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Law IV. The separation of the upper and compression of the lower particles diminishes to the tentre. The angles of the wires show the diminution.

No. II.—Proofs. The rod with the wires contains a simple proof of all four laws. The wires are acted upon by the lines of particles only. When, therefore, the wires are displaced, the particles too must be displaced, aud in a similar manner.

Therefore, where the rods indicate separation, •impression greater or less, similar action must have been effected (on a very reduced scale) in the flexed body.

No. II.—Applications.

App. I.—Neutral Axis is that part of the body which is neither dilated nor compressed. And Ibis neutral axis, practically extends to some depth at either side of the strict, absolute centre line.

App. II.—Hollow Beam. Under certain conditions a hollow beam is stronger than a solid one of the same dimensions. This question occurs again, jnst now, only what flexibility contributes to the explanation? 1. In the hollow beam the real effective part is retained, that is : the upper set of particles, which resist by their molecular attraction, and the lower set, which resist by their opposition to compression. The beam, then, has in certain dimensions, all that contributes to its strength, although much diminished iu quantity of matter.

2. The weight of all the matter in the neutral axis is removed; so much bending force is thus avoided.

3. No crushing force is directly conveyed £om the upper to the lower set of particles

bottom of the beam—except at the sides;" th» rest of the bottom of the beam is only acted upon through its cohesion to its edges, or sides. There is, therefore, introduoed a resistance to flexure, represented by the whole length of the bottom of the beam, inside its perpendicular walls. On the contrary, in the solid (warn each vertical layer transmits a portion of the force, directly crushing the lower particles.

App. III.—Girders. The great horizontal beams of iron now so common, have a forii, the section of which is like the letter H laid flat, |—|, Fig. 39. Their resistance depends in pat on the same principles as those just explained for the hollow beam. The centre rib takes the place of the two sides.and is strengthened as required by angle pieces, Fig 39, a a. The girder is not of necessity

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§ III.—Elasticity

Elasticity is, to a certain point, the same as flexibility, but it has something additional, by which it differs essentially. When drawn away from their position of rest, elasticity gives to the particles the power of returning to that position. This power is called "restitution." Almost all bodies are elastic, more or less ; some very little, others very much so.

Elasticity is a subject of great consequenre. which has been investigated by deep mathematical study and innch experimental research Here, of course, but the grcnt general principles

There are font sorts of elasticity—elasticity of compression^ of traction, of fiexion, and of torsion. The two first, are familiar iu balls, strings of india-rnbher; the third, in rods of wood, steel, &o.; the fourth, is that observed in cords of every sort, when twisted. Torsion-elasticity is well shown to the eye by an elastic rod with wires, • >r a thick cord. Holding the rod or cord perpendicular, the wires show the changed position of the particles when the rod or cord is twisted, Fig. 41. In all, the disturbed particles return to

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their position of rest, whether forced from it by compression, by drawing a«nnder, by bending, or by twisting. All four belong to the solid state, the first only to the liquid and gaseous.

No. I.—Laws. These regard particularly the fourth sort of elasticity.

Law I. The laws of flexibility are true of elasticity, inasmuch as the sort of elasticity will permit. There is separation, compression, and neutral axis, where the source, cause, mode of action of the elasticity, can produce these effects.

Laws II. In torsions elasticity the force of torsion is proportional to the angle of torsion. When the cord is twisted, the particles move horizontally, forming an angle with fche'r original position. The wires represent each a line of particles from the centre to the circumference of the corll. The angle with the position of rest is called the angle of torsion, «, Fig. 42.

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Law III. In torsion elasticity the oscillations are isochronous. No mat tcr how great or small, the distance (within certain limits) at either side of the point of rest, to which the particles move, the swing is effected in equal times.

No. II.—Proofs. Experimental only.

Law I. Compression-elasticity, does not admit the separation of particles. The existence of this source is usually shown by dropping an ivory ball upon a block of marble, damped with some dark thick fluid. The spot upon the ball will be found to be much larger than the point on which the ball would stand if laid gently down. The ivory must, then, have been compressed. Its form remains still true; itmu-t, therefore, have been restoredIn traction-elasticity there can be no compression. The particles arc all drawn away. A Btrip of india-rubber under traction is interesting. By marking some one spot its changes can be observed. It grows thinner, u. the force increases,

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from action, to its first form, h. In torsion, elasticity, the compression which takes place is not in the snme direction ns in flexibility. This is made evident by the wires, Fig. 41 ; at both sides the wires stand out.

The compression begins at the axis of the cord, decreasing horizontally to the circumference This compression makes itself felt by the resistance to further twisting, which a cord soon offers. There is, too, a certain spiral motion which adds to the compression. To have lieen mentioned, most be enough for this spiral winding.

Law II. Found to be experimentally true; a known force 2, 3, n times greater, was found equal to the torsion of an angle 2, 3, n times larger. Some idea of the apparatus in the Applications.

Law HI. Shown by Conlomb to be the ca The experimeuts do not belong to mechanics, least, the forces brought into action.

