opportunity of distinguishing himself, and is supposed to have died in or soon after 1661. (Campbell's Lives of the Admirals; Charnock's Biographia Navalis.) STE. CROIX, DE, GUILLAUME EMMANUEL JOSEPH GUILHEM DE CLERMONT LODE'VE, an eminent French historian, was born at Mormoiron near Carpentras, in the Comtat Vénaissin, January 5, 1746, of a noble family. He was educated among the Jesuits of Grenoble, and afterwards, in 1761, entered the army with a captain's commission in the French cavalry, and as aide-de-camp to his uncle the Chevalier Sainte Croix. On the death of his uncle the same year, he changed his regiment and obtained a company in the Grenadiers de France; after serving between six or seven years he left the army, and, abandoning an active life, gave himself up entirely to the study of history. His literary labours soon obtained him distinction, and he had the honour of being crowned three times by the Académie des Belles- Lettres, of which he was made a foreign member in 1772, being at that time resident at Mormoiron, not then in the French dominions. In 1770 he married Mad'lle d'Elbène, by whom he had two sons and a daughter. In 1791 he shared in the troubles of the times, being driven from his home and thrown into prison; his property was sequestered and his papers and books destroyed. He succeeded by means of a disguise in escaping to Paris soon afterwards. These calamities were followed by severe domestic afflictions which embittered his latter years. He died March 11, 1809. ་ Sainte Croix's works are numerous. Besides contributing many articles to the Journal des Savans,' the Magasin Encyclopédique,' the Archives Littéraires,' the Mémoires de l'Académie des Belles-Lettres,' he published the following works:- Examen Critique des Anciens Historiens d'Alexandre le Grand,' Paris, 1775; second edition, Paris, 1804. This work was translated into English by Sir Richard Clayton, 1793. 'L'Ezour-Vedam, ou Ancien Commentaire du Vedam, contenant l'Exposition des Opinions Religieuses et Philosophiques des Indiens,' Yverdon, 1773. De l'Etat et du Sort des Colonies des Anciens Peuples,' Philadelphie, 1779. Observations sur le Traité de Paix conclu en 1763 entre la France et l'Angleterre,' Yverdon, 1782. Mémoires pour servir à l'Histoire de la Religion Secréte des Anciens Peuples, aux Recherches Historiques sur les Mystères du Paganisme,' Paris, 1784. Histoire des Progrès de la Puissance Navale d'Angleterre, Yverdon, 1782. Les Anciens Gouvermens Fédératifs, et de la Legislation de Crète,' Paris, 1798. (For further information see the Notice Historique, by Sylvestre de Sacy, prefixed to the Catalogue of Sainte Croix's library, Paris, 1809, 8vo.) STE. MARIE, at present the only settlement which the French preserve on the eastern coast of Madagascar, is an island whose centre is in 16° 45' S. lat. and 50° 55′ E. long. It is called by the natives Nossi-Ibrahim, and is separated from Madagascar by a channel, which in its narrowest part is about three miles across, but towards the northern extremity of the island widens to ten miles. The island extends in length from south-west to north-east about thirty miles, and varies in width between five and eight miles. The circuit is nearly seventy miles. A narrow arm of the sea, not far from its most southern extremity, separates a small part of the island at high water from the remainder. The southern part of Sainte Marie is surrounded by a reef, rising above the level of the sea, but there are several openings in it, three of which are deep enough for large vessels. The shores of the island are in general low and swampy, except in a few places of small extent, where they are of moderate elevation. The interior consists of hills apparently isolated, but arranged in chains in the direction of the island from south-west to north-east. The highest of them are from 160 to 200 feet above the sea-level. Their slopes are gentle, and admit cultivation to the very summit; some of them are used as pasture-ground. The soil is bad, except a narrow tract in the interior, which may cover one-fifth of the area of the island, and which is regularly cultivated by the natives. The channel which divides Sainte Marie from Madagascar is a vast roadstead, with good anchorage, and safe, even during the western gales, which rarely occur. this channel an inlet enters eastward into the island, which is more than 2200 fathoms long, and about 1100 fathoms wide, and constitutes the harbour of Port Louis. At the entrance of the harbour is a small island, called L'Ilot Ma From dame. The passage south of this island can only be navigated by small vessels, but the northern passage is deep enough for frigates. At this place the French settlement has been made, as it affords a safe anchorage for several vessels, and as the interior of the harbour is almost entirely filled up with shoals. The island is watered by many streams, and the water is generally good. The climate is very moist. The wet season begins in March and continues to August. In May, June, and July, it rains nearly without