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Law III. The central particles are neither separated or compressed. This law is not to be taken in a very strict sense, which would be that only one single file of particles was in the unaffected condition. But so small is the greatest separation, that a considerable portion of the body may be looked upon as practically unaffected.

FIG. 37

38

Law IV. The separation of the upper and compression of the lower particles diminishes to the centre. 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, compression 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 this 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, just 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 in 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 from the upper to the lower set of particles the bottom of the beam-except at the sides; the rest of the bottom of the beam is only acted upon through its cohesion to its edges, or sides. There is, therefore, introduced 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 beam 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 form, the section of which is like the letter H laid flat, H, Fig. 39. Their resistance depends in part 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

flexibility, but it has something additional, by Elasticity is, to a certain point, the same as 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 consequence, which has been investigated by deep mathematical study and much experimental research Here, of course, but the great general principles

There are font sorts of elasticity-elasticity of compression, of traction, of flexion, and of torsion. The two first are familiar in balls, strings of india-rubber; the third, in rods of wood, steel, &c.; 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, or 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 asunder, 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 their original position. The wires represent each a line of particles from the centre to the circumference of the cord. The angle with the position of rest is called the angle of torsion, a, Fig. 42.

FIG. 42

Law III. In torsion elasticity the oscillations are isochronous. No matter 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 1. 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; it must, therefore, have been restored. In traction-elasticity there can be no compression. The particles are all drawn away. A strip of india-rubber under traction is interesting. By marking some one spot its changes can be observed. It grows thinner, as the force increases,

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from action, to its first form, b. In torsion, elasticity, the compression which takes place is not in the same direction as 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 been mentioned, must 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, a times larger. Some idea of the apparatus in the Appli

cations.

Law III. Shown by Coulomb to be the ca The experiments do not belong to mechanics, least, the forces brought into action.

No. III.-Applications.-From amongst 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 out 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 handle 6 of the axis a above, from which the thread hangs. The handle 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 complete 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 upper torsion scale, the axis, and nippers to hold the silver wire. The lower torsion scale is usually on the cage. All other details are dependent upon electrical principles. And here occurs the first occasion of warning the student against over-attention to the unimportant details of instruments. See, make 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, the instruments under 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; in the confusion the mind does not see where the essentials are, and after a most careful setting forth ot even the smallest points of height, position, material, and general action &c., the explanation of an apparatus may

FIG. 45

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conclude without one of the main princples having rod is formed with a straight rack gearing with a
been touched.
toothed segment on the beam. The back of the
Examiners in mathematics will sometimes pre-rack works against a roller A.
sent a proposition with the figure reversed from 135. A parallel motion commonly used for
its usual position. How many candidates has stationary beam engines.
not this immaterial change completely puzzled? 136. An arrangement of parallel motion for
So in scientific apparatus, change the position, side lever marine engines. The parallel rods
form, &c., leaving the essentials; many who have connected with the side rods from the beams or
stored up pages of details, will no longer re-side levers are also connected with short radius
cognise the instrument, to acquire the explanation arms on a rock-shaft working in fixed bearings.
of which may have cost them much labour.
137. Parallel motion, in which the radius rod is
App. II.-Inelastic Bodies. It is impossible connected with the lower end of a short vibrating
to have a body hard, and perfectly free from rod, the upper end of which is connected with the
elasticity. Of this remarkable proofs later in ex-beam, and to the centre of which the piston-rod
periments, which for their perfection require such is connected.
bodies. Perfect elasticity is equally impossible. 138. Another modification, in which the radius
Full restitution takes place, but its external effect bar is placed above the beam.
is not fully produced; more strictly the "ex-
ternal 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 sound. At the curves of the "line," the inside wheel produces a torsion in the axle, which little by little disintegrates the metal, and so 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 safe. guard 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 20000lb., exhibited an elasticity which recalled that of compressed air. (To be continued.)

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139. Parallel motion for direct action engines. In 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 B C.

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

147. 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 seats 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.)

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. ON THE RELATIVE SAFETY OF DIFThe 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 before the Institution of Civil Engineers on

the shaft 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 pump-
ing engine on the atmospheric principle, with
chain connection between piston rod and a seg-
ment at end 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 framing and at the other with a vibrating
piece on top of piston-rod.