No. III.—Applications.—From amotigBt these the principal one is, the explanation of Coulomb's instrument, in its mechanical essentials. These well understood now there is so much done and ont of the way for electrical study later.

App. I.—Torsion Balance.—An index A is delicately suspended by a thread free from all torsion, Fig. 44. Under the index is a graduated circle, the degrees of which show the angles of torsion. In some experiments it is required to make the index stand at a certain angle. This is effected by turning the handlo b of the axis a above, from which the thread hangs. Thchandle of that axis carries an index over a small graduated circle. When the upper torsion is used, it must enter into the calculations of the force under examination. It may sometimes be more than one turn, one comp'ete revolution of the handle. The divided circle serves for any number of turns. At first sight the torsion balance here shown may seem to differ very much from the Coulomb instrument, Fig. 45. Still there is not the smallest essential difference, in as much as they are torsion balances. A cage of glass A, protects the index from the air currents. From the cage rises the tube which supports the upp;r torsion scale, the axis, and nippers to hold the silver wire. The lower toraion scale is usually on the cage. All other details are dopendent upon electrical principles. And here occurs the first occasion of warning the student against over-attention to the unimportant details of instruments. See, make r i c . As

sure of all the essentials required by the principles. These, well understood, and retained, the explanation of the instrument is fixed in the mind. The details may or may not be remembered; this will not affect true knowledge, tho instruments uuder new forms can still be recognised and examined. Besides, in too much attention to details, there is sometimes this danger, that the memory is over-burdened with them j in the confusion the mind does not see where the essentials arj, aud after a most careful setting forth ot even the smallest points of height, p nition, material, and general action &c, the explanation of au app.ir.itus m ly

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conclude without one of the main princplcs having been touched.

Examiners in mathematics will sometimes present a proposition with the figure reversed from its usual position. How many candidates has not this immaterial change completely puzzled f So in scientific apparatus, change the position, form, &c, leaving the essentials; many who have stored up pages of details, will no longer recognise the instrument, to acquire tho explanation of which may have cost them much labour.

App. II.—Inelastic Bodies.—It is impossible to have a body hard, and perfectly free from elasticity. Of this remarkable proofs later in experiments, which for their perfection require such bodies. Perfect elasticity is equally impossible. Full restitution takes place, but its external effect is not fully produced; more strictly the " external action " of even the most perfect elasticity is never complete—proved experimentally in Dynamics.

App. III. Torsion shock.—Under this title, a French engineer explained, some time ago, the cause of the occasional breaking of railway carriage axles, which may have appeared to be quite Bound. At the curves of the "line," the intide wheel produces a torsion in the axle, which little by little disintegrates the metal, and Bo insidiously, that none of the ordinary methods of examinations, indicate the flaw which has extended from the outside to the centre. Close scrutiny and experiments have confirmed the theory of Mr. Lucas. The chief remedy he proposes, is to have the axles made of welded bars. Any flaw will thus show at once. Another safeguard is to have the wheels so hung that they can move with different velocities. The experiments of Mr. Lucas are very interesting and instructive, but are too long to be produced here.*

App. IV. Cork springs.—Cork would seem to possess a remarkable power of endurance, in bearing compression, elasticity ; so much so that it has been substituted for indian rubber in railway 'springs. The cork prepared for this use, under a pressure of 200001b., exhibited an elasticity which recalled that of compressed air. (To be continued.)

MECHANICAL MOVEMENTS-! (Continued from page 248.) ~i QQ A Parallel motion used only in parXOO» ticular caBes.

134. Shows a parallel motion used in some of the old single-acting beam-engines. The piston

* "Lcs Mondes." Vol. I., p. 290, III. p. 343. t Extracted fioni a compilation by Mr. H. T. Baow.v, Editor of the " American Artisan."

rod is formed with a straight rock gearing with a toothed segment on the beam. The back of the raok works against a roller A.

135. A parallel motion commonly used for stationary beam engines.

136. An arrangement of parallel motion for side lever marine engines. The parallel rods connected with the side rods from the beams or side levers are also connected with short radius arms on a rock-shaft working in fixed bearings.

137. Parallel motion, in which the radius rod is connected with the lower end of a short vibrating rod, the upper end of which is connected with the beam, and to the centre of which the pistou-rod is connected.

138. Another modification, in which the radius bar is placed ahove the beam.

139. Parallel motion for direct action engines. Iu this, the end of the bar B C is connected with the piston-rod, and the end B slides in a fixed slot D. The radius bar F A is connected at F with a fixed pivot, and at A midway between the ends of BC.

140. Another parallel motion. Beam DC with joggling pillar-support B F which vibrates from the centre F. The piston-rod is connected at C. The radius-bar E A produces the parallel motion.

141. "Grasshopper " beam engine. The beam is attached at one end to a rocking pillar A, and the abaft arranged as near to the cylinder as the crank will work. B is the radius bar of the parallel motion.