intermission, and sometimes in August. But even between August and the end of February rain is frequent. The number of days on which rain falls is stated to vary between 220 and 240, and it is presumed that there are few places on the globe on which a greater quantity of rain descends. The heat is excessive in January and February, when the thermometer sometimes rises in the afternoon to 100°, and varies during the remainder of the day between 88° and 92°; but in the night it descends considerably, so that at sun-rise it sometimes is at 70° and even 68°. During the rainy season the winds in general vary between south-west and south-east, and only occasionally blow from the east or north-east, generally in February and March. In the dry season the winds vary between southeast and north-east; they rarely blow from the south or south-west. The land-breeze, which blows from Madagascar, is felt during the night and early part of the day, and the sea-breeze sets in at noon. In the dry season the breezes are very feeble, but during the rains they nearly always blow with great force. The hurricanes, which are so terrible on the islands of Mauritius and Bourbon, are felt much less in Madagascar, and particularly at Sainte Marie. The natives rear cattle and cultivate rice, mandioc, and some other vegetables; and a great part of the island is covered with forests, which contain many timber-trees. The French settlers introduced several tropical productions, and they continue to cultivate coffee, cloves, sugar, and some vegetables. A considerable number of the natives live on the produce of their fisheries, fish being very abundant at cerlain seasons of the year. Dried fish is exported to several places in Madagascar. The population is composed of natives and foreigners In 1836 it was below 5000 individuals, of whom about 4000 were natives, or Malgashes, as the inhabitants of Madagascar are called. There were also 67 persons paid by the French government, 13 European settlers, and some blacks. The foreign population did not exceed 700 individuals. The French have built a few houses at Port Louis, and fortified L'Ilot Madame. In the vicinity of Port Louis are a few plantations, in which the French cultivate the articles of export for the European market. The natives inhabit about 40 villages chiefly situated in the interior of the island The commerce of Sainte Marie is not important, and s only carried on with the islands of Bourbon and Madagascar. Bourbon receives from Sainte Marie and other ports of Madagascar, cattle, beef, pork, suet, a few hides, landturtles, game, rice and paddy, and some timber; and sends to it some cotton-stuffs of French and English manufacture, spirits, salt, soap, some articles of hardware, arms, and crockery. The French began to form settlements on the eastern coast of Madagascar as early as 1642, but they had no stability, as they were frequently changed, abandoned, an! again taken possession of. The few places occupied by the at the beginning of the present century were taken by the English in 1811, and destroyed. In 1818 and 1819 the French again took possession of Sainte Marie, Titingve Foul Point, Fort Dauphin, and Sainte Lucie; and in 1821 Sainte Marie was regularly settled by a colony of seventynine persons. But a war soon broke out between the Frenc and Radama, the king of Madagascar, who took Fort Dauphin and the other places in 1825, with the excepti of Sainte Marie. In 1829 the French began an active war with the queen of Madagascar, who had succeeded Radam and retook several places, as Titingue, Foul Point, an Tamatave; but after the revolution of 1830, all these poss→ sions were again abandoned, and thus Sainte Marie las remained the only French settlement on Madagascar. (Notices Statistiques sur les Colonies Françaises, Pars 1840.) STEALING. [LARCENY.] STEAM is the name given in general to the vapou::> arising from moist or liquid bodies when subjected to the action of heat: in the mechanical applications however water is the liquid used; we shall therefore, in this article, treat steam as the vapour of water. Steam, naturally, like other gases, is transparent and colourless, its visibility in air arising from its partial liquefaction, whence arise small vesicles of water enclosing steam, which are capable of reflecting light. From their great number, the light which they reflect or transmit, coming to the eye from all angles of incidence, and being a c. nbination of the primary coloured rays, is white, as in snow, the foam of a cascade, &c. 8 343.6 119.2 As the application of heat has generally an expansive effect on bodies, so water converted into steam occupies more than 1700 times its former space. The action of heat in liquefying ice, on the contrary, slightly diminishes its bulk, which remarkable exception to the general effects of caloric is explained by an alteration of the relative positions of the solid elementary crystals of ice in passing into its liquid form, and arising from the repulsive action of heat; Temp. Elast. Temp. Elast. Temp. Elast. Temp. Elast. for