act.

The engine so constituted is commonly called the Atmospheric Engine, because the power is derived from the pressure of the air, the steam being used merely to form a vacuum against which the atmosphere is to As this engine constituted a very important era in the history of steam, a short account, with a diagram of it, is subjoined, more especially as it shows in very bold contrast the many and great advantages that resulted from the application of the genius of Watt to the steam-engine. There are three essential parts in the engine the boiler, in which the steam is produced; the cylinder, in which it is condensed; and the beam, where its movements alternate with the ascent and descent. The boiler (B) is placed over a proper furnace, and built in with bricks. The summit of the boiler has a pipe or tube which communicates with the cylinder (C), situated immediately above. The

communication between the cylinder and the boiler is protected by a valve (V), called the regulator, or regulating-valve, so that the admission of steam may be regulated at will. The boiler is provided with gaugecocks (G G), as already noticed under the head of boiler, and also with a safety-valve (S V), which is not loaded to any great extent, as the engine works at a low pressure. The cylinder, which is placed above, is made of cast-iron, and nicely bored, so as to permit the free working of the metallic piston, but at the same time to prevent the access of air or steam. The piston (P), in short, works like the piston of a common syringe. There are four apertures in the cylinder, while it is also open at the summit. There are, first, that marked V, the valve of communication between the boiler and cylinder; second, that at the left inferior angle in the opening of the pipe (A), which transmits the water for condensation, armed with the stop-cock (R), named the injection-cock. This pipe leads from the cistern (C), which is kept constantly supplied with cold water by the action of the small pump attached to the beam, raising the water and carrying it along the tube or water-pipe (E).

At the opposite angle below H, an aperture is observed, being the commencement of the eduction pipe, by which the water injected for condensation is removed to a cistern beneath. This pipe is conveyed a considerable way down into the cistern, and is protected with a valve at its extremity, opening outwards, so as to permit free passage of the water from the cylinder, but none to regurgitate from the cistern. The fourth aperture opposite the opening of the injection pipe (H) is also supplied with a valve opening outwards. It is commonly known by the name of the blowing-valve or

snifting-calve. It is through this valve that any air: the cylinder is expelled before the engine starts.

On a large support (K) a beam (1) is placed trae versely across, which moves on an axis at I. T. beam has one arched head at either extremity, to b of which chains are attached. On the one immediat above the cylinder the chain is continued down to " piston-rod (M), into which it is fixed, so that, as the pis ascends and descends, there will be a similar moves of the arched head of the beam. To the other end chain is connected with the pump-rod, by whic water is to be brought up. But the pump-rod is heavy, so that it naturally draws down this extrem. : and elevates the piston-rod.

The mode in which this engine is worked is the f lowing:-The fire being properly raised and ster freely formed, the valve (V) is opened, to allow i entrance of the steam. The snifting-valve (H} ar forced open, and the air escapes along with the stes until the cylinder is full of steam. The regulator-v2. (V) is now shut, and the stop-cock (R) on the pipe being opened, the cold water is injected, and conder on the steam. But as a vacuum is effected by the e densation of the vapour of the water, the pressure the air, now acting with a force equal to fifteen pouc on the square inch on the surface of the piston, carr it down to the bottom of the cylinder, and conseqĮ Las raises the other end of the beam to which the pum rod (N) is attached. In this manner the water is ra from the mine; and by a repetition of the movem. already noticed, a constant discharge of water rest. There were not a few difficulties or impedimente in free working of this engine, one of the most labor of which was the incessant attendance of a persar open and shut the stop-cocks alternately as it was r quired. This was accomplished by catches (serggat worked by the beam, or strings connected wah lever of the valves and the beam-an invention of a ke Humphrey Potter, to avoid the trouble that ens attendance on the levers demanded. By meam «. plug frame fixed to the beam, invented by Beighton, = engine was made to work the valves with great reg larity-a most important practical advance in mak the steam-engine work itself, and adjust its own valer The analogous part of this machinery in the modr double-acting engine is to be observed in the eceratri This, the atmospheric or Newcomen's engine, ka many and very striking advantages over all previously proposed. It may, indeed, be consider the basis of the engine subsequently modelled by Wa But there were very serious defects in it, which t reader will in some measure be acquainted with fr the history of Watt's engine. It is here sufficient bred to enumerate them. Much steam must, then, bel during the process of the heating of the cylinder afeach condensation; for it must always at least be ran to the temperature of the steam before the steam eu as such, continue in it, and be in any degree efficie and on the other hand, the cold air which follows u descent of the piston must necessarily withdraw a er siderable portion of heat. By the calculations of Wa it was estimated that three times as much steam wi expended in this manner as would have been equal i work the engine-a loss, therefore, equal to 75 per en Nevertheless this, as has been correctly observed, wi the first really efficient steam-engine; that is, the fir engine which could be applied profitably and safely | the most important purposes for which such macha were required at the time of its invention."

