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engine, Mr. Watt makes use of a very large fly-wheel G ; on theaxis of which is a small concentric toothed wheel, H. A similartoothed w heel, I, is fastened by straps to a rod coming from theend of the beam, so that it cannot turn round on its axis, but mustrise and fall with the motion of the great beam. A bar of ironconnects the centres of these two small toothed wheels, so thatthey cannot quit each other. When, therefore, the beam raisesthe wheel I, it must move round the circumference of the wheelH, and turn it together with the fly ; and it will be evident, uponconsideration, that the fly, driven in this manner, will make tworevolutions for every one of the wheel I. This mode of movingthe fly, is preferable to a crank ; as it goes with twice the velo-city. This contrivance is called the sun and planet-wheel, fromthe resemblance of the motion to that of those bodies. Thevalves of this steam-engine are all puppet-valves, as these are foundleast liable to be out of order. The mode of operation in Mr.Watt's engine, is as follows : Suppose the piston at the top of thecylinder, in the situation represented in the plate, and the low’erpart of the cylinder filled with steam. By means of the handle E,open the steam-valve a, and the eduction-valve d, the levers ofwhich are connected together; there being now a communicationbetween the cylinder and the condenser, the steam instantly rushesinto the condenser, leaving the cylinder empty ; whilst at the-same time the steam from the boiler, entering by the steam-valvea, presses upon the piston, and forces it down. As soon as thepiston has arrived at the bottom, the steam-valve c, and the educ-tion-valve b, are opened, whilst the valves a and d are shut; thesteam therefore immediately rushes through the eduction-valve b,into the condenser, whilst the piston is forced up again by thesteam, which is now admitted by the steam-valve c. Fig. 2,which is a section of the steam-pipes, taken at right angles to thatin fig. 1, shews this more distinctly; i is the pipe which conveysthe steam from the boiler; a and c are the steam-valves, and band d the eduction-valves. By attending to the operation in boththe sections, the reader will easily understand it. It appears atfirst a little confused, by there seeming to be only one steam-pipefor communicating between the cylinder and the’ condenser; butthe difficulty is cleared up, by representing both tiie pipes, as infig. 2. Fig. 3, is a longitudinal section of the boiler, representingthe mode of supplying it with water, and the safety-valve andcocks, / is a small cistern, which is supplied with water from thehot-well, as represented in fig. 1 : from the bottom of this cistern,a pipe goes down almost to the bottom of the boiler, whore itturns up a little, to prevent the entrance of the steam which risesfrom the bottom. From the side of this cistern, is supported aSmall lever, to one end of which is fastened a wire, that carries astone which hangs in the water of the boiler ; the other end of thefever supposing also by a wire, a valve that shuts the top of thepipe that goes down from the cistern. Now, supposing the stonejust at the surface of the water, and balanced by a weight at theopposite end of the lever ; it is evident, that by {lie laws of hydro-statics, already explained, a certain part of the weight of (lie stonewill be supported by the water, so long as it continues immersedin it; hut if a part of the water evaporate by boiling a propor-tional part of the stone will he above the water, consequently thestone will bear more upon the lever, and raise the weight at theother end; but in raising that weight, it also opens the valve inthe small cistern, and admits water until it stand at the same heightin the boiler as before, and then the valve and the stone beingagain in equilibrio, the valve remains shut until a new quantity isevaporated. By this means the supply of water is very gradual,"however, and not by fits and starts, as here described for the sakeof illustration. It is found by experience, to he a much bettermethod than a ball-cock, and the regular supplying of the boilerwith water is of the first importance. As a check upon this, andto know perfectly the height of the water in the boiler, there aretwo cocks, g and h, one of which readies nearly to the surface oftiie water when at its proper height, and the oilier enters a littlebelow tiie surface. It is evident, that if the water be at the justheight, and you open g, steam will issue ; and if h be opened, wa-ter will be driven out by the pressure of tiie steam. But if watercome out from g, then the water must be too high in the boiler;and if steam issue from li, tlu-n the water is loo low. By thismeans, it is easy to know at all times the exact height of the waterin the boiler, i is a safety-valve, to prevent the bursting of the