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

145. Inverted oscillating or pendulum engine.

The cylinder has trunnions at its upper end and
swings like a pendulum. The crank-shaft is be-
low, and the piston-rod connected directly with
crank.

146. Table engine. The cylinder is fixed on a table-like base. The piston-rod has a cross-head + Extracted from a compilation by Mr. H. T. BROWN, working in straight slotted guides fixed on top of

Editor of the "American Artisan."

FERENT MODES OF WORKING COAL.
HE following is the substance of a paper read

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 in coal mining from accident, there was 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 29 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 board 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 in

he longwall system of the Midland Counties. In he latter case, as the faces advanced, pack walls of roof rock, or bind, were built at regular interals, and whenever a sufficient width of opening was obtained, the roof settled down, with or without fracture, upon these packs. Accidents by falls might fairly be brought to the test of figures; for although the roofs of various seams might differ much, the averages of large districts were likely to be uniform. Of a gross tonnage of 198,636,043 tons obtained by pillar work in 1866, 1867, and 1868, the casualties by falls were 814, or 231,739 tons of coal for each life. Of a gross tonnage of 22,899,000 tons extracted by the longwall plan, the casualties were 75, or one life for every 305,320 tons. If the latter ratio existed in pillar work, the casualties would have been reduced from 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 been excluded; as in North Staffordshire, Cheshire, and Shropshire, both modes 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 greater safety of longwall mines from falls was owing to the narrow width of the working places, to the constant change to a new roof, so that there 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 fragments, 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 under sized 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, the author 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 were proposed, and were more or less adopted, the effect 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 subject to a local failure of ventilation, in which the discharge of gas was best regulated, in which large accumulations of gas were prevented, and in which the superintendence of the workmen could be most thorough. In an unbroken coalfield, the free hydrogen might be assumed to be distribute evenly over smali areas, and each ton of coal would have a certain proportion diffused through it. If this were liberated only in the coal actually cut, and when it was cut, the amount of ventilation could be exactly regulated to the production of the mine; and the mode of work, so long as ventilation was possible at all, would be immaterial. The firedamp lying in coal seams possessed considerable mobility amongst the particles of coal, and as it was often at a pressure in excess of the atmosphere, it travelled through the coal for some 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 the amount of resistance which the gas met in permeating the coal. It also varied according as the openings were broadways or endways of the seam. In all probability it was three or four times the greatest on the end of the coal, as the clevage planes were to a certain 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 the heading, and in such a heading a large 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 should be made in different localities. to ascertain (1) the quantity of gas given off per square yard of freshly cut surface, (2) to what extent this varied on the face or end, (3) in what ratio this discharge diminished with time of exposure, (4) to what extent barometrical changes effected the discharge of the gas, and (5) by what amount the pressure of gas increased, as measured from the exposed surface inwards to the solid seam. It was believed such experiments would show that from 50 per 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.

the extension of longwall working. It was satisfactory to be able to add that, on economical grounds, it was daily gaining in favour, and that 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 we shall give next week, was read at the same meeting by Mr. Emerson Bainbridge.

THE SCENERY AND GEOLOGICAL STRUCTURE OF A COUNTRY. *

(Continuel from page 220.)

ND all that is necessary to understand what As the free hydrogen gas came not only from I am putting before you to-night is, not the hewn coal, but also from the solid seam, it was so much technical knowledge, as that you should important that the surface exposed to the air make good use of your eyes. This explanation should be as small as possible. It was argued which I have offered to you of the carving out of that every mode of pillar work liberated three, the earth's surface was, for some time, vehemently five, or ten times the amount of gas per ton hewn opposed, on the ground that the forces were not in the solid than was liberated by a system of equal to the task, and that they must have been lengwall work. If, therefore, the diluting power helped very much by the action of the sea. This of the air current was the same in both cases, objection was very satisfactorily met by a three, five, or ten times more would be necessary reference to the country of Auvergne, in the south in pillar mines than in long wall mines. In a of France. That country was a long time ago mine under the author's charge, this excessive the scene of volcanic action, and it is still studded discharge of gas in pillar roads was very notice-over by volcanic cones, which may be roughly able. Long after a headway was driven, the gas described as heaps of ashes, which have accumuoozed out of the sides of the headway, and might lated in piles all round. These cones were so be beard at a considerable distance. In the long-friable that if they had been submerged beneath wall faces this was not perceptible, as the gas the sea they must have been worn away to sand given off there was that due merely to the coal and mud. Therefore, if we can point out there he wn. any valleys hollowed out since the period of Again, in pillar mines from six to twenty times volcanic action, our case will be pretty clear. as much surface coal was exposed as in longwall In the valley, then, a volcanic eruption has taken nines, and therefore such mines were from six to place, and through that valley lava has poured twenty times more subject to the effect of changes and completely filled it. Subsequently a fresh in the atmospherical pressure. channel has been cut in it, at first partially, and afterwards quite as far down as the old rock. Now, we are quite sure that the new channel must have been cut out since the time of the eruption, and that the sea has never flown over it. That which these forces did at Auvergne, they are competent to do elsewhere.