142. Old-fashioned single-acting beam pumping engine on the atmospheric principle, with chain connection between piston-rod and a segment at eud of beam. The cylinder is open at top. Very low pressure steam is admitted below piston, and the weight of pump-rod, &c, at the other end of beam, helps to raise piston. Steam is then condensed by injection, and a vacuum thus produced below piston, which is then forced down by atmospheric pressure thereby drawing up pump-rod.

143. Parallel motion for upright engine. A A are radius-rods connected at one end with the raming and at the other with a vibrating piece on top of pistou-rod.

144. Oscillating engine. The cylinder has trunnions at the middle of its length working iu fixed bearings;, and the piston-rod is connected directly with the crank, and no guides arc used.

145. Inverted oscillating or pendulum engine. The cylinder has trunnions at its upper end and swings like a pendulum. The crank-shaft is bolow, and the pistou-rod connected directly with crauk.

146. Table engine. The cylinder is fixed on a table-like base. The piston-rod lias a cross-head working in straight slotted guides fixed on top of

cylinder, and is connected by two side connecting rods with two parallel cranks on shaft under ttw table.

117. Section of disc engine. Disc piston, seen edgewise, has a motion substantially like a coin when it first falls after being spun in the air. The cylinder-heads are cones. The piston-rod is made with a ball to which the disc is attached, said ball working in concentric Beats in cylinder-heads, and the left-hand end is attached to the crank-arm or fly-wheel on end of shaft at left. Steam is admitted alternately on either side of piston.

148. Mode of obtaining two reciprocating movements of a rod by one revolution of a shaft, patented in 1836 by" B. F. Snyder, has been used for operating the needle of a sewing machine, by J. S. McCurdy, also for driving a gang of saws. The disc A on the central rotating shaft has two slots a a crossing each other at a right angle in the centre, and the connecting rod B has attached to it two pivoted slides c c, one working in each slot.

(To be continued.)

ON THE RELATIVE SAFETY OF DIFFERENT MODES OF WORKING COAL

THE following is the substance of a paper read before the Institution of Civil Eugiueerson May 3rd by Mr. G. Fowler, on the above subject It was maintained by the author, that whilst there was no possibility of freeing the workmen engaged iu coal mining from accident, there wag reason to hope for a considerable diminution in the proportion of those killed to the number employed. It was the purport of this communication to show that the mode of getting coal had considerable effect on the safety of the workmen. The accidents incidental to mining were classified by Her Majesty's Inspectors of Coal Mines under five heads, as arising from explosions, from falls of roof or of coal, in shafts, from miscellaneous causes underground, and on the surface. It appeared that in the years 1866, 1867, and 1868, out of a total of 3686 casualties, 1091 were the result of explosions, and 1255 of falls, or respectively 20 per cent, and 34 per cent, of the whole ; the remaining 37 per cent, being attributable to the other causes, which were not influenced by the mode of working. The different methods of getting coal, which were described in detail, were the practical application of two distinct principles. One idea was to remove the coal at two operations, and this was practised in the '-oard and pillar work of the North of England, in the bank work of Yorkshire, and in the stall work of South Wales. The other idea was to remove the whole of the mineral at one operation, as exemplified ia the longwall system of the Midland Counties. In the latter ease, ns the faces udvauced, packwalls of roof rock, or bind, were built at regular intervals, ami whenever a sufficient width of opening was obtained, the roof settled down, with or without fracture, upon these packs. Accidents by falls might fairly bo brought to the tost of figures; for although the roofs of various t-eams might differ much, the averages of large districts wero likely to be uniform. Of a gross tonnage of 198,630,043 tons obtained bv pillar work in lSfiii, 1867, and 1808, the casualties by falls wore 814, or 231,739 tons of conl for each life. Of a gross tonnage of 22,899.000 tons extracted by the longwall p'.an, the casualties were 75, or one life for every 305,320 tone. If the latter ratio existed in pillar work, the casualties would have been reduced front 814 to 614, or a saving of 200 lives. In these calculations certain coal fields, which yielded about three-tenths of the produce of the whole kingdom, has beon excluded ; as in North Staffordshire, Cheshire, and Shropshire, both modef of working coal were adopted, and the same was the case in Scotland. The mortality from falls was greatest in South Staffordshire, where the lofty cavernous openings killed off 1 man for every 214,517 tons of coal raised, or an excess over the ratio of 35 per annum. There, too, the coal was obtained by both methods, but the greatest number of accidents took place in the thick seam, which was worked in pillars. The greator safety <if longwall mines from falls was owing to the narrow width of the working places, to theconstant change to a new roof, so that thero was not time for atmospheric action, which greatly weakened the roofs of many mines, and to the small extent of open mines, which permitted a more thorough examination. It might be thought that in longwall work, the constant settlement or bending down of the roof would be attended with danger, but. practically that was not the case. If a fracture occurred, it was not by the running down of a number of loose fragment?, but a general settlement took place gradually, accompanied with so much noise that warning was given, when the workmen retired. The excessive mortality of some pillar districts was owing to the weak undersized pillars, which were crushed and sank into the floor, and induced a weak jointy state of the roof. The goodness of a roof as often depended npon the way in which it was managed, as upon the character of the material of which it was composed.

