STEAM-ENGINE, PERKINS'S

STEAM-ENGINE, PERKINS'S. In addition to the detailed theory of STEAM, and of the STEAM-ENGINE,* such as it has already been furnished by the science of a Robison, and the ingenuity of a Watt, some further theoretical investigations and computations are still required from time to time, in order to keep pace with that restless spirit of incessant speculation and practical improvement, to which Great Britain is indebted for so much of its prosperity; and none of the inventions which have of late been introduced are so striking, so promising, or at first sight so paradoxical, as those which have resulted from the skill and enterprise of Mr Perkins. In order that we may be prepared to appreciate the merit of these inventions, and to discriminate between the effects which are the results of new philosophical principles, and those which are only obtained from a high perfection of mechanical execution, and a just confidence in the admirable art with which the materials are selected and tempered; it will be necessary to recur to the theories of heat and of steam which have been already founded on acknowledged facts, and to inquire how far they will carry us in explaining all that has hitherto been made public respecting the machinery which has excited so much attention, before we allow ourselves to reject the principles [of Atmology] at present admitted, as either erroneous or insufficient.

Respecting the density, or specific gravity, of steam at different temperatures, there appear to have been three different opinions; that of Mr Wolf, who fancied that it was nearly equal in all cases, and that the difference of elasticity depended only on the immediate effect of heat; that of Mr Southern, who inferred from his experiments that the density was simply proportional to the elasticity, without any regard to the effect of temperature; and thirdly, the earlier and more probable theory of Mr Dalton, which is intermediate between them, and which at-

tributes the same effect to heat in the expansion of pure steam that it possesses with respect to all other gases, amounting for each degree of Fahrenheit to about $\backslash frac\{1\}\{375\}$ of their bulk at 52°, or to $\backslash frac\{1\}\{375\}$ at 212°, provided that no additional quantity of moisture be present to furnish fresh steam by its evaporation. The elasticity might be computed from Dr Young's formula $E\; =\; 1781\; (1\; +\; .006f)^2$, in which $f$ is reckoned from the freezing point, and which agrees sufficiently well with the latest experiments of Southern and Ure, as well as with the earlier ones of Watt, Schmidt, Biker, Bétancourt, Robison, Dalton, and others; and we might transform it into the more convenient expression $E\; =\; (1\; +\; .0029f)^2$, for the number of atmospheres expressing the elasticity, reckoning the $f$ from 212°. But since Mr Southern's experiments, as described in Robison's System of Philosophy, were carried to higher temperatures than any of those which had been before made public, it will be safer to adopt an exponent approaching more nearly to that which he has employed, and to make $E\; =\; (1\; +\; .004f)^2$. Notwithstanding, however, the apparent accuracy of these experiments, it seems impossible to rely on them with perfect confidence; they give, for example, the absolute specific gravity of steam at 212°, $\backslash frac\{1\}\{1200\}$; while Sir Humphry Davy's very accurate observation of the perfect identity of the space occupied by oxygen and hydrogen before and after deflagration, makes it only $\backslash frac\{1\}\{3750\}$, which agrees very nearly with the experiments communicated by Mr Davies Gilbert to Dr Young (Nat. Phil. II. p. 397), as well as with the much older determination of Desaguliers.

We obtain, upon these principles, the following Table of the elasticities and densities:

There is another of Mr Southern's opinions, which it appears to be almost as difficult to reconcile with other considerations as his conclusions respecting the absolute and relative densities of steam: he imagines that the heat consumed in the formation of steam is almost all employed in the simple conversion of the liquid into a gas, without any great deduction for that which is required for its expansion; while, on the other hand, if we adopted Dr Young's computation, deduced from the effect of expansion and compression on the velocity of sound, we should be obliged to infer, that almost the whole of the heat is required for the expansion only, and very little, if any, would be left to supply what Mr Southern calls the constitutional heat of the steam, in contradistinction to the heat of expansion. Dr Young's book is so little known, though it has been sixteen years before the public, that it will not be superfluous to extract from it the computation in question, which bears immediately upon the utility of all engines with high pressure.