On inspecting the maps of mines worked by different pillar methods, aud 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 that 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 the ventilating current would be proportional to the discharge.

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 denudaThe ease with which a mine could be ventilated, tion. That is the reason why Wales and Scotland and the freedom from local derangement, would remain hilly and mountainous, and parts of depend much upon the cubic contents of open England are flat and comparatively tame. Again, mine, upon the freedom from stoppings, doors, &c., wherever we find a different degree of hardness in and upon the general simplicity of the arrange- the rocks, we always find a striking variety of ments. For a like extraction of coal, the cubic land contour. Take the country about Bath, or contents of pillar mines were from ten to twenty the moorland tracts of the north of England, or times the amount of 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 the ventilating channels, and constant brattices were essential. It would be seen, by examining the reports, how numerous were the accidents from defective brattices.

the high land of Derbyshire and Yorkshire. There 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 come to look a little into the structure of the eountry, we find that wherever a river runs across a ridge, it flows in, narrow gorge, and wherever it leaves such ridges, and enters a valley, it widens. These ridges, from the fact of their being formed of a hard substance, have been able to defy the impress of the water. A large por tion 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 lie very nearly flat, any single bed will reach over a large portion of country; and if the rocks are soft, we shall have, in addition, an undulating and feeble-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 greater variety of rocks, and also more varied features of country; and, as I quoted just now, we find ridges with valleys lying between them.

In South Wales the working places were driven into the solid coal, and when finished had no channel left for a steady through current, and thus the chance of their harbouring fireddamp was very great. In the North of England there were none of these dumb points, but the cubic contents of boards, in which there was no sensible current, was often very large. Whatever might be the difference of opinion with regard to barometrical changes in mines, it was reasonable to suppose that they would exert the least influence where the surface of coal which might exude gas was the least. The 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 settleI will now lay before you one or two instances ment. These goaves thus became so many gas in which beautiful scenery is associated with holders. The longwall mode of work also admit- geological structure, and endeavour to point out ted of the nearest approximation to goaf ventila- the connection between the two. I once had the tion. The only open parts were the edges, and as good luck to get a view at Buxton, Derbyshire, these were cut through with roads, a constant curso far that I could actually see the sea at Liverrent could be maintained along them. It was pos- pool. I stood on a wild, rugged hill country, sible, in a properly laid out longwall mine, to keep overspread with heather, and tenanted only by the goaf clear of gas as far back as it was open. grouse and mountain sheep. This hilly country In pillar workings there was no possibility of send-ended off sharply and abruptly in a bold line of ing air into the goaf, and it thus became charged with gas. It was therefore submitted, that the safety of mining operations might be increased by

GREEN, Esq., M.A., Cambridge, F.G.S. (of her MaA lecture delivered at St. George's Hall, by A. H. jesty's Geological Surveys), March 20, 1870.