In the further history of steam-engines, we fall on t era of Watt, the atmospheric engine having been ver extensively used for a period of nearly one hundr years. The modifications it afterwards underwent hu been sufficiently explained in the history of Wai engine, and it is therefore unnecessary to extend the observations further.

Printed and published by W. and R. CHAMBERS, Edinburgh. Sold also by W. S. ORR and Co., London.

but nearly determinate. They usually run east and west, and dip or underlie either towards the north or south; while the non-metalliferous veins, which run north and south, dip either towards the east or west. The cases in which metalliferous veins assume a north and south direction are few, and chiefly foreign. It frequently happens that the metalliferous lodes, as we have said, cross each other; and, as a leading fact, the intersection of two lodes at a small angle is productive of good ore. Should, however, a copper lode pass through a tin lode, the copper lode invariably divides the tin lode, and generally heaves it out of its course, to the frequent perplexity and loss of the miner. All mining experience of a general character is, however, sometimes set at defiance; for, in the small space of one little hill, instances may be found in which veins of almost every description dip or underlie in almost every possible direction, traversing each other in such a manner as completely to baffle the miners; but it is an ascertained fact, that there are seldom or never, in the same district, two series of metalliferous veins running at right angles to each other. As a tolerable average, we may assume the direction of the Cornish lodes to be about four degrees south of true west, and their dip or inclination to average sixty or seventy degrees from the horizon. Taken on the whole, the lodes appear tolerably straight both in direction and inclination, but when examined in detail, they exhibit almost continual curvatures and irregularities in both respects, although these flexures would seem to be projected on certain lines which manifest considerable constaney. The length of no one lode has as yet been satisfactorily traced. Some of them, indeed, have been followed for two or three, or even four or five miles; but no instance has occurred in which a vein has been known to stop; nor has the miner ever yet seen the bottom of one, although there are several mines in Cornwall upwards of 1000 feet in depth from the surface, and two or three about 1300 feet deep. The lodes differ exceedingly in respect of width, in which, indeed, they vary from a mere line to forty or fifty feet. On the average, they may be assumed to be three feet and a half wide. Lodes of from one to three feet in width are usually less intermixed with foreign and troublesome substances than those which are wider. A vein of tin in a mine called Whealan Coates was only three inches wide, and yet proved so rich as to be worth working. Some of the veins containing copper in Herland mine did not exceed six inches in width; and after continuing this thickness for a few fathoms, eventually passed away east and west in mere strings; but they yielded copper of a very rich character. In the next hill there was also a very productive copper vein of from twelve to twenty-four feet in width.

The compositions of the lodes or veins are as variable as the nature of the rocks through which they pass. By far the greatest portion of them, however, is earthy matter of the nature of the contiguous rock, but also containing large intermixtures of quartz. These ingredients occasionally occur in separate veins, but for the most part they are mingled without regularity or order, and throughout them are dispersed the metallic ores. Sometimes these are aggregated very thickly, and very generally occur in large irregular lumps or patches, called bunches, connected with each other by small veins of ore. At other times the ore is very sparingly sprinkled through the whole of the earthy matter of the vein, and in some rare instances it forms the larger part of its contents. The sides of metalliferous veins are generally very determinate, being covered by a hard dark-coloured crust, called by the miner the walls of the vein.

traced, is about south-east and north-west. T dimensions are variable, being perhaps on an aver about two feet; their dip, too, fluctuates, but, asag ral rule, it is greater from the horizon than that lodes. The clay with which the flucans are flet variably partakes of the same character as the e guous rock. Tin and copper ores are occas found in small quantities in the cross veins, a two or three instances silver and its ores have or in them to some amount. The chief metallic pro however, of this class of veins, is lead ore; but th seldom yield in the neighbourhood of lodes which t been productive of other metals. Indeed it is, a have said above, a general law in Cornwall, that • series of productive metalliferous veins, at right an to each other, are very seldom or never found in same district. Both the lodes and the crossramify and divide; and whilst the part which m place is large will sometimes, within a short distan dwindle and die away, the portion which is smail, wa the other is rich, will often, within a small space, large and become productive.