With respect to explosions.theauthor contended that the mode of getting coal had more influence on this question than was usually allowed : and whilst fans, safety lamps, the absence of gunpowder, and all sorts of precautionary expedients wci e proposed, and were more or less adopted, the ■effeot of the mode of working, perhaps the most important of all, had been lost sight of. It might be safely laid down, that that mode of working was the safest from explosion which admitted of the most perfect ventilation, which was the least «u ijeot to a local failure of ventilation, in which the discharge of gas was beet regulated, in which large accumulations of gas were prevented, and in which tho superintendence of the workmen could be most thorongh. In an unbroken coalfield, the free hydrogen might be assumed to be distribute evenly over srnali areas, and each ton of coal ■would have a certain proportion diffused through it. If this wero liberated only in the coal actually cut, and when it was cut, the amount of ventilation could be exactly regulated to the production of tho mine ; and the mode of work, so long as ventilation was possible ut all, would bo immaterial. The firedamp lying in coal seams possessed considerable mobility umoDgst the particles of •coaL and as it was often at apressnre in excess of the atmosphere, it travelled through the coal for pome distance towards a point of discharge. The rapidity with which a given area was so drained, no doubt varied in some proportion to the difference between the initial pressure of the gas and that of the atmosphere, and theamouut of resistance which the gas met in permeating the coal. It also varied according as the openings were bmadways or endways of the seam. In all proliability it was three or four times the greatest on tho end of the coal, ns the clevnge planes were to u cortttin extent channels for the passage of the gas. Thus, in a headway on the end of the coal, the discharge of gas was most abundant at the back of the heading ; while in board ways it was most perceptible at the sides of tho heading, and in such a heading a largo part of the gas would probably be let off for some yards on each side. When the excess of pressure was relieved, the

discharge might be supposed to vary with the changes in the barometrical pressure. It was suggested that experiments shonld bo mado iu different localities, to ascertain (1) tho quantity of gas given off per square yard of freshly cut surface, (2) to what extent this varied on the face or end, (.'!) in what ratio this discharge diminished with time of exposure, (4) to what extent barometrical changes effected the discharge of tho gas, and (5) by what amount the pressure of gas increased, as measored from the exposed surfaco inwards to tho solid seam. It was believed such experiments would show that from 50 por cent, to 75 per cent, of the gas contained in the coal lying 10, 20 and 30 yards on each side of a boardways heading, was liberated when and after this was driven.

As the free hydrogen gas came not only from the hewn coal, but also from the solid seam, it was important that tho surface exposed to the air should be as Bmall as possible. It was argued that every mode of pillar work liberated three, five, or ton times the amount of gas per ton hown in the solid than was liberated by a system of longwall work. If, therefore, the diluting power of the air current was the samo in both cases, three, five, or ten times more would he necessary in pillar mines than in longwall mines. In a mine under tho author's charge, this excessive discharge of gas in pillar roads was very noticeable. Long after u headway was driven, the gas oozed out of the sides of the headway, and might be beard at a considerable distance. In the longwall faces this was not perceptible, as the gas given off there was that due marely to the coal hewn.

Again, in pillar mines from six to twenty times as much surface coal was exposed as in longwall mines, and therefore such mines were from six to twenty timos more subject to the effect of changes in the atmospherical pressure.

On inspecting the maps of mines worked by different pillar methods, and comparing them with the diagram showing a like extraction of coal by longwall, it was clear how large a proportion the gas discharged in the former must bear to lh.it in the latter. It was frequently argued that this gas drainage was desirable, but it was submitted that before such a course could be with propriety recommended, it was necessary to show that tho ventilating current would be proportional to the discharge.

The ease with whioh a mine conld bo ventilated, and the freedom from local derangement, would depend much upon the cubic contents of open mine, upon thefrcedom from stoppings, doors, etc., and upon the geueral simplicity of the arrangements. For a like extraction of coal, tho cubic contents of pillar mines were from ten to twenty times the amount o'. properly designed longwall mines, and the drawings showed clearly the relative simplicity of each. In every pillar mine the workings were driven in advance of tho ventilating channels, and constant brattices were essential. It would be seen, by examining the reports, how nnmorous were the accidents from defective brattices.