it is easy to conceive how an alteration in the axes of a multitude of such infinitely small crystals, produced itself by repulsion, would bring the whole to occupy a smaller space than before. (Biot, Physique.) the elastic force of the vapour of water in mercurial inches, The following is a table abridged from Dr. Ure's, showing with the corresponding temperatures, in which, it may be observed, that Dr. Dalton is confirmed in giving some clastic force even at the temperature of freezing: When ice or snow is in the process of liquefaction, a mercurial thermometer plunged in it will remain constantly at the same height, whatever heat is applied, until the whole mass is dissolved. This heat, latent to the thermometer, but measurable by a calorimeter, is the caloric of liquefaction. Continuing after this stage to apply more heat, the thermometer in the water will be observed to indicate rising temperatures proportional to the surplus of heat thus given. If the heat be applied to the bottom of the vessel containing the water, the lowest stratum of the water expanded by the heat becomes specifically lighter than the incumbent strata; it therefore rises, making way for a descending current of the colder parts, which in their turn rise, and thus the heat becomes diffused through the whole mass. Upon a further application of heat, globules of vapour formed at the bottom rise along the sides of the vessel, but become liquefied in reaching strata of inferior temperature. When these bubbles become larger and more frequent, their condensation is attended by a series of sounds commonly called singing; and after they have acquired sufficient heat to reach the surface, and sufficient elasticity to overcome the pressure of the atmosphere, the vapour passes into the air in the form of steam, and the water is then said to boil. The further application of heat converts gradually the whole of the water into steam, during which the thermometer again becomes stationary, showing the absorption of latent heat; but after this stage has been completed, it proceeds again to indicate degrees of temperature nearly proportional to the surplus heat then applied. Hence we have two fixed points for the thermometer; that of melting ice, 0° Centigrade, or 32° Fahrenheit; and that of boiling-water, 100° Centigrade, or 212° Fahrenheit. [THERMOMETER] The point of ebullition will occur at lower temperatures by diminishing the pressure, 30 inches being the ordinary height of the barometer [BAROMETER]; we may diminish its altitude by ascending a mountain, or we may draw away a portion of the air by means of the AIR-PUMP; steam will then be produced at a proportionally lower temperature. When we continue to apply heat to ordinary steam, under a cons ant bulk, its elasticity rapidly increases, and it is then termed high-pressure; steam of the ordinary temperature being termed low-pressure. The following table gives the temperature of boiling water and the corresponding pressures of the air as observed by Dr. Dalton and Sir J. Robison, those of the former are marked by the letter D, those of the latter by R: 12.05 270° 86.30300 6° 140.90 273.7 91 20 302 144.30 277 9 97 80 303 8147'70 280 101.90 305 150 56 283 8107 70 306 8154 40 285 2112 20 308 . 157 70 55 65 32° 0.20 170° 75 0.86 200 23 60 290 120 15 311 4 164 80 167 00 Various tables of the same kind have been constructed and published by the French academicians at the desire of government, and also by a committee of American gentlemen, which however do not harmonise with the first-named tables. Various empiric formula have been attempted, and some on particular hypotheses have been calculated to represent the relation between the elastic force of steam and the temperature. As they all deviate from the observed results at very high or very low pressures, we shall here mention only a few of the more celebrated. Laplace's formula (from Dalton's experiments): =(V).... (Poisson, Mec., vol. ii.) If therefore the absolute heat were constant (i.e. the sum of the latent and sensible heat), we should upon determining y by observation, have the complete solution of the relation between the elasticity and temperature for each gas. The results in the case of steam are, as above mentioned, however, but approximations, and in extreme temperatures by no means close. Sir J. Lubbock has therefore modified the hypothesis, by supposing that the expression for the absolute VOL. XXII.