structed a machine meant to serve as a pump, and in which there was a detached boiler wherein steam was generated. This engine, as well as Papin's little model, presents a vertical metal cylinder, closed below, open above, and a piston, fitting closely, intended to ascend and descend through the whole length of the cylinder. In the one apparatus, as well as in the other, as the steam gains a free admission into the lower part of the cylinder, it fills it, and thus counterbalances the pressure of the external air; the ascending movement of the piston is effected by means of a counterpoise. Finally, in the English engine, in imitation of Papin's, as soon as the piston has arrived at the limit of its ascent, the steam which had helped to elevate it, is cooled down; a vacuum is thus made throughout the whole space which the piston had just traversed, and the weight of the external atmosphere causes it forthwith to descend.*

* Switzer, in his system of Hydrostatics, already quoted, after giving a detailed account of Savery's Engine, says, p. 335,—“ To finish this long account of the surprising Engine for the raising Water by Fire, I produce this last improvement of it by Mr. Thomas Newcomen, which makes it undoubtedly the beautifullest and most useful Engine that any age or country ever yet produced." Then after describing Newcomen's Engine, he says, p. 341,—“ What I have to add in this place is, that as the best and most useful improvements which have been discovered either in Art or Nature, have, in process of time, been liable to improvement; so this of the Fire-Engine has been subject to the same. For this ingenious gentleman, to whom we owe this late invention, has, with a great deal of modesty, but as much judgment, given the finishing stroke to it. It is, indeed, generally said to be an improvement to Mr. Savery's Engine; but as I am well informed that Mr. Newcomen was as early in his invention as Mr. Savery was in his, only the

To effect the proper cooling, Papin, as you already know, was contented with removing the

latter, being nearer the Court, had obtained his Patent before the other knew it, on which account Mr. Newcomen was glad to come in as a partner to it."

Newcomen and Cawley became associated with Savery in the Patent obtained in 1705. The engine has, however, always borne the name of Newcomen.

The improvements introduced into it were very considerable, compared with the scheme of Papin, or the engine of Savery. In those first made, one cylinder was placed within another, and the interstice was filled with cold water, which effected the condensation with less trouble than the affusion of water. The piston was tightened by packing with leather or rope, and by a stratum of water upon it. A separate boiler was used, as had been done by Savery, for the generation of the steam, and the consumption of water in it was supplied by a pipe from the top of the piston. A snifting-valve was applied for blowing out the air, and an eductionpipe for getting rid of the water arising from condensation, with improved mechanism of cocks and valves. But the great invention for rendering the power applicable to practical purposes, consisted in a working lever, or great beam moving on a centre, one end being connected with the piston-rod, by means of an arch and chains, and the other with the pump to be worked by it, having a counterpoise for the piston. The subsequent introduction of the cold water into the inside of the cylinder, and the working of the cocks or valves from the great beam, as stated in the text, were important points; and, by the last, the steam-engine was rendered a self-acting machine. The details underwent much improvement in the hands of Beighton, and finally of the great engineer Smeaton. See the particulars in Farey, Treatise on the Steam-Engine, pp. 138-204. The historical and mechanical information there given by this author, and indeed down to p. 308, will be found very deserving of an attentive perusal.

It results from the facts we have adduced in the notes pp. 41 and 50, that of the above machine, the first idea of the cylinder, packed piston and rod, and the use of the pressure of the atmosphere as a power, belong to Otto Guericke; the forming a vacuum by the condensation of steam in the cylinder to Papin; the separate boiler, and perhaps parts of the mechanism of the valves, &c. to Savery. But, whether the ironmonger and the glazier of Dartmouth were

fire which heated the base of his little metal cylinder. Newcomen and Cawley adopted a process in all respects greatly preferable; they caused an ample quantity of cold water to flow into the circular space contained between the outside of the cylinder of their engine, and a second cylinder, a little larger, which enclosed the first. The cold was gradually communicated to the whole thickness of the metal, and at last reached the steam itself.*