In South Wales the working places were driven into the solid coal, and when finished had no chanuel left for a steady through current, and thus the chance of their harbouring firedamp was vory great. In the North of England there were none of these dumb points, but the cubio contents of boards, in which there was no sensible current, was often very large. Whatever might bo the difference of opinion with regard to barometrical changes in mines, it was reasonable to suppose that they would exert thejloast influence where the surlace of coal which might exude gas w is the least. Tho proportion which the surface exposed in pillar mines bore to that in longwall mines was from 10-20 to 1. The goaf of a longwall mine became approximately solid as the coal was extracted over large areas, and thus permitted of a general settlement. In pillar mines the tendency was towards the formation of many small goaves, where there could be no surface settlement. Those goaves thus became so many gas holders. The longwall mode of work also admitted of the nearest approximation to goaf ventilation. The only open parts were the edges, and as thsse were cut through with roads, a constant current could be maintained along them. It was possible, in a properly laid out longwall miue, to keep the goaf clear of gas as far back as it was open. In pillar workings ther> was no possibility of sending air into the goaf, and it thus became charged with gas. It was therefore submitted, that tbo safety of mining operations might be increased by

tho extension of longwall working. It was Mtisfactory to be able to add that, on economics. I grounds, it was daily gaining in favour, and thac simplicity, compactness, small cubic contents of open mine, small exposure of coal surface, regular gas discharge and thorough ventilation could be best attained in longwall mines.

Another paper on " coal mining." an abstract of which wo shall givo next week, was read at the same meeting by Mr. Emerson Bainbridge.

THE SCENERY AND GEOLOGICAL
STRUCTURE OF A COUNTRY. •

(Conlinue.l from page 220.)

A ND all that is necessary to understand what Jt\. I am putting before you to-night is, not so much technical knowledge, as that yon shonld make good use of your eyes. This explanation which I have offered to you of the carving out of tbo earth's surface was, for some time, vehemently opposed, on the grouud that the forces were not equal to the task, and that they mast have been helped very much by the action of the sea. This objection was very satisfactorily met by a reference to tho country of Auvergn», in the south of France. That country was a long time ago the scene of volcanic action, and it is still studded over by volcanic cones, which may be roughly described as heaps of ashes, which have accumulated in piles all round. TheM cones were so friable that if they had been submerged beneath tho sea they must have been worn away to sand and mud. Therefore, if we oats point out there any valleys hollowed out since the period of volcanic action, our case will be pretty clear. In the valley, then, a volcanic eruption has taken place, and through that valley lava has poured and completely tilled it. Subsequently a fresh channel has been cut iu it, at first partially, and afterwards quite as far down as the old rock. Now, we arc quite sure that the new channel must have been cut ont since the time of the eruption, and that the sea has never flown over it. That which these forces did &t Auvergne, they are competent to do elsewhere.

From this short sketch of the manner in which the shape of the ground has been produced, you will have gathered that one important element is the relative hardness of the rocks. The harder rocks are best able to hold out against denudation. That is the reason why Wales and Scotland remain hilly and mountainous, and parts of England are flat and comparatively tame. Again, wherever we find a different degree of hardness iu the rocks, we always find a striking variety of land contour. Take the country about Bath, or the^moorland tracts of the north of England, or the high land of Derbyshire and Yorkshire. Thore we find the characteristic features of the country to be a number of long, straight, terraced lines of hill, running roughly parallel to each other, with deep valleys between them, and when we couao to look a little into the structure of the eountry, wo rind that wherever a river runs across a ridge, it flows in, narrow gorge, and wherever it leaves such ridges, and enters a valley, it widons. These ridges, from the fact of their being formed of a hard substance, havo been able to defy the impress of the water. A large portion of the rocks, as you are aware, are composed of beds, or layers of strata lying above one another. In some cases these layers lie flat or horizontally, in others they lie at angles. It is quite clear that if the beds Ho very nearly flat, any single bed will roach over a large portion of country ; and if tho rocks are soft, we shall have, in addition, an undulating and feeblo-looking country; but if the rocks are hard, the valleys will tend more or less to be steep-sided, and approach the form of gorges. Again, if the beds, instead of being flat, are tilted, we shall cross a greator variety of rooks, and also more varied features of country; and, as I quoted just now, wo find ridges with valleys lying between them.

I will now lay before you one or two instances iu which beautiful scenery is associated with geological structure, and endeavour to point out the connection between the two. I once had the good luck to get n view at Buxton, Derbyshire, so far that I could actually see the sea at Liverpool. I stood on a wild, rugged hill country, overspread with heather, and tenanted only by grouse and mountain shoep. This hilly oountry euded off sharply and abruptly in a bold line of

• A lecture delivered at St. George's Hall, by A. H. Gbeen, K»q., M.A.. Cambridge, K.G.8. (of her Majesty's Geological Surveys), March 20,1S70.