-3 P heat ought to contain one term proportional to the temperature; thus, according to his view we should have V=C+D (1+a0), which combined with the former equations (supposing p' to be the pressure corresponding to e) gives and comparing the results of this formula with observations of Dr. Ure, from 123° to 224° Fahr., the errors are all within the limits of observation, some positive and some negative. (Heat of Vapours, by J. W. Lubbock, Esq., Lond., 1840.) It has recently been observed that the discharge of steam from boilers is accompanied by a development of electricity. The facts are at present too little methodized to be introduced here. The reader may consult on this subject the Phil. Mag.; and Sturgeon's Annals. STEAM-ENGINE. In conformity with the plan of this Cyclopædia, a general outline of the principles of the engine will be here given, the reader being referred to different articles connected with the subject, or to works written specifically on the steam-engine, for more detailed information. The claim to the invention of the steam-engine has been made a subject of national contention; but the conclusion, arrived at from the discussions which this has originated, seems to be, that, in common with all other important applications of physical principles, no individual can lay claim to the invention. Whatever may have been the nature and date of its origin, it has been reared to its present gigantic stature by the fostering care of different countries, and, without detracting from or underrating the efforts of others, England may be justly proud of her share of the glory, a share readily conceded by our competitors. Considering therefore dispute the as unprofitable, and the discussion of dates of patents and improvements as uninteresting, we shall incorporate all that is requisite of the history of the engine with our account of it. A steam-engine may be defined generally, an engine by which the force arising from the properti of elasticity and of instantaneous condensation possessed by steam are transmitted to produce a continuous rotatory motion, either of a fly-wheel to constitute a reservoir of power for the purposes of driving machinery, or for any other uses that power may be put to. Admitting this definition, the earlier steam-engines, as they are commonly called, those of the Marquis of Worcester (1663), and the improved forms of it contrived or suggested by others, and even Captain Savery's (1698), which was long employed in this country, were only pumps for raising water: a partial vacuum was formed in close vessels by the condensation of steam within them, the atmospheric pressure raised the water to a certain height; from whence it was forced higher by the elasticity of the steam admitted to act on its surface. Passing over all these therefore as foreign to our subject, the first engine which it is necessary to describe is that of Newcomen (1705), as constituting the connecting link between the steam-pumps alluded to, and the modern engine, of which it contained the germ, and into which it was converted by the genius of WATT. In the subjoined diagram, A represents a cylinder open at the upper end, fitted with a piston B, and rendered airtight by having water on it to the depth of several inches: the piston-rod was suspended by a chain from the arched end of a beam C, turning on an axle, and having a pump rod at its other extremity, loaded so as to counterpoise the weight of the piston, and to raise it to the top of the cylinder. This cylinder was placed over the boiler D, with which it communicated by a steam-pipe E, furnished with a cock F to open or close the passage. G is a cistern fixed above the cylinder, to the bottom of which a pipe H passed, also provided with a cock I. cock F was closed and I opened, a jet of cold water from the cistern G rushed into the cylinder, condensing the steam. and thus forming a partial vacuum beneath the piston,* the pressure of the air on its upper surface forced it downwards, and caused the pump at the other end of the beam to raise an equivalent weight of water to a height equal to that through which the piston moved: the injected water and condensed steam-water flowed off into the cistern L through K, as the air had previously done. The cock I was now closed, and F opened, and the action was repeated, and when this engine was first introduced, it was the duty of an attendent to open and shut these cocks alternately; but subsequently lever handles to open and shut the cocks were acted on by pins or cams, carried by a rod suspended from the beam; and the engine became self-acting. This improvement was rudely made in the first instance by a boy named Potter, for the purpose of saving himself trouble; was subsequently perfected by an engineer named Beighton in 1718. Newcomen's engine was successively improved upon by Smeaton, Brindley, and other engineers, previous to Walt's time, and from its intrinsic merits it remained in genera use under the appropriate name of the 'atmospheric engine during the greater part of the last century, but was only used for pumping water: its existence was further prolonged by the important improvements we are about to describe, and possibly one or two may still be found in our remoter mining districts, neglected or in ruins, witnesses to the rapid mareb of our mechanical skill within the last fifty years. It hence appears, that in Newcomen's engine, the steam was solely employed to produce a partial vacuum by its condensation, its elastic force at high temperatures not being made use of; and a great waste of heat, or fuel, was occa sioned by this condensation taking place within the cylinder. for the consequent reduction in