As these two series of veins, the lodes and the e veins, run at right angles to each other, they of es frequently meet and intersect. In a few instances: lodes traverse the cross veins, but in far the gres number of cases the cross veins cut through the a Occasionally, the cross vein simply intersects the but more generally displacements, provincially ter.. heaves, attend their contact. These heaves, althou usually only amounting to displacements of a few 3or fathoms (a fathom being equal to six feet), yet some cases turn off 20, 30, or 40 fathoms, and in instance to the extent of 72 fathoms. If, for exam: a cross vein, in its north or south course, meets lode containing copper or tin, the last seems to ba been split, as it were, into numerous little branche the first, which generally pursues its uninterr?" course straight forwards. Another effect, tos, much more extraordinary kind, is produced by the a trusive cross vein. In searching for the tin or sp. lode on the other side of this north and south ve lengthened period frequently elapses before the taga can be discovered. Notwithstanding the experien the miners, forty years have sometimes passed before the search, though carried on with viguer 1o great expense and labour, has proved successi i by no means a simple task for the mining engineer u lay down a law for the recovery of the lode. Insane have been known in which it has been again found. or even 450 feet north or south of its original overs The cross vein will not perhaps generally intersect !: lode exactly at right angles, but its inclination to t course of the lode will usually be such as to proderma the intersection an obtuse angle at one side of the lou and an acute at the other side; and it is thought, bi the most experienced observers, as well in Saxony in Cornwall, that the second portion of the lode . more frequently be discovered on the side of the catam angle, formed by it with the cross course, than en in side of the acute angle. In other words, on whichever portion of the lode we approach the cross vein, the other portion will be found towards the same hand, name the right hand. There are other kinds of interraştım to which metalliferous veins are subject, though far less extensive in their agency than the cross vers These are denominated slides, and generally consis clay or argillaceous matter. Their direction is near; parallel to that of the contiguous lodes, but their de underlie being either greater than, or opposed to, of the latter, they intersect them either in a borzen or more or less inclined direction. Slides are curITUT in Cornwall, and occur also in Mexico and other me ing countries.

surface. Silver is found in some foreign mines at a depth of from two to three hundred feet, while the silver mines of Mexico are of a much more considerable depth. Tin is also found at shallow depths, of which the great lode of the Charleston mines in Cornwall furnish a good example. Lead is usually met with at a very trifling depth, and slightly spotted veins of it are sometimes to be observed in the sides of brooks, and in the rocky channels of rivers. Copper, on the contrary, generally lies deep, and the enormous deposits of this metal found in Cornwall are generally situated two or three hundred feet below the surface. Where tin and copper are found together in the same vein, the tin commonly occupies the upper part, and disappears at the depth at which the copper is discovered. Sometimes, however, the ores of both metals occupy the vein together to a great depth, as at the Poldice mine near Redruth. On referring to the known depths to which different metals extend, it will be found that those which commonly lie near the surface, as lead, zinc, gold, and occasionally tin, do not generally penetrate to any great depth; while those which lie deeper, as copper and silver, are worked in the bottoms of our deepest mines. This coincidence may be the result of a natural law, or it may be apparent, and consequent only upon the limit of our experience and knowledge.

Cavities or open spaces frequently occur in metalliferous veins, which may reasonably be conjectured to be such portions of the original fissures as have not, owing to local causes, been filled up in the same manner as the remainder. These cavities are very irregular both in size and form, but yet, in their size, appear to bear some relation to that of the vein in which they are situated. They are probably the secret laboratories in which nature has perfected some of the most beautiful productions.of the mineral kingdom. The whole of their interior is generally lined with various substances, often crystallised in beautiful groups of dazzling brilliance. It has been elegantly though fancifully observed, that minerals are the flowers of rocks; and might we not, in pursuance of the same idea, call these cavities the gardens of the mineral kingdom? It is from these open spaces that most of the mineral specimens are collected for the cabinets of the curious and the wealthy.