"We may deduce from" Mr Dalton's "experiment," says Dr Young (Vol. II. p. 409), "an acceleration of about $\backslash frac\{1\}\{2\}$ to be added to the calculation of the velocity of sound; and, since the results of [direct] experiments on sound require an acceleration of $\backslash frac\{1\}\{2\}$, or only $\backslash frac\{1\}\{2\}$ more, which has been ascertained with great accuracy, it may be fair to allow the supposition of Laplace and Biot, that the whole acceleration of sound is owing to this cause. We may, therefore, make the exponent of the density $\backslash frac\{1\}\{2\}$ for expressing the change of capacity, and the heat produced $1450\; (x^\{\backslash frac\{1\}\{2\}\}\; -\; 1)$; which, when the density is doubled or halved, becomes 131.2°; [and] a compression of $\backslash frac\{1\}\{100\}$ will produce a heat of 1°.

"Now, it appears from experiments on the sounds of different gases, and from the sound of a pipe in air of densities the most various, that the correction of the velocity of sound is nearly the same in all; hence it may be inferred, that the heat produced by condensation follows nearly the same law with respect to all gases. This principle may, therefore, probably be extended to steam. Supposing the conversion of water into steam to absorb as much heat as would raise its temperature 940°, we may call its capacity at 212°, 1.60, and may calculate a table for other temperatures, assuming, with Mr Dalton, that its simple expansion by heat is equal to that of air. Mr Watt has shown, by direct experiment, that steam has a greater capacity, as its temperature is lower.

"Hence, if a steam-engine work with double atmospheres, the heat being about 247°, it will require 1.87 times as much water, of which the capacity [as steam] is 1.48; and its excess above that of water $\backslash frac\{1\}\{2\}$ as much as at 212°; it will therefore absorb about 752°, and the heat required for raising water from 100° will be as 1.87 (147+752) to 112+940, or nearly as 8 to 5, while the effect is doubled.

"Robison says, that four ounces of water, at 100°, will condense in a second nearly 200 cubic feet of steam, reducing its expansive force to $\backslash frac\{1\}\{2\}$. If this is correct, it sets at defiance all theories of capacity. The only distant analogy that can be found for it, is

the facility with which rarified air is found to carry off heat, which would induce us to suppose, that the capacity of a given bulk of air is much less affected by its density than this calculation appears to demonstrate;" and this apology, if we allowed it any weight, would carry us still farther from Mr Southern's opinion."

The two steps by which Dr Young's argument proceeds are, first, the effect of compression on air, as deduced from Laplace's ingenious and fortunate suggestion respecting the velocity of sound; and, secondly, the identity of this effect, as observed by himself and others, in air of different densities, and in gases of different kinds; whence he infers, that the same law may probably be extended to steam; that is, as far as we can depend on such analogies as have been employed in the doctrine of the capacities of bodies for heat, which, however, appear to be by no means unobjectionable. A remarkable consequence of this mode of computation would be, that water might be converted into steam of its own density almost without any expense of heat whatever for what Mr Southern calls its constitutional heat; for if we make $x\; =\; \backslash frac\{1\}\{2\}$ in the formula $1450\; (x^\{\backslash frac\{1\}\{2\}\}\; -\; 1)$, we have 907° for the change of temperature attending such a compression or expansion; and the result would be still greater, if we supposed the heat evolved to be expressed by $n\; \backslash log\; x$, taking $n$ above 400°, for the multiplier of the common logarithm of the density. Probably, indeed, both these modes of computation give a change of temperature greater than the truth for the effect of any very considerable compression or expansion of steam; but the lowest estimate that we can form of this effect will still make it probable that steam is always cooled by its expansion below the temperature which would allow it to subsist in its rarified state as pure steam; or, in other words, that a deposition of moisture is always immediately produced by its expansion, and that steam is rendered drier by compression, supposing no heat to be absorbed or emitted in either case. The very low temperature at which a thermometer stands, in the steam that issues out of a boiler under a very high pressure, is a further proof of the great increase of its capacity, or of the great absorption of heat occasioned by the expansion.

Mr Perkins's invention, as described in the Edinburgh Philosophical Journal for July 1823, appears to consist in substituting for the boiler a strong vessel of gun metal, which he very properly calls a generator, intended to subject the water to a heat of between 400° and 500°, and allowing it to escape only through a valve loaded with a weight equivalent to 35 atmospheres, while it is furnished with a safety valve loaded to 37, and with a gage indicating, by means of a portion of compressed air, the actual pressure of the steam produced by it; and even the water that returns to the generator is subjected to a pressure of five or six atmospheres, which keeps it at a temperature of more than 300°. The generator contains eight gallons of water, or 2352 cylindrical inches, while the piston is two inches in diameter, and the cylinder 18 inches long, containing 72 cylindrical inches, or $\backslash frac\{1\}\{3\}$ as much as the generator: affording a stroke of 12 inches, with a pressure

initially of about 430 pounds on each square inch, that is, about 1300 pounds in the whole, or probably somewhat less, since the valve of the generator cannot be supposed to remain open long enough for the pressure on each side of it to become equal. The danger of explosion, which has hitherto prevented the general employment of engines with very high pressures, is here avoided by the great strength and the moderate dimensions of the apparatus; and besides the safety valves, a thin ball of copper, or a safety bulb, is provided, which will only sustain the pressure of 1000 pounds on the square inch, while every other part of the vessels is calculated to sustain 4000.

Mr Perkins's mode of applying heat to the water appears in reality to be extremely advantageous, not so much from any evidence that has been produced respecting the performance of the engine, as from the fact asserted by the writer in the Journal, that "as much low pressure steam, of four pounds on the square inch, may be generated by one bushel of coals," employed upon three tubes of gun metal, communicating, by means of a loaded valve, with the boiler of a common engine, as by nine bushels applied in the ordinary manner. It has been conjectured that the heat is more easily communicated by the generator to the water, on account of the more intimate contact which subsists between them, and the absence of any nascent bubbles of steam in the neighbourhood of the common surface: but it appears to be more probable, that by far the most material part of the advantage depends upon the more perfect combustion of the coal at a high temperature near the external surface of the thick boiler: for it has been sufficiently proved by Sir Humphry Davy and others, that an intense combustion evolves a much greater quantity of heat from the same materials than a more languid oxygenization.

However this may be, there is nothing paradoxical, and nothing marvellous in the operation of the engine itself, nor in the rapid production of a quart of steam from eight gallons of water "almost red hot." The writer in the Journal has fancied that the water, as it "flashes out," may rob the neighbouring particles of their heat, so as even to leave them below the freezing temperature, while it is itself much above the boiling: but this supposition is in direct oppo-

sition to every thing that we know of the properties of heat. If we reason at all in natural philosophy, we must reason from analogy: such a phenomenon as this would be incredible, even if it were asserted upon the direct and clear testimony of the senses; but to assume it as a probability, without pretending that it has been observed, appears to be setting completely at defiance the obvious dictates of common sense. There is no other instance whatever in which a body, contiguous to another, and exceeding it in thermometrical temperature, does not communicate heat to it rather than receive heat from it: nor, on the other hand, is there any one fact besides, that could be imagined to favour the hypothesis of a sudden and general communication of heat between the distant parts of a material substance, like that which appears to take place with regard to the electrical fluid.

The quantity of water, thrown out at each stroke, appears, from the table of densities inserted in this article, to be $\backslash frac\{1\}\{40\}$ of the quantity that would fill the cylinder, that is $\backslash frac\{1\}\{5000\}$ of the content of the generator, and its conversion into steam at any temperature would not require, at the utmost, more than $940^\backslash circ$ of heat, that is, a quarter of a degree for the whole of the water in the vessel, and possibly not above one fifth of a degree: and it is obvious that the parts nearest the surface might easily furnish this with sufficient rapidity, and yet without approaching very near to the "freezing temperature."

But whatever mistakes may have been made respecting the principles of Mr Perkins's engine, there can be no reason to doubt of its possessing some considerable advantages in practice, which he will probably soon be enabled to establish, by the accurate test of its performance in pumping up water, as compared with that of the engines at present in common use, which are said to be capable of raising a weight equal to that of the coals consumed to the height of above half a million of feet: so that, in order to make good the expectations that have been held out to the public, he will have to raise an equal weight to about five million feet; and when this has been performed, he will have triumphantly redeemed the pledge that has been given on his behalf.