temperature caused a partial condensation of the next charge of steam, till the latent heat given out by this condensing steam had raised the temperature of the cylinder again to that of the boiler. more steam was therefore requisite than would otherwis have been necessary. The first and most important of Watt's improvements on the engine consisted in effecting the condensation in When the piston was depressed to the bottom of the a separate vessel, termed the condenser, which commucylinder, it drove out all the air before it, which escaped at the orifice of a pipe K into the water of a smaller cistern L:provement on that previously employed by Savery, which was by the externa This mode of effecting the condensation was considered an important in the cock F being next opened, the steam from the boiler affusion of cold water over the steam-vessel; nevertheless, this latter principer filled the cylinder as the piston rose again from the action has been partly re-introduced in the form of a condenser, patented by Mr. of the counterpoise; as soon as it arrived at the top, then, which is employed in many steam-vessels, and appears to be facreasing in use. nicated with the cylinder. This condenser being filled with steam from the boiler at the same time with the cylinder, the jet of cold water, admitted into the former only, effected the condensation of the whole volume of steam, both of that in the cylinder as well as of that in the condenser, in conformity with the well known principle in physics, that an action originated in any part of a homogeneous fluid is almost instantaneously communicated throughout its mass. To effect still further the object of this separate condensation, Watt placed his condenser in a cistern, the temperature of which was kept constant by a fresh supply of cold water, brought from a well by a pump, to be presently mentioned; for otherwise, the heat given out by the condensing steam would, by heating the vessel and the water surrounding it, have prevented the rapid or almost instantaneous condensation necessary to the efficient action of the engine. To comprehend the necessity for a rapid condensation, it must be remembered that the effective power of the engine depends on the pressure on the piston minus any resistance it encounters and on the space through which it moves. If the steam could be instantly converted into water, and so entirely removed, a perfect vacuum would be formed beneath the piston, in which case, there being no resistance from this source to overcome, a maximum of power would be obtained: but if the condensation be slow, or only partial, since the piston will begin to move the instant there is any inequality in the pressures exerted on its opposite sur faces, its motion will be retarded, or the power diminished, by the resistance to compression offered by the uncondensed steam; and although that resistance would tend to diminish as the condensation proceeded, yet the space occupied by the steam diminishing in consequence of the descent of the piston in nearly the same proportion, the resistance would be nearly constant through the whole of that descent. On the other hand, to maintain the temperature of the cylinder as high as possible, Watt, at first, cased it in wood to retard the radiation, and subsequently surrounded it by a second iron cylinder, admitting steam from the boiler between the two. This casing, or jacket,' as it is termed, is not used in most modern engines made since Watt's time; for reasons which will hereafter appear: and the effects of radiation from the surface of the cylinder are now chiefly guarded against as much as possible by keeping that surface bright and smooth. The second of Watt's improvements on Newcomen's engine consisted in closing the cylinder at top, the pistonrod being made to pass through a cylindrical neck in that top, termed a stuffing-box, from the passage being rendered steam-tight by a stuffing of tow saturated with grease, which by its lubrication diminished the additional friction resulting from this arrangement. The object of this alteration was to admit of the elastic force of the steam being employed to impel the piston downwards, instead of simple atmospheric pressure: for this purpose the steam was admitted from the boiler above the piston at the same moment the condensation took place in the condenser; the steampassage being made double for the purpose, so that the communication with the condensor could be cut off when that with the cylinder was opened, alternately. When the piston had descended to the bottom of the cylinder, the counterpoise at the pump-rod raised it again, as in Newcomen's engine; but to allow of this upward motion, it was necessary to remove the steam which was above the piston, and this was done by allowing it to pass under the piston, and into the condenser through a passage opened at the proper instant for this purpose. Such is the general principle of Mr. Watt's single-acting engine, which hence became a steam-engine, and was no longer an atmospheric one. By a further improvement