bills to the west. Looking over these hills I saw rise of the ground is shown by the increased
another range, the Peckforton Hills, and far away depth of the cuttings, and at last we shoot into a
I caught a glimpse of the sea. There was a rough, tunnel. On coming again into daylight, and
rugged, wildness around me which contrasted looking back, we find the outlet of the tunnel to
strongly with the peaceful stillness of the broad be situated on a steep hill side, which runs away
plain. I enjoyed it all the more because every in a well-marked bank or escarpment, east and
one of these elements of beauty conveyed to my west, as far as the eye can reach. We next cross
mind a history of the way in which these par- a strip of nearly flat ground, whose shape shows
ticular features had been produced. I knew that that it is formed of a soft and uniform bed: it is
the foreground stood up high because it was made indeed occupied by a clay, known as the Gault,
of hard rocks; I knew that it was rugged because which comes out from below the chalk. From
its rocks were ef unequal hardness; and I the plain the ground again begins to slope up in
knew that the broad plain before me was low- a gentle incline, terminated on the south by a
lying because the strata of which it was composed steep escarpment; this land, we know from these
was more easily and largely swept away than the features, must be formed by a band of harder
hard rocks I was standing on. The keen moun-rock coming out from below the Gault: the beds
tain air had quickened my appetite, and the sheep are known as the Lower Green sand. The strips
I saw around me reminded me of the excellence of country occupied tby the chalk and Lower
of the moorland mutton: I recollected that Wales, Green sand resemble one another in having a
the Lake country, and other hilly districts, were long gentle slope to the north and a steep descent
similarly distinguished. The superiority arises to the south, but the incline of the Lower Green
probably from several causes, the nature of the sand is traversed by a number of east and west
food, the pure air, and the active habits rendered ridges with valleys or flats between, while that of
necessary by the rugged grouad; and all these in the chalk has a far more even surface. Beyond the
the end depended on the fact that the rocks of the escarpment of the Lower Green sand we run over
district were hard. By contrast, the great plain a broad flat formed of the soft clay known as the
called up pleasant remembrances of Cheshire Weald clay, and note that to the south of this
dairies and Cheshire cheese, and the existence of flat the ground again begins to rise with a gentle
these is due to the luxuriant pasturage afforded slope in such a way that we infer that another
by the rich, marly subsoil. The main physical hard bed is coming out from beneath the Weald
and geological features of the view I have just clay, and on reaching the higher country we find
described are shown in a somewhat exaggerated our suspicion correct, for we observe in the cut-
form in the sketch section Fig. 1.
tings, sandstones and shales, known as the

so exactly resembles that which we see when water rolls off a flat country, where we see little stream lets running down into brooks, that we cannot resist the conclusion that the whole of this extraordinary assemblage of precipitous ravines is a river system. Some persons have conjectured that these vast chasms were owing to earthquakes, but there is no trace whatever of them.

I must say something of the action of another denuding force-viz., rain. The effect of rain falling in any large quantity is to wash in and reduce this steepness caused by the action of rivers. Springs will burst out at different depths of the steep sides of the river, and break off great masses of the groundwork; and so we see that though a river channel, if left to itself, must inevitably cut a trench-like form, yet it is impossible 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 months of the American exploring expedition, rain fell on only six days. The result is that the action. of the rivers being uninterfered with, they have in process of time cut down these extremely deep gorges. But in countries like our own, where other agencies are at work, as fast as the trench is cut down by the river, it is widened again by these other actions. The rate of this widening process must depend very much on the nature of the rocks of which its sides are composed. If they are soft, they will be easily worked off, but if the river should cut across hard strata, the sides will

A similar prospect is seen 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 Buxton, and from it the ground falls away rapidly to the west, along a steep hillside, which from a distance seems to rise 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 sight, clustered apparently on a small space, 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, out of which the lovely little group of the Lake Mountains stands up with charming distinctness. You will at once see the reason for each feature 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 the hill country have been thrown side by side with the softer beds of the low ground, and that while on one side the latter have been largely denuded, the solid strata on the opposite side have been left standing in a bold face, the steepness of which sub-aerial action has somewhat lessened 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 left behind. So far we have
found the dip to be steadily to the south, and
have passed continually on to lower and lower
beds. After passing through another tunnel we
find that the country beyond possesses just the
same gentle incline sonthwards 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 we passed over in the first half, only
that the order has been reversed.

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

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

Passing to the consideration of the second class

80

be able to stand better. For example, a railway cutting in hard rock can have nearly vertical sides. The case of the river Derwent, north of Matlock, in Derbyshire, shows that when we come to look at in the great wall of hills there is an opening-a gap, which was excessively narrow that when they wished to carry a turnpike road through it, they found there was not room, and they were obliged to blast the rock. If we examine the structure of the country, the explanation of this is extremely simple. The broad valleys are in soft shale, and the impediment to the river is formed of hard 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; and, to my mind, it was increased by the knowledge I possessed on this subject.

(To be concluded.)

MORTAR-JOINTS IN BRICKWORK.-The quantity of mortar which may safely and with advantage be used with bricks must in a great degree depend

Let us now turn to another district, not quite, of denuding agents-viz., sub-aerial forