Difcovery of native Gold in Ireland.

227

laft (1795), and continued till the 18th of October, when a party of the Kildare militia arrived, and took poffeffion by order of government; and the great concourse of people, who were bufily engaged in endeavouring to procure a share of the treasure, immediately defifted from their labour, and peaceably retired.

Calculations have been made, that, during the foregoing period, gold to the amount of three thousand pounds Irish fteriing was fold to various perfons; the average price was three pounds fifteen fhillings per ounce; hence eight hundred ounces appear to have been collected within the short space of fix weeks.

58 ΤΟΥ

ΤΟΓ

The gold is of a bright yellow, perfectly malleable; the specific gravity of an apparently clean piece 19,000. A fpecimen affayed here by Mr. Weaver in the moift way produced from 24 grains, 22 grains of pure gold, and 143, of filver. Some of the gold is intimately blended with, and adherent to quartz; fome (it is faid) was found united to the finegrained iron-stone, but the major part was entirely free from the matrix; every piece more or less rounded on the edges, of various weights, forms and fizes, from the most minute particle up to 2 oz. 17 dwt. ; only two pieces are known to have been found of fuperior weight, and one of thofe is five and the other twenty-two ounces.

The bearings are all taken by the compafs, without allowing for the variation. William Molefworth, Efq. of Dublin, in a letter to Richard Molesworth, Esq. F. R. S. writes, that he weighed the largest piece of gold in his balance, both in air and water, and that its weight was 20 oz. 2 dwt. 21 gr. and its fpecific gravity to that of sterling gold, as 12 to 18. Alfo, that Richard Kirwan, Esq. F. R. S. found the specific gravity of another specimen to be as 13 to 18. Hence, as, the gold was worth 41. an ounce, Mr. William Molefworth concludes that the fpecimens are full of pores and cavities which increase their bulk, and that there are fome extraneous substances, such as dirt or clay, contained in those cavities.

This opinion was difcovered to be well founded, by cutting through fome of the small lumps.

Stanesby Alchorne, Esq. his Majesty's affay-mafter at the Tower of London, affayed two fpecimens of this native gold. The first appeared to contain in 24 carats,

21 of fine gold.

17 of fine filver.

of alloy, which feemed to be copper tinged with a little iron.

The fecond fpecimen differed only in holding 21 instead of 21 of fine gold. Major John Brown, of the royal engineers, tranfmitted to the Right Hon. Thomas Pelham a sketch of the fpot where the gold was found, which Mr. Pelham permitted to be engraved for the ufe of the Royal Society, and of which fig. 5. plate IX. is a copy, one shird of the fize.of the original.

Gga

An Account of the Principles and Effect of Steam Engines, which act by means of a Piston. With Defcriptions of the Atmospherical Engine of NEWCOMEN and CAWLEY; the Engine of WATT, which works in Vacuo; and a new Engine by Mr. SADLER, in which the direct Action of Steam and the Preffure of the Atmosphere are combined.

SOME

OME account of the original fteam-engine of the Marquis of Worcester, and its fubfequent improvements, has been given in a former paper in this work. One of the chief imperfections of that engine was then fhewn to confift in the direct action of the fteam, for forcing, being neceffarily more than equal to the weight of the column of water required to be raised; in confequence of which, it became in moft cafes an indifpenfable condition, that the boiler and veffels fhould be very strong, as well as that a large quantity of fuel fhould be confumed, to produce fteam fufficiently denfe. It is probable that these inconveniences may have early directed the thoughts of various ingenious men to the application of a pifton, though the difficulties of the undertaking feem to have retarded this purfuit for a confiderable time. The firft fteam-engine, with a pifton, made in 1707 by Papin †, was little calculated to remove thefe difficulties; and it is to Newcomen and Cawley that we are indebted for the application of a pifton with machinery, by which the indirect action of fteam little stronger than the atmosphere, or rather the direct action of the atmosphere, is made to act with fafety and effect against the most fevere preffures. It appears that they had brought their engine, about the year 1713 t, to a degree of perfection little different from those which are to be seen at present. A particular defcription of this engine, with drawings of its parts, and a confiderable portion of the history of its invention, are to be met with in Defaguliers's Courfe of Experimental Philofophy, and it has also been described in many other books. For these reasons, I fhall in this place give merely a verbal account of its principles and mode of operation.

Suppofe a very large fyringe to be fet upright, and a pifton or plug inferted at the upper end, the ufual aperture being supposed to be at the lower extremity. If this laft aperture be open, the pifton will defcend by its own weight, neglecting the effect of friction at its circumference. But let it be imagined, that the pifton is supported by a counter-weight at the oppofite extremity, by a lever or by any other means. In this cafe the piston will not defcend unless more weight be added to it. Among the various ways of applying fuch a weight, there is one which confifts in exhaufting the air from the internal part of the cylinder beneath the pifton. For, if this were done, it is evident that the whole preffure of the atmosphere, which amounts to about twelve pounds on every circular inch, will become active upon the upper furface. If the vacuum were to be produced by means of an air-pump, it may eafily be allowed, that the labour of effecting it would be at leaft equal to that of any work which might be performed by the fubfequent defcent of the pifton. We have feen that, in Savery's engine, the operation of fteam is twofold; namely, by the direct preffure from its

Philofophical Journal, I. 419.

+ Loco citato, p. 421.

Detauliers in his Lectures, ii. 467, fays that it had been in ufe near thirty years at the time he wrote, and the imprimatur to his work bears date November 17, 1743. See also p. 532 of the fame volume

elafticity,

Steam Engines which work by a Pifton.

229 elafticity, and by the indirect confequence of its condenfation, which affords a vacuum. This laft is the only principle difplayed in Newcomen's engine. In order to produce the vacuum at pleafure, it becomes requifite that various apertures fhould be formed at the bottom of the cylinder or fyringe we have been speaking of :-one to communicate fteam from a boiler; another to admit a jet of cold water, to condenfe that fteam during the interval in which the communication from the boiler is cut off; a third provided with a pipe called the eduction-pipe, to carry off the condensed fteam and injection water; and lastly, a small lateral aperture or valve through which the permanently elastic fluid, which cannot descend through the eduction-pipe, may be driven out. This laft is called the fnifting clack. By these provifions the operation is made to take place as follows: The piston being up, the steam-cock is opened and fteam iffues from the boiler, which being less than half the weight of common air, rifes to the top and expels the air through the eduction-pipe, of which the lower extremity is covered with a flap valve in a trough of water. When the noife of its escape is heard the steam-cock is fhut, and the injection-pipe being opened throws a ftream of cold water against the bottom of the piston. The fteam becomes immediately condenfed, and the preffure of the atmosphere forces the piston down into the vacuum. Upon its progrefs downwards the injection pipe is closed; and when it has arrived nearly to the bottom of the cylinder, the steam-cock is again opened. The elastic fteam then not only fills the fmall space between the cylinder and the bottom, but its preffure affifts the eduction water to pass off through its pipe, and drives the difengaged elaftic fluid through the fnift. In this ftate therefore the team is fomewhat ftronger than the atmosphere, and counterpoifes its action on the upper furface of the pifton; whence the pifton itself rifes by the action of the counter-weight, and regains its original pofition at the top of the cylinder. A fecond repetition of the process, namely, of shutting off the fteam and injecting cold water, causes it again to defcend, and in this manner the alternations may be continued without limit.

As the preffure requifité to work an engine of this kind with speed, and to overcome the friction of its parts, may be estimated at about 71⁄2 pounds for every round inch, the effect of a pifton three feet in diameter, with ten ftrokes of fix feet each per moment, will be to raife a weight of near half a ton at the rate of one foot per fecond. Such a power will therefore be very confiderable.

It is to be understood, that the opening and fhutting of the fteam and injection cocks are performed by apparatus fixed to the working beam, in such a manner as to strike the levers of those cocks at the precife inftants of time when their effects are required to be produced. The attendant has no other office to perform than keeping up the fire.

This curious and moft eminently ufeful engine became an object of the researches of Jaines Watt, then of Glasgow, but fince affociated with Matthew Boulton of Birmingham. It would be fuperfluous for me to enter into any general account of the fcience and intelligence of either of these engineers, or of the great advantages which fociety must ever derive when the spirit of enterprise is joined to the exertions of mental ability. Neither can I here attempt to detail the variety of new refults which are exhibited in the specifications lodged by Watt in the Chancery offices concerning this object. My prefent limits and the attention which the world has already paid to these eminent men, and the products of their enlightened

230 lightened activity, render both unneceffary. I fhall therefore confine myfelf to remark, that the great features of improvement made in the engine of Newcomen by Watt are, first, that the elafticity of the steam itself is used as the active power in his engine; and fecondly, that befides various other judicious arrangements for the economy of heat, he condenses the fteam, not in the cylinder, but in a separate veffel.

Defcription and Effect of the Steam Engine of Watt and Boulton.

In the great cylinder or fyringe, concerning which we have spoken, in defcribing the engine of Newcomen, let us fuppofe the upper part to be closed, and the piston-rod to flide air-tight through a collar of leathers. In this fituation, it is evident, that the piston might be depreffed by throwing the fteam upon its upper furface, through an aperture at the fuperior end of the cylinder. But if we fuppofe the exterhal air to have accefs to the lower furface of the piston, we shall find, that steam no ftronger in its elasticity than to equal the weight of the atmosphere would not move the pifton at all; and confequently that this new engine would require much denfer fteam, and confume much more fuel than the old engine. The remedy for this evil is to maintain a conftant vacuum beneath the piston. If such a vacuum were originally produced by steam, it is certain that its permanency could not be depended on, unless the engine contained a provision for conftantly keeping it up. Mr. Watt's contrivance in his fimpleft engine is as follows: The fteam is conveyed from the boiler to the upper part of the cylinder through a pipe, which also communicates occafionally with the lower part, and beyond that space with a veffel immersed in a trough of water; in which veffel the condenfation is performed by an injected ftream of cold water. This water is drawn off, not by an eduction-pipe but by a pump, of which the stroke is fufficiently capacious to leave room for the elastic fluid, separated during the injection, to follow and be carried out with the injection water. Suppofe now the pifton to be at its greatest elevation, and the communication from the boiler to the upper as well as to the lower parts of the cylinder to be opened. The fteam will then pass into the whole internal part of the engine, and will drive the air downwards into the condenfer, and thence through the valves of the air-pump. In this fituation, if the communication from the boiler to the lower part of the cylinder be stopped, and an injection be made into the condenser, a vacuum will be produced in that vessel, and the fteam contained in the lower part of the cylinder and communication pipe will expand itself with wonderful rapidity towards the condenser, fo that in a period of time too minute to be appretiated, the whole of the fteam beneath the pifton will be practically condensed. The fteam which continues to act above the pifton will immediately deprefs it into the vacuum beneath; at the fame time that by connection with the external apparatus the pifton of the air-pump alfo defcends in its barrel. When the ftroke is nearly completed downwards, the requifite part of the apparatus fhuts the communication with the boiler; opens that between the upper and lower parts of the cylinder and condensing veffel; and turns the injection-cock. At this very inftant the pifton lofes its tendency to defcend, because the fteam preffes equally on both furfaces, and continues its equality of preffure while the condenfation is performed. It therefore rifes; the injection is stopped; and the air-pump making its ftroke fuffers the injection water and a confiderable part of the elastic fluid to pass through its lower valve. The vacuum is thus kept up through the whole internal capacity of the engine. As foon as the pifton has reached the upper part of the cylinder, the communication to the under part of the cylinder is ftopped, and that with

Comparison of the Steam Engines of Newcomen and of Watt.

231

the boiler opened as before; the confequence of which is, that the pifton again descends, and in this manner the alternations repeatedly take place.

The principal augmentation of power in this engine, compared with that of Newcomen, appears to arife from the cylinder not being cooled by the injection water, and its being practicable to ufe fteam fomewhat ftronger than the preffure of the atmosphere. In general thefe engines are worked by steam, which would fupport a column of four or five inches of mercury befides the preffure of the atmofphere, and I have fometimes feen the gage as high as eight inches. But whether the comparative profit of the additional work was such as to repay the extraordinary confumption of fuel in fuch cases may be doubted; and indeed I fuppose this practice is never adopted but when the work of a manufactory is fuch as to exceed the power or rate of the engine conftructed for performing it.

The most ample information refpecting thefe improved steam engines, which is to be met with in print, is found in the Nouvelle Architecture Hydraulique of Prony.

When we contemplate thefe engines together with the fimpler engine of Savery, one of the moft ftriking differences, is, that the latter is not embarraffed with the weight and expence of the beam and counterpoife. I believe that the fly was first introduced by Meflrs. Watt and Boulton, as an equalizer of the action in fteam engines, which in various conftructions renders a counterpoife unneceffary, but is faid to diminish the power by loading the work with friction and refiftance. Of this however I can fay nothing from my own experience or obfervation. The counterpoife is alfo unneceflary in the engines called double engines, in which the steam is made to act alternately on each side of the piston, by proper communication from the boiler, while the fpace into which the pifton is to move is made to communicate with the condenfing veffel, where a conftant injection is maintained. I was alfo informed, in the year 1794, that fome engines had been conftructed without the beam; but the architect from whom I had the information did not ftate the particulars.

The injection water and elaftic fluid are difpofed of in Newcomen's engine with scarcely any lofs of power, because the former flows fpontaneously through the eduction-pipe, and the latter is driven out by a very flight effort of the fteam against the atmosphere. Watt's engine could not have an eduction-pipe unlefs the column were at least 34 or 35 feet long, to exclude the atmosphere, and even in that cafe there could be no fnift to deliver the elaftic 'fluid. It was therefore neceffary there fhould be an air-pump to carry off both; and this is a load upon the engine equal to the amount of the friction of these additional parts and the whole weight of the eduction water. I am not well informed of this last quantity; but. fuppofe it to be about one eightieth * part of the power of the engine.

The fteam-engine for which a patent has lately been granted to Mr. Sadler, is worked. without a lever or beam, and confequently has not the inertia of that heavy mafs to overcome. The greateft part of its action is performed by the elafticity of fteam; but the most remarkable character of his invention is, that part of the fteam previous to its condenfation is. employed a fecond time in another cylinder, the piston of which is subject to the preffure of the atmosphere. By this fecond application it not only performs the office of an air-pump. in a very accurate manner, but likewife adds to the total amount of force in the machine..

Suppofing the injection water to be about of the magnitude of the cylinder within, and that the ftroke would raife of the fame magnitude of water through its own length..

Plate X