the counterpoise at the pumprod was done away with, which obviously had been so much added to the unproductive work of the engine, since this weight had to be raised in addition to that of the water. The upward stroke of the piston was now produced by admitting the steam below it, to act by its elasticity, as it had previously done above when causing the piston to descend: thus the engine became double-acting, and assumed that essential general principle which it has ever since maintained, although all the details of its construction bave been improved upon by successive engineers. One cubic inch of water occnpies 1711 cubic inches of space, in the form of steam at 212°; consequently, the space occupied by the water after the condensation may be neglected in the computation. The changes in the engine introduced by Watt created the necessity for two pumps, and commonly three, which are worked by rods attached to the beam: the first of these is the hot-water or air pump, intended to remove the air, condensed water, and steam from the condenser, in which they would otherwise accumulate, and finally stop the action; for this water cannot flow away into an open cistern, as it had done in Newcomen's engine, since by Watt's principle it is essential that the condenser should be as steam-tight and as perfectly closed as the cylinder, or else the steam could not exert a pressure greater than that of the atmosphere, as it is intended to do in order to increase the effective force of the engine. The second is a force-pump, required to return the water, drawn from the condenser by the first, back to the boiler, as will be hereafter explained; and the third, termed the cold-water pump, is that alluded to in a preceding paragraph as supplying the cold-water cistern which contains the condenser. Having thus explained the general principle of the engine, some of the details of its construction must now be considered, and the piston [HYDRAULICS] may claim our first notice, both from its paramount importance and the practical difficulties to be overcome in its formation. In hydraulic machines, all vessels, pipes, valves, &c. must be made water-tight: in Bramah's pump, for example, the efficiency of the engine entirely depends on the accurate fitting of parts moving in contact, which must be perfectly water-tight, though subjected to a pressure of many hundred pounds on each square inch of surface, and the utmost perfection of skill in workmanship is requisite to ensure this: this difficulty is obviously considerably increased when steam or gases are the fluids to be dealt with. Now the piston of a steam-engine must be steam-tight, and yet move with a minimum of friction in the cylinder; and as this latter, from defective workmanship, can never be a perfectly true one, the cylindrical periphery of the piston must be so contrived as to be capable of adapting itself to every inequality in the surface against which it slides: this is effected in common pistons by their being made two inches or more less in diameter than the cylinder, leaving a projucting flange at the bottom, which, together with a top plate bolted to them, accurately fits the cylinder. Tow or soft rope, saturated with grease, is carefully wound round the cylindrical body of the piston, between the upper plate and lower flange; the former is then drawn up by screws, compressing the intervening packing till it perfectly fits the cylinder, and yet by its elasticity allows of its adapting itself to the inequalities in the surface. The upper one of the annexed figures will explain the details of the construction of the ordinary piston." But as the friction of this common piston is necessarily very considerable, the better class of engines have usually what are called metallic pistons, of which there are different kinds, invented by Cartwright, Jessop, Barton, and others; the body of these pistons is metal, made in pieces or segments, acted on by springs radiating from a centre; so that while the friction is diminished by both surfaces being metallic, the piston, owing to its construction, can adapt itself to the irregularities of the cylinder. It is found in practice that these metallic pistons wear for a long time, and do not of course require the frequent repacking necessary to those with tow or hempen stuffing. The adjoining lower figures represent a plan and elevation of an improved form of Barton's piston, to explain the principle. In Newcomen's engine the steam was admitted to the cylinder, and the communication again cut off by means of a cock of the common construction; but a more efficient contrivance is requisite when the steam is to be admitted alternately both above and below the piston, and to the condenser, as it is in engines since the introduction of Watt's improvements. This can be accomplished by a four-passage cock, originally invented by Leupold in 1720, and since improved by Bramah and others. The principle of a four-way cock will be understood by the annexed figure of the plan of one employed by Messrs. Maudslay and Field in their