boiler by the steam growing too strong ; k is the pipe which con-veys tiie steam to tiie engine. Fig. 4, is Mr. Cartwright’s steam-engine, the construction of which evinces much ingenuity, a isthe cylinder, which is supplied with steam from the boiler throughthe pipe b; c is the piston in the act of going up ; d is the pipethat conducts the steam into (lie condenser e, which consists of twocylinders, one within (he other, leaving a small space betweenthem, into which the steam is admitted; while the inner cylinderis filled with cold water, and also the external cylinder surroundedby the same ; so that, by this means, a very large surface of steamis exposed, though no water is suffered to come into actual con-tact with it. To the bottom of the piston, c, is attached a rod,with another piston, e, working in the pipe d. When the pistone arrives at the bottom of the cylinder, a valve which is in thepiston, is opened by its pressing against the bottom, and opens acommunication with the condenser, whilst the spring k, fixed tothe rod of the piston, shuts tiie valve which admits the steam fromthe boiler. The steam, therefore, being thus condensed, runs intothe lower pipe/. The piston c, arriving at the bottom of the pipein which it works at ti,c same time with c, presses upon the con-densed water, shuts the valve/, and forces the water up the pipeg, into the box h. The air which is disengaged from the water,rises to the top of the box, and, by its elasticity', forces the waterthrough the pipe i, which carries it back again into the boiler.When the air accumulates in the box to such a degree as to de-press the water, the ball-cock falls with it, and opens a valve intlip top of the box, which suffers some of the air to escape.\Y hen all the steam is condensed, the motion of the fly attached tothe machine brings the.piston up again, its valve now remainingshut by its weight. On arriving at the top, it presses up thesteam-valve, which admits tiie steam from the boiler to force itdown as before. I and m are two cranks, upon whose axis aretwo equal wheels working in each other, for the purpose of con-verting the perpendicular motion of the piston-rod into a rotatorymotion, for working tiie machinery attached to it. But the mostvaluable part of this engine is in the construction of the piston,which Mr. Cartwright made wholly of metal, and so as, by meansof springs, to fit tiie cylinder very exactly. This not only savesthe expence and trouble of packing, which they are obliged fre-quently to renew in all other engines, but also saves a great dealof steam, on account of the more accurate fitting of the piston.As it is evident from its construction, that the whole of the steamis brought back again into tiie boiler, it affords the means of em-ploying ardent spirit instead of water, and thus saving a great dealof fuel. This machine seems to he peculiarly applicable to pur-poses requiring only a small power, as it is not expensive, andoccupies little room. It would far exceed the limits of this work,to enter into an examination of all the steam-engines invented bydifferent persons. It is sufficient to mention, that no engine ofthis kind has been found, upon careful trial, to be superior to thoseof Mr. Watt.

STEATITE , in mineralogy, a species of the talc-genus, gene-rally of a white colour; but it presents several varieties through(lie greys, greens, and reds. It occurs massive, disseminated, mcrusts, and crystallized. Tiie crystals are six-sided prisms, acu-minated on botli extremities by six planes. Specific gravity 2.C.The constituent parts are, according to Klaproth ,

Silica.48.0

Magnesia .20.5

Oxide of iron. 1.0

Water.15.5

Alumina.14.0

99.0

Loss. 1.

100

It occurs in beds and veins in serpentine, also in irregularly shap^pieces, imbedded in rocks belonging to the floetz formation.is found in Norway, Sweden, several parts of Germany , and >the united kingdom of Great Britain ; also in China , ft is 11Sin the manufacture of porcelain. ..j,

STEATOMA, [rt«itu l u.«,] a kind of encysted tumour, con5 p,g

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