from 12 atmospheres to 24, the in crease of temperature necessary is Fahr., and if we may rely on their formulaf given for computing the pressures at temperatures above 24 atmospheres, it will require an increase of Fahr. to the temperature to augment the pressure from 25 to 50 atmospheres.
The folloWing little table of differences, by Profess or Walter 11 Johnson, is derived entirely from that part of the table of the French Academicians con structed From their experiments, and shows that the fourth stage of doubling pressure, six. From 12 to 21 atmospheres, the increase of temperature is i of what it is at first, or from I.', to 3 atmospheres.
Thus although the general position of Mr. Evans is true, viz. that while the temperature is 'increased arithmetically, the elastic power of the steam is in creased geometrically: yet the particular law of that in crease, as laid doss II by Mr. Evans, is not sustained.
In conformity with this general position, NIr. Per kins, p. 306, considers the observation which he has made upon elastic steam generated with enormous heat, as leading to the following result : That while the temperature rises in an arithmetical ratio, the expan sive force will be that of an increasing ratio, and the increment of fuel will be a decreasing ratio.
Mr. Henwood, an advocate of the low pressure en gine, stalest that the average duty of Mr. Watts' en gines was about 25 millions lbs. lifted with one bushel of coals, and according to Mr. Far•y, all the water works in London are now served by Watt's engines, working low pressure steam, acting expansively in one cylinder, and the performance of the best of them is about 25 millions. Mr. John Taylor states, that ac cording to the official monthly report of the Mine agents in Cornwall. one engine (high pressure)of Mr Woolf at Wheal Towan. raised in 1828, on an aver age 77,290,000 ; in 1829, it was 76,235,307 lbs. with one bushel of coals. During an experiment by Mr. Rennie with the same engine, it raised 92.327,000 lbs. with the same quantity of fuel:§ and Sir H. Davy states in the Trans. of the Royal Soc. for 1827 and 1829, that this engine performed a duty in the whole month of December 1829, exceeding the average of 17 en gines on Mr. IVatt's construction in 1198, by a pro portion of nearly 4 to Farty gave his opinion before the committee of the House of Com mons, that " the difference in cost, between the quanti ty of coals consumed by the engines now in use (which are all on Mr. Woolf's system) and by an equal force of engines such as were in use before he went to Corn wall, in 1813, would absorb the profit of all the deep mining that is now carried on in Cornwall." Nothing, it would seem, can more fully demonstrate the superior power and economy of high pressure engines.
Mr. Thomas Lean, the professional inspector of steam engines in Cornwall, stated to a committee of the House of Commons, of England, in 1817, that " high pressure engines save at least two-fifths of the whole consumption of coals in Cornwall." In p. 397 the discovery that steam at a very high temperature will not scald, is attributed (but erro neously) to Mr. Jacob Perkins, for it was first acci dentally made (in this country) sixteen or seventeen years since, at the Fairmount Waterworks, on Schuylkill, by a workman named Kissick, who had the care of the high pressure engine which was used at the time, to raise water from the river for the sup ply of the city; and who has at present the charge of the powerful water wheels erected for the same pur pose. Having opened the guage cock for the water, to ascertain the height of it in the boiler, he received a stream of high steam in his face, and thought him self ruined, but in a minute or two he was agreeably disappointed, on finding no inconvenience from it, except the trickling down his face and breast, of the water from the condensed steam.
The fact being thus ascertained, the workmen of Mr. Evans made no difficulty about caulking boilers, while high steam from them was pouring out on their faces and hands, through leaks. A short time after the first discovery, an occurrence similar to Kissick's happened to Mr. Eckfeldt, at the mint. The aperture
of the guage-cock of the high-pressure engine, used for driving the machinery of the mint, was originally at the bottom, but in consequence of a deposition from the water in the boiler, it became closed. The place of the aperture was therefore changed to the front, in order that a wire might be easily inserted to clear the passage when necessary. Some time after, the cock was turned, and Mr. Eckfeldt, who was standing about two feet from it, received the full blast of the steam directly in his face: at the moment he was much alarmed, but in turning round, he was agree ably surprised to find he was not scalded. The writer made the experiments in the presence of Mr. Eck feldt, on the 20th of April, 1831, while the engine was at work, with a pressure of nearly 150 lbs. to the inch, the usual extent to which it carried. He held his open hand first at the distance of 18 inches from the cock, while the steam was rushing with tremen dous force from the aperture in the cock, and finding it little more than warm, he placed it within nine inches of the cock, and even then the sensation ex cited was not more than that of an agreeable warmth. Oliver Evans mentioned the fact, in a hand bill, on the 28th of October, 1817, that high steam does not scald, see p. 431: this was before Mr. I'erkins left Philadelphia, which he did in May 1819. Mr Brun ton, an experienced English enginee.r, gave another proof of the fact to the committee of the House of Commons, in and Mr. Vivian also said, on the same occasion, that " the steam from low pressure scalds much worse than the steam from high pres sure."t Mr. Perkins, however, confirmed the fact in Eng land, by a set of ingenious experiments, as related in p. 397, and explains it thus: "I have frequently served, that when the stop cock of a high-pressure boiler was opened, whether at the water or steam cocks, the temperature was lowered in proportion to the height of the steam. He ascribes the phenomenon to the great force and rapidity of motion of the steam causing the atmospheric air to be driven before it, evidently tending to produce a partial vacuum, to which the surrounding atmosphere would rush in, and diminish." This explanation, however, is only in part true, because the diminution of temperature of the steam is observed in tubes when the atmospheric air is excluded. A very neat set of experiments by Mr. Evart, of Liverpool, show that the reduction of the temperature of steam, is owing to the great ex pansion of it, and the recovery of its capacity for heat. He ascertained that the heat of the boiler was and that of the issuing steam 185°4 thus con firming a well-known law, viz: the diminution of the capacities of bodies for heat when condensed, and their increased capacity for it when expanded. This law equally applies to fluids in an aeriform state, and hence the temperature of steam condensed in a boiler is raised, and lowered if allowed to escape and ex pand.
The following papers may be found useful to those who are desirous to enquire into the subject of steam engines.
1. On the comparative power of steam engines, and rule (with example) to find the power of a steam en gine on Watt's principle, in horse power. Franklin Journal, vol. iii. p. 333.
2. On the relative proportions of the various parts of the Boulton and Watts', or low pressure engines; the fuel required for working engines of different powers, and the effect produced in pumping water or grinding wheat. First published in the Franklin Journal, vol. iii. p. 336.
3. Specification of a patent for an improvement in the application of the escape heat from the high pres sure engine, by A. Brown, Onondago county, New York. Ibid, vol. iv. new series, p. 273.
4. On the economy of using highly elastic steam expansively, by Jacob Perkins, Ibid, vol. iv. 1827, p. 24. Remarks on the same, p. 120.
5. Method of using heated air, gases, elastic fluids, and products of combustion, which are available to the increase of steam power, by M. Ward, Baltimore. Ibid, p. 49.
6. Account of Mr. Perkins' new high pressure steam engine, vol. iii. 1827, with plates vol. iv. p. 39, p. 239, p. 349, by Mr. John March, p. 414.
7. Dolittle on securing a constant and uniform sup ply of water in steam engine boilers. Silliman's Journal for 1827, vol. xiii. p. 64. Remarks on the same by Dr. Jones, Franklin Journal, vol. iv. p. 347.
8. On the true mode of computing the power of a high pressure steam engine, by Charles Potts. Ibid, vol. v. new series, p. 111. Remarks on the same, pp. 251, 398.
9. On the difference between the absolute efforts employed to move a locomotive engine, when the force proceeds in one case from the engine itself, and in the other from a stationary engine, supposing the place over which the motion takes place to be horizontal, by Charles Potts. Ibid, vol. v. p. 246.
10. Arago, on the explosion of boilers. Ibid, vol. v. and vi. new series.