small engines. B CDE A is a portion of the cylinder; B, the steam-pipe; C, D, E, three passages, one communicating with the top, another with the bottom of the cylinder, and the third with the condenser; F is the fourpassage cock, which by turning alternately to the right and left establishes a communication between one of the former with the latter passage. The lower figure represents the conical valve with its side apertures, and that at the top, by which the steam enters. Watt employed flat conical valves for the purpose under consideration, which were raised or depressed by cranks acting on a guide-rod at right-angles to the plane of the valve, which therefore did not turn on a hinge like the common clackvalve of a pump; in some of his engines the valves were raised or depressed by toothed sectors acting on a rack in the guide-rod, so that the valve might rise from its seat without altering the parallelism of its plane. Two such valves were mounted in one box, one above the other, the guide-rod of the lowest passing through that of the upper. Chelsea water-works. C, part of the cylinder; P, the piston; T, the 'plug-tree;' G, the gearing handles, which are struck by the tappets on the plug-tree, and thus open and close the valves V; S, the steam-passage to the upper and lower parts of the cylinder; D, the passage to the condenser. In most engines of the present day however the slidevalve, as it is termed, has superseded the use of the others: a perfectly flat surface slides on another, terminating the orifices which are to be opened and shut; such is the general principle, but the forms and arrangements are too numerous to be mentioned The figure subjoined shows a part of the cylinder of an engine with box-slide valves, now much used. S, the orifice of the steam-pipe; the steam passes to the upper part of the cylinder at D, the lower passage E being shut off in the position of the valve shown and shaded in the figure; the slide is moved by the rod R, and it is shown in its second position in dotted lines, in which position it will be seen that the steam can then enter beneath the piston, while the passage P to the condenser is in turn in communication with the upper part of the cylinder by means of the tube T of the slide. The characteristic and most valuable part of this principle is this, of making part of the slide act as a pipe to connect the two parts of the cylinder alternately with the condenser. The steam by pressing on the slide in the common form enormously increases the friction with the surface against which it acts, and also produces rapid wear of the parts; this defect is remedied in the box-slide and all others which possess this peculiarity. Another form of slide-valve is shown in the diagram, page 480. Slide-valves were proposed by Murray, in 1799, but were abandoned, till improved workmanship allowed of their being more perfectly made; they have been successively improved in principle by Murdoch, Bramah, Millington. Maudslay, and Seward, the slides of the last named being now much used in marine engines. It has been mentioned that the alternate action of the valves in the atmospheric and Watt's engines was produced by pins, or tappets, adjusted on a rod called the plug-tree, suspended from the beam; as the plug-tree moved up and down with the beam, the tappets struck the ends of bent levers or cranks, which raised or depressed the valves in proper succession: some of these levers were so formed that the tappet by pressing against them might keep the valve closed during the greater part of the stroke of the piston, and others required an intermediate shorter lever, or claw, to act on the valve-rod; so that the whole arrangement was inevitably complicated and cumbrous. But when the slidevalve superseded Watt's double conical valves, and the steam passages could be opened and closed by the motion of one rod only, connected with the slide, this motion could be readily produced by what is termed an excentric, which for this purpose usually consists of a circular plate of metal, keyed excentrically on the shaft of the fly-wheel, and working within a ring attached to the end of a frame intended to move a crank directly connected with the slide-rod at its other extremity. As the shaft revolves, the excentric plate imparts an alternating motion to the frame, which, transmitted by the crank, alternately raises and depresses the slide rod, The principle of the excentric is one of the most valuable of those mechanical contrivances by means of which a continuous circular can be converted into an alternating rectilinear motion. tion of the piston from the top to the bottom of the cylinder and back again. The term stroke is technically used by engineers to express the whole mo + Messrs. Maudslay and Field have however retained the plug-tree and tap pets and the conical flat valves in the large condensing engine erected by them for the Chelsea Water-Works at Pimlico in 1837; which replaced an original engine of Boulton and Watt, probably the last to be seen in the neighbourhood of London, |