Papin's machine, thus improved in the mode of cooling or condensing the steam, engaged the attention of proprietors of mines to a very great degree. It spread rapidly through some of the counties of England, and was there of considerable

acquainted with what Otto and Papin had described in languages probably to them unknown, can only now be guessed at. That they knew something of what Savery had carried into practice six years before is likely, although Switzer appears to consider that they invented the whole. In the history of arts and sciences there have been many cases of apparent coincidence of inventions, of which, the theory of fluxions, bringing into opposition the great names of Newton and Leibnitz, forms the most illustrious instance.-TR.

*

Savery had already had recourse to a stream of cold water which he threw on the outer sides of a metallic vessel, to condense the steam which that vessel contained. Such was the origin of his partnership with Newcomen and Cawley: but it must not be forgotten that Savery's patent, his engines, and the work in which he describes them, are many years later in date than the papers written by Papin.-M. ARAGO.

66

But, as Papin's first process was conducted in a way which M. Arago admits was scarcely tolerable even in an experiment intended to verify the accuracy of a principle," and at such an expense of fuel and labour, as to make it confessedly of no use in practice; and, as on the appearance of Savery's invention, Papin abandoned his own, and imitated the new one, but without success; we beg to refer, for a statement of the dates and circumstances of the respective inventions, to the notes given above at pp. 41 and 50.—TR.

use.

The slowness of its motions, unavoidably consequent on the length of time which the steam required to cool, and lose its elasticity, was however the subject of deep regret. Accident fortunately pointed out a way of remedying this inconvenience.

In the beginning of the eighteenth century, the art of boring large metal cylinders, and of closing them hermetically by means of moveable pistons, was yet in its infancy. Thus in Newcomen's first engines, the piston was covered over with a layer of water, intended to fill the interstice between the circumference of that moveable part and the internal surface of the cylinder. To the great amaze of the constructors, one of their engines began one day to move with much more than its ordinary rapidity. After repeated investigations, it was clearly proved, that on that day the piston had a hole in it; that cold water fell into the cylinder in very small drops, and that in passing through the steam, they caused it rapidly to disappear. From this accidental observation, is to be dated the complete abandonment of the application of cold from without, and the adoption of the rose-head for the purpose of injecting a shower of cold water through the whole interior of the cylinder at the instant marked for the descent of

*

* In French, pomme d'arrosoir. The term rose-head seems to be derived from the French arrosoir or rosée, or perhaps from the Latin ros. In the first engines in which the injection was introduced into the cylinder, the water appears to have spurted straight up from the end of the injection pipe.-TR.

the piston. The alternate motion thus acquires all the rapidity desirable.

Let us see whether accident has not also had some share in another equally important improve

ment.

Newcomen's first engine required the closest attention on the part of the person who had constantly to open or shut certain cocks, either to admit the steam into the cylinder, or to inject the cold shower which was to condense it. This person happened on one occasion to be a lad of the name of Henry Potter.* This boy, unhappy at hearing the joyous cries of his companions then at play, ardently longs to go and join them, but the work with which he had been entrusted would not admit of half a minute's absence. His brain becomes excited, passion inspires him with genius,— he discovers relations of parts of which till then he had no suspicion. Of the two cocks, the one should be opened at the moment when the beam, which Newcomen first introduced with so much advantage into his engines, has completed its descending movement; and, similarly, it must be shut at the close of the contrary movement. Of the other cock, the management is directly the reverse. The positions of the beam and of the two cocks have a necessary dependence on each other. Potter seizes on this observation; he perceives that the beam may be used to communicate

* In Desaguliers, vol. 11. p. 533, edit. 1744, quoted by Stuart, p. 66, by M. Arago in the Annuaire for 1830, p. 210, &c. &c., this idle but ingenious boy is called Humphry Potter.—TR.