bills to the west. Looking over those hills I saw another range, the Peckforton Hills, anil far away I caught a glimpse of the sea. There was a rough, ragged, wildness around me which contrasted strongly with the peaceful stillness of the broad plain. I enjoyed it nil the more because every one of these elements of beauty conveyed to my mind a history of the way in which these particular features had b?en produced. I knew that the foreground stood up high because it was made of hard rocks ; I knew that it was rugged because its rocks were ef unequal hardness; and 1 knew that the broad plain before me was lowlying because tho strata of which it was composed waa moro easily and largely swept away than the hard rocks I was standing on. The keen mountain air had quickened my appetite, and the sheep I saw around me reminded me of the excellence of the moorland mutton: I recollected that Wales, the Lake country, and other hilly district!, were similarly distinguished. The superiority arises probably from several causes, the nature of the food, the pnre air, and the active habit« rendered necessary by the rugged ground; »ud.ail these in the end depended on the fact that the rocks of the district were hard. By contrast, the great plain called np pleasant remembrances of Cheshire dairies and Cheshire cheese, and the existence of these is due to the luxuriant paituxage afforded by the rich, marly subsoil. The «nain physical and geological features of the view I have just described are shown in a somewhat exaggerated form in the sketch section Fig. 1.

rise of the ground is shown by the increased depth of the cutting?, and at last wc shoot into a tunnel. On coming again into daylight, and looking back, we find the outlet of the tunnel to be situated on a steep hill side, which rnns away in a well-marked bank or escarpment, east and west, as far as the eye can reach. We next cross a strip of nearly flat ground, whose shape shows that it is formed of a soft and nniform bed : it is indeed oconpicrt by a clay, known as the Ganlt, whioh comes out from below the chalk. From the plain tho ground again begins to slope np in a gentle incline, terminated on the sonth by а steep escarpment; this land, we know from these features, must be formed by a band of harder rock coming out from below the Gault : the beds are known as the Lower Green sand. The strips of country occupied tby the chalk and Lewer Green »and resemble one another in having a long gentle slope to the north and a steep descent to the south, hut the incline of tho Lower Green sand is traversed by a number of east and westridges with valleys or flats between, while that of the chalk;hae/i far more even surface. Beyond the escarpment of the Lower Green snnd we run over a broad flat formed of tho soft clay known as the Weald clay, and note that to the south of this flat the ground again begins to rise with a gentle slope in such a way that we infer that auother hard bad is coming oat from beneath the Weald clay, and on reaching the higher country we find our swpicion correct, for wo observe in the cuttings, sandstones and shales, known as the

so exactly resembles that which wo see when wutei rolls off a flat country, where we see little stream lets running down into brooks, that we ennnnt resist the conclusion that the whole of this extraordinary assemhUgo of precipitous ravines is n. river system. Some persons have conjectured that, these vast chasms wore owing to earthquakes, but there is no trace whatever of them.

I must say something of the action of another denuding force—viz., rain. Th; effect of rain falling in any largo quantity is to wash in and reduce this steepness caused by the action of rivers. Springs will hurst ont at différent depths of tho steep sides of the river, and break off great masses of the groundwork ; and so wo see that though а river chauiud, if left to itself, must inevitably cut a tronch-Jike form, yet it is impossihle that it can keep this shape for any length of time. The trench must spread out before long into a wide valley. The country where those canons occur is a district of very little rain. During six monthe of the American exploring expedition, rain fell on only six days. Tho result is that the action of the rivers being unintcrfered with, they have in process of time cut down these extremely deep gorgos. But in countries like our own, where other agencies aro at work, as fast as the trench is cut down by the river, it ¡a widened again by these other actions. Tho rate of this widening process must depend very much on tho nature of the rocks of which its siiies arc composed. If they are soft, they will be oasily worked off, but if Непуст should cut асго.-s hard strati, tho sides will

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A similar prospect is «ean from the top of Mickle Fell in the extreme north-west of Yorkshire. The hill is situated on the western edge of a wild and rugged moorland district formed of the same rocks as are found in the neighbourhood of Bnxton, and from it the ground falls away rapidly to the wast, along a iteep hillside, which from a distance aeems to riso almost like a vertical wall from the broad vale of Eden which lies beneath. On the far side of this valley the Lake Mountains rise in eight, clustered apparently on a small врасе, but with each peak and each broadbacked hill clearly and distinctly marked. Here again we have the wild and rugged foreground contrasted with the broad, peaceful valley, ont of which the lovely little group of the Lake Mountains stands up with charming distinctness. You will at once see the reason for each fc;itnre in the view, and I need only mention that the bold, walllike, eastern flank of the Eden valley is a fault by which the hard rocks of; tho hill conntry have been thrown side by sido with the softer beds of tho low ground, and that while on one side the latter have been largely denuded, the solid strata on the opposito side have been left standing in a bold face, the steepness of which впЬ-oérial action hoe somewhat lessoned but not entirely removed. Fig. 2 shows the shape of this country.

Hastings sands, dipping so as to pass below the plain we have just loft behind. Bo for we have found the dip to be steadily to the sonth, and have passed continually on Xo lower and lowor beds. After passing through another tunnel we find that the country beyond possesses jnst the same gentle incline southwards as the chalk country near Croydon had to the north. In fact we have no doubt that in the latter part of our journey we have had exactly the same succession of strata as wo passed over in the first half, only that the order has been reversed.

