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model, capable of producing a force equal to 14 pounds on every inch of the piston, and which did not require more than onethird of the steam used in the common atmospheric engine to produce the same effect.

“ It will be evident that this was as near an approximation towards perfection as could possibly have been expected ; and indeed much more than was likely to be effected in a large engine, as the vapour left beneath the piston possessed only 1-15th part of the elastic force of the steam employed to form the vacuum.

“Having discovered that the great waste of caloric in the old engine, arose from the alternate heating and cooling the cylinder, by the admission and subsequent condensation of the heated steam, Mr. Watt perceived that to make an engine in which the destruction of steam should be the least possible, and the vacuum the most perfect, it was necessary, that the cylinder should remain uniformly at the boiling point; while the water forming the steam was cooled down to the temperature of the atmosphere. To effect this, he employed a separate condensing vessel, between which, and the hot cylinder, a communication was formed by means of a pipe and stop cock.

66 To understand the action of this engine, we may employ a common syringe, connected with a boiler, as in the atmospheric engine, and furnished with a pipe passing into an airtight vessel, immersed in water for the purpose of condensation.

“ If the piston be then raised, and the communication with the condenser cut off, the steam will speedily expel the air ; when this is effected, the further admission of steam must be prevented, and the communication with the condenser opened. The steam will now expand itself, passing down the pipe and entering the condenser; the moment, however, that it comes in contact with the sides of the cold vessel, it will be condensed and a vacuum formed; and this process will continue to proceed, so long as any steam remains beneath the piston.

“ The only objection that offered itself to this admirable mode of condensation, arose from the difficulty experienced in getting rid of the water and air that remained in the condensing vessel. When steam was generated from water that had been freed from air by long boiling, a considerable advantage was obtained ; and it was found that a power nearly equal to the entire pressure of the atmosphere was produced. The great advantage thus obtained will be sufficiently obvious, when it is known that, in the engines previously constructed, the elasticity of the steam arising from the heated'injection water remaining at the bottom of the cylinder, was equal to one-eighth of the atmospherical pressure, and consequently destroyed an equal proportion of the power of the engine.

"The mode of condensing the steam, by the application of cold water to the outside of the condenser, was soon found in convenient from the great size and expense attendant on the use of this apparatus; and Mr. Watt introduced an internal jet of cold water, which, striking against the steam, instantaneously reduced it to its original bulk, and thus formed a vacuum. To draw off the condensing water, as well as to get rid of the air that was extricated during condensation, he found it necessary to employ a small pump, worked by the engine, the size of which was proportioned to the amount of air and water generated in the condenser. In one of the early engines upon this construction, erected at Bedworth, three air-pumps were used; two below, worked by chains connected with the beam, and a third, placed above, which received the hot water raised by the others. In the engines now constructed, only one air-pump is employed, and this fully answers the intended purpose.

Another improvement introduced by Mr. Watt, consistedin surrounding the upper part of the cylinder with a cap, through a hole in the centre of which the piston rod worked air-tight. The force of steam was then substituted for that of the atmosphere, and at a pressure of more than fifteen pounds on the square

inch; so that when a vacuum was formed beneath the piston, steam of considerable impellent force was entering the upper end of the cylinder, by means of a pipe connected with a boiler.

“By thus substituting the force of highly elastic vapour, for the ordinary pressure of the atmosphere, the upper and under side of the piston were preserved at the same temperature, and the supply of steam being regulated by the width of the aperture, any given amount of force might readily be produced. In the atmospheric engine this could not be effected, as the whole pressure of the atmosphere was made to act on the piston, the instant the vacuum was formed by the condensation of the vapour beneath ; so that in the event of a pump-rod breaking, by which the elevation of the water might be impeded, and the labour of the engine taken off, the rapid descent of the piston would evidently cause the destruction of the entire apparatus.

“Soon after the completion of his first model, Mr. Watt erected an engine for his friend Dr. Roebuck of Kinneil, near Borrowstownness, with whom he was afterwards associated in the manufacture of his improved engine : the latter gentleman, however, in 1774, disposed of his share of the business to Mr. Boulton, of Soho."

Want of room prevents our making any additional extracts from Mr. P.'s work, or attempting an enumeration of the various engines he describes, which could only be satisfactorily accomplished by reference to the numerous plates employed for their illustration. But it may be adviseable before we finally dismiss the subject, to briefly notice another work of a more general nature, but with much higher pretensions, announced as far back as 1816, though but just published. We allude to the new edition of Professor Robison's Mechanical Philosophy, edited by Dr. Brewster. The article Steam Engine, after having been revised by the late Mr. Watt and the learned editor, has been put forth by Mr. Murray, as "the only account of the steam engine that can be relied upon.” What claims it possesses to this title, may easily be seen by reference to a very simple fact. The last steam engine described in Professor Robison's Mechanical Philosophy, was erected for the Albion Mills, in 1788, since which period we find, by turning to Mr. Partington's appendix, that more than one hundred patents have been enrolled, many of which are of the utmost importance.

Transactions of the Cambridge Philosophical Society, Vol. I.

Part II. 1822.

From an accidental cause, we omitted to notice the first part of this Society's Transactions; we, therefore, take an early opportunity of giving a brief sketch of the contents of the present part.

I. Analysis of a Native Phosphate of Copper from the Rhine. By F. Lunn, Esq.

As this paper has been given entire in the Annals, it is unnecessary to notice it upon the present occasion.