Situation of Metalliferous Deposits. Metalliferous veins, and, indeed, metalliferous deposits generally, are found traversing a great variety of rocks stratified and unstratified, and appear to belong equally to formations of igneous and sedimentary origin. Although not confined to that class, they may be said to exist most abundantly in the older and more crystalline rocks, and usually in those bordering upon mountain chains. They are mostly found in those situations where a junction occurs between two contiguous rocks; where different rocks are interstratified, or where they are broken or dislocated by faults and cross courses. Thus, in Great Britain, the tin and copper mines of Cornwall and Devon are situated in granite and different varieties of slate, belonging chiefly to the primary class, and are most productive near the junction of these strata. Many of the metalliferous deposits of Wales and the north of England, producing both lead and copper, are also contained in slaty rocks, which are generally referred to the greywacke series. Our great deposits of lead, however, are contained in the lower rocks of the carboniferous series, which is the case in the lead mines of north Wales, of Derbyshire, Yorkshire, and the great lead mine district of the north of England. Above the carboniferous series no metalliferous deposits of any value occur in this country. The metalliferous deposits of Scotland occur in primary and transition rocks; and those of Ireland occur mostly in granite, mica slate, and clay slate, although extending also into the carboniferous series.

The mining district of the west of England may be considered to commence at Dartmoor, and terminate at the Land's End. The surface is gently undulating,

the loftiest hills rarely exceeding 1000 feet above the level of the sea, whilst the greater number of them range from 500 to 700, and the plains at their bases are usually from 100 to 200 feet above high water. The highest peaks are for the most part granite, whilst the lower hills and most of the plains consist of various descriptions of slate. The granite may be considered to present six patches of large dimensions, as Dartmoor, &c.; and three eminences of minor dimensions, in which we include St Michael's Mount. All the other parts of Cornwall (except the Lizard district, which is composed of serpentine) may be said to consist of slate of various kinds. The granite is commonly coarsegrained, and of porphyretic structure; the slates, in general, partake of the character of felspar, and are of a compact structure when near the granite, and otherwise when at greater distances. Both the granitic tracts and the slates in their vicinity are intersected by veins or dykes of a porphyretic felspar rock, provincially termed elran. These dykes or veins have, in a few cases, been traced for miles, and they pass uninterruptedly through both granite and slate; their usual direction is about 20 degrees south of west, and they are generally several fathoms in width.

The sulphuret of zinc (blende, or the black jack of the Cornish miners) occurs very abundantly in the mineral veins of Cornwall, being, however, more frequently associated with the ores of copper than those of tin. The conditions favourable to the production of tin and copper lodes have been favourable also to the appearance of the sulphuret of zinc or blende, which, however, frequently occurs in the continuation of tin or copper lodes, far beyond those localities where the tin or copper can be profitably raised. Viewed on a large scale, blende is one of the most widely distributed ores in Cornwall; while the carbonate of zinc, or calamine, is a very rare one in this district. The sulphuret of zine is not, however, an ore much worth raising at present for profit.

Cobalt is a rare ore in this district, and does not seem to be accompanied by any marked geological conditions, as far as regards its occurrence in any parti cular class or kind of rock. The same may be said of bismuth; and nickel is a very uncommon metal too, and has only been found in any workable quantity near St Austell.

The siloci of Devon and Cornwall has for the most part been obtained from those ores of lead (argentiferous galena) in which its presence has been detected. Silver ores-proper have, however, been obtained in several mines, in lodes or cross-courses in the grauwacke. Such ores have consisted of native or pure silver, and silver in various states of chemical combination with sulphur, arsenic, &c.

MINING OPERATIONS. Preliminary Proceedings.

When the mineral contents of a spot are entirely unknown, the operations instituted for the discovery of lodes must be founded upon the general presumptions furnished by geological science in connexion with mining experience, as mineral deposits usually present no precise traces of their existence at the surface. The first objects of pursuit, in such circumstances, to the Cornish miner, are what he denominates shode or shoadstones. These stones are partially rounded and apparently water-worn, and are found on the surface or at very small depths below it. Their mineralogical characters nearly resemble those of the contents of the lodes in the vicinity, of which they are presumed to be portions removed by diluvial action. As, however, the shoad-stones contain tin ore, a careful search for them has been constantly kept up, and their increasing scarcity will probably render this mode of discovery impracticable. When they were uncollected, the examiner might commence marking their presence at any given spot, and then trace them to where they appeared in the greatest abundance, which situation was pro