We now see the whole geological history of tho district. The different beds, Chalk, Ganlt, Lower Green sand, Weald clay, and Hastings sand, once stretched in level sheets one above another over tho wholo area; they were then bent np into the shape of an arch, and as they rose into that form the higher portions wero stripped off by denudation, and beds lower and lower in the series wero laid bare towards the centre of the area.

This is the way in which I parsed my time during a journey from London to Brighton a few days ago. I found it very pleasant, and I shall be still more pleased if what I have said enables any of my hearers to whilo away unavoidably idlo hours in an equally agreeable manner.

Passing to the consideration of the second class

be able to stand better. For example, n railway cntting in hard rock can havenearly vertical cides. The case of the river Derwent, north of Matlock, in Derbyshire, showe that when we come to look at in the greet wall of hills there is an opening—a gap, which was so excessively narrow that when they wished to carry a turnpike road through it, they found there was not room, and tbey were obliged to blast the rock. If we examine the strncture of the country, the explanation of this is extremely simple. The broad valleys aro in softshale,and the impediment to tho river is formed of bard limestone. There is no doubt that originally the river flowed along this flat track, where it cut a deep, narrow, trench. Where the clays were easily worn away, the valley was widened out, but along the harder rocks it kept its narrow form. The contrast between the valley and the hills rising boldly across it is very beautiful indeed j and, to my mind, it was increased by the knowledge I possessed on this subject.

(To be concluded.)

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Let us now turn to another district, not quite so strongly featured as the last, but where tho outlines of the surface givo rise to a charming variety in the scenery, and are in the most unmistakeable wayjdependent on geological structure. Tho district I mean is the Weald of Kent and Sussex, and we may easily noto all its main features in a railway journey aoross it, say from London to Brighton.

of denuding agents—viz., sub-aerial forces, we shall find it convenient to begin with rivers. If a river is left to itself it will inevitably cut a narrow steep-sided trench. To tost tho truth of this we must try to find a district in which rivers are free from interference—that is from rain. Such a district is the country draiood by the groat Colorado river of the West in North America. The whole of this vast territory is a desolate

upon the quality of the mortar nnd the pnrpofn to bf ierved by the brickwork. Some mortars swelMexpand; in use; other» shrink. Tbc best samples of mortar in sotting become hard and tough: poorsnmples remain soft, aud crumble on exf os-ire. A thin bed of tli« host, mortar for such a work us a tall chimney would not be so strong as a thick bed. because In a thin bed there will be parts where the best bricks will he in contact, even where ¿in. thickness of bed may have been specified for, and this thickness of lied and joint may show on the face of the work. With common.

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As far as Croydon there is nothing very marked, but after passing that station the traveller enters a country whoso general form is that of a broad plain sloping up gently to the south, and cuttings and other openings show him that it is mode np of chalk. The general smoothness of this incline is broken by many deep valleys, but looked at on a large scolo it has the shape I bave just mentioned. . .\s we journey or, the general

plafean, traversed in every direction by profound gorges called canons, in which nil the drainage of the country rnns off. It is hardly possible to convey to the mind an adequate idea of the enormous eizes of these gorges. Tho Great Canon is 238 miles long, and from 2.100 to -lOUOft. in vertical depth. I think I speak correctly when I say that in some places they are only 200 or 300ft. агеоя. The arrangement of the whole

bricks, a bed of Jin. of mortar will leave rongh p-ojeettng portions of tho bricks in contact. Good mortar, when eet. Is, as we have jnst said, hard and tough.; aud to eeoure the whole strength it is capable of giving, the entire bed and joint must he full, so that tho whole area of hods and joints of bricks shall be cemented by intervening mortar.

THE GBEASE TKEE OF CHIVA, mentioned p. 203, last number, Is ihn SlUUnfin ublfrm. Michx., belonging to the .Spurge order, or Euphnrbuicca;. Particulars vide Tichnologis!, vol. iv., p. 33. —Bksmardir. _

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1, represent! i plan of setting diamonds in an am u!:ir cutter, and will be easily understood. Fig. 2 represents wh t is termed a " No. 1 prospecting drill," S3 called because of its general use in testing the character and value of mines and quarries It consists of a small, upright boiler, to one side of which is firmly bolted the cast-iron frame which supports the engine and swivol drillhead, gears and screw shaft, as shown in the engraving. The engine—an oscillator of from five to seven horse-power—is shown at A. 1$ is the screw-shaft with drill passing through it. This shaft is made of hydraulic pipe from 5ft. to "ft. in length, with a coarse thread cut on the outside. This thread, a portion of which is shown in the cnt, runs the entire length of the shaft, which also carries a spline by which it is feathered to its npper sleeve-near. This gear is double and conuects by its lower teeth with the bevelled driving-gear, and by its npper teeth with the release-gear E. This release-gear is feathered to the feed shaft F, at the bottom of which is a fractional gear fitting the lower gear on the screw-shaft, which has one or more teeth less than tbo fractional gear, whereby a different feed is produced. This fractional gear is attached to bottom of feed-shaft F by a friction-nut, thns producing a combined differential and fractional feed, which renders the drill perfectly sensitive to the character of the rock through which it is pa-sing, and maintains an uniform pressure upon the same. The severe and sudden strain upon the cutting-points incidental to drilling through soft into hard rock with a positive feed is thus avoided. The drill proper (passing through the screw-shaft B) consists of a tubular boring bar, made of lap-weld pipe, with a steel bit or boringhead D, screwed on to one end. This bit is a steel thimble about 4in. in length, having throe rows of black diamonds in thoir natural rough istate firmly imbedded therein, so that the edges of those in one row project forward from its face, ] while the edges of those in the other two rows project from the enter and inner peripherics re-' spectively. The diamonds of the first mentioned' row cut the path of the drill in its forward pro-1 gress, while those upon the outer and inner periphery of the tool enlarge the cavity around the same, and admit the free ingress and egress of the water as hereafter described. As the drill passes into the rock, cutting an annular channel, that portion of stone encircled by this channol is of conrse undisturbed, and passes up into the drill in the form of a solid cylinder. This core is drawn out with the drill in sections of from 8 to 12ft. in length.