II. Upon the regular Crystallization of Water, and upon the Form of its primary Crystals. By Dr. E. D. Clarke.

This communication of the late and lamented Professor is accompanied by a plate, which is indeed requisite to the perfect understanding of his views.

After mentioning various authors who have treated on the same subject, and described the appearances which crystallized water assumes, Dr. Clarke concludes his memoir with observa ing: “ It is presumed, therefore, that the question respecting the crystallization of water may be set at rest by these phenomena; because it is now no longer a mere inference deducible from observing the intersection and disposition of the spiculæ exhibited by water when frozen upon the surfaces of other bodies, and in its approach to crystallization; but it is a decided fact, shown by regular crystals of ice, that the compound we call water, or hydrogen oxide, crystallizes both in hexahedral prisms and in rhombi, having angles of 120° and 60°; and that the latter is its primary form. The manner too in which these forms have been displayed may guide to the crystalline forms of other bodies, by inducing a careful examination of the surfaces, points, and interstices of all minerals when they are found as stalactites. The stalactite formation is of all others the most likely formation to bear marks of a regular crystallization ; because it is the result of a process in which the particles of bodies are not carried by a too sudden transition from the fluid to the solid state ; but gradually approach, and become united by virtue of their mutual attractions, as the moleculæ of the fluid which had separated them go off by evaporation or by vther causes. And in further confirmation of this, it may be urged, that when the crystallizacion of the stalactite carbonate of lime, and of other stalactites, especially chalcedony, had been considered as impossible formations, contradictory to the laws by which Nature acts in the stalactite process, yet the primary form of the carbonate of lime is nevertheless exhibited by the stalactites of the cavern of Antiparos, and the primary form of the hydrates of silica by the stalactites of blue chalcedony brought from the Hungarian mines."

III. On the Application of Hydrogen Gas to produce a moving Power in Machinery; with a Description of an Engine which is moved by the Pressure of the Atmosphere upon a Vacuum caused by Explosions of Hydrogen Gas and Atmospheric Air. By the Rev. W. Cecil, ÑA. Fellow of Magdalen College, and of the Cambridge Philosophical Society.

The author of this paper observes that “two of the principal moving forces employed in the arts are water and steam. Water has the singular advantage that it can be made to act at any moment of time without preparation ; but can only be used where it is naturally abundant." A steam-engine, on the contrary, may be constructed at greater or less expense, in almost any place; but the convenience of it is much diminished by the tedious and laborious preparation which is necessary to bring it into action. A small steam-engine, not exceeding the power of one man, cannot be brought into action in less than half an hour; and a four horse steam engine cannot be used under two hours' preparation."

The engine in which hydrogen gas is employed to produce moving forces was intended to unite the two principal advantages of water and steam so as to be capable of acting in any place without the delay and labour of preparation.

The general principle of this engine, as described by Mr. Cecil, is founded upon the property which hydrogen gas mixed with atmospheric air possesses, of exploding upon ignition, so as to produce a large imperfect vacuum. *If two and a half measures of atmospheric air be mixed with one measure of hydrogen, and a flame be applied, the mixed gas will expand into a space rather greater than three times its original bulk. The products of the explosion are a globule of water, formed by the union of the hydrogen with the oxygen of the atmospheric air, and a quantity of azote, which in its natural state (or density 1) constituted •556 of the bulk of the mixed gas; the same quantity of azote is now expanded into a space somewhat greater than three times the original bulk of the mixed gas; that is, into about six times the space which it before occupied;


its density is, therefore, about one-sixth, that of the atmosphere being unity.

According to Mr. Cecil, if the external air be prevented by a proper apparatus from returning into this imperfect vacuum, the pressure of the atmosphere may be employed as a moving force, nearly in the same manner as in the common steam engine ; the difference consisting chiefly in the manner of forming the

Mr. Cecil then enters into an estimate of the power resulting from such a vacuum by comparing the effects of equal bulks of steam and hydrogen ; this it will be impossible to comprehend without the diagram by which it is illustrated ; but the author concludes, that " it appears by calculation that any quantity of pure hydrogen gas will produce more than five times the effect of the same bulk of steam; and in practice the disproportion of their effects is still greater. It is here supposed, that steam produces by condensation a perfect vacuum equal to its own bulk; but this is far from being the case : much of the power is lost by needless condensation by the escape of steam through the piston, besides a considerable deduction for working an air pump, and two water pumps, which are necessary to a steam engine.

This paper is accompanied with a drawing and explanation of a model of a gas engine. The drawings are adapted to the Isometrical Perspective of Prof. Farish. There is also a drawing of one of a different construction which Mr. Cecil has introduced on account of its simplicity.

The paper concludes with some observations upon the use of the explosive force of gunpowder as a moving force, and with showing that it cannot be practically useful, for several reasons, and particularly from the corrosion of metals by the sulphur. contained in the gunpowder, and by the sulphuric acid which is produced during combustion.

IV. On a remarkable Peculiarity in the Law of the extraordinary, Refraction of differently-coloured Rays exhibited by certain

Varieties of Apophyliite. By J. F. W. Herschel, Esq. FRS. of London, Edinburgh, and Gottingen, &c. &c.

In this paper Mr. Herschel refers to the figures contained in the first part of the Transactions ; and as without these, it would be imperfectly intelligible, we shall not attempt any analysis of


this paper.

(To be continued.)

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