The sides of the hollow bit arc one-fourth of an inch thick, and the diamonds of the inner row project abouione-eighth of an inch, so that the core or cylinder produced by a two-inch drill (the ordinary size for testing), is one and a quarter inches in diameter.

Inside the bit D is placed a self-adjusting wedge which allows the core to pass up into the drill without hindrance, but which impinges upon and holds it fast when the action of the drill is reversed—thns breaking it off at the bottom and bringing it to the surface when the drill is withdrawn.

In order to withdraw the drill it is only necessary to throw out the releaso gear K by sliding it up the feed-shaft F to which it is feathered"! when the drill runs up with the same motion of the engine which carried it down, hut with a velocity s:xty times greater—that is, the speed with^which the drill leaves the rock, bringing the

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coro with it, is to the speed with whioh it ponetratesjit as sixty to one—the revolving velocity in both cases being the same.

The drill-rod may be extended to any desirable length by simply adding fresh pieces of pipe. Common gas pipe is found to serve admirably for this purpose, the successive lengths being quickly coupled together by an inside coupling four inches long, with a hole through the centre of each to admit the water. The drill is held firmly in its place by the chuck G at bottom of screw shaft.

The small steam pump C C is connected by a rubber hose with any convenient stream or reservoir of water, and also with the outer end of the drill-pipe by a similar hose having a swivel joint, as shown in the cut. Through this ho*e a s}in. stream of water is forced by a pump iuto the drill from which it escapes between the diamond teeth tit the bottom of the bit D, and passes rapidly out of the hole at the surface of the rock carrying away all the grit and borings as fast as produced. Where water is scarce or difficult of access, a spout is laid from the mouth of the hole to the tank |or reservoir, and a strainer attached to the connecting hose, so the same water may be used over and over again with but little loss.

Messrs. Severance and Holt, of New York, are ihe manufacturers of Lesehot's patent diamond pointed steam drills.

A THEORY OF NEBUL/E AND COMETS.*

WILL yon allow me to lay before your readers a theory regarding nebula? and comets which I believe is in a great degree new, and which appears to mo to afford a satisfactory explanation of the phenomena observed in those remajkable bodies?

The theory may be stated as follows :—There exist in space large masses of matter in a gaseous, non-lnminons. and therefore invisible state. This

* By A. S. Davis. B.A.. Mathematical Master, Loeds School, In the Phttotophical MagasUe.

matter Is necessarily very rare, because there is no solid nucleus to condense it by its attraction.

As long as a mass of such matter remains by itself, it continues gaseous, non-lnminous, and invisible. It will, however, sometimes happen that two masses of gas having a chemical affinity for each other rush together under the influence of their mutual gravitation. When this occurs, chemical combination will take place, with a consequent evolution of light and heat, and a nebula will begin to be formed. Chemical action will only take place where the gases become mixed —that is, about their common bounding surfaces; and the greater part of the matter will remain invisible. The apparent shape, then, of a nebula will by no means indicate the real shape of all the matter composing it, but will rather reveal the form of the common bounding surfaces of the different masses of gas.

The gases, so long as they remain separate, cannot condense into a liquid or solid state; but the compound formed by the chemical union m»y only be able to exist as a gas so long as it remains at a great heat. After the compound has formed, it will cool down, and may ultimately assume the liquid or solid state.

When liquid or solid matter is formed, it will gradually aggregate and gravitate towards the centre of the nebula, and will form into a nucleus there. It is probable that chemical action will not be the only source of light and heat, but that thoy will also be produced by the loss of motion of the solid and liquid parts moving through the gases.

This theory attempts to explain how nebulas originally came into existence and assumed the shapes they actually have. And in this it differs from that of Sir William Herschel, who only speculated on what would be the subsequent motions and behaviour of nebulous matter after it had once been formed.

I will now proceed to examine what account this theory renders of the phenomona observed in nebula'.

And first, I think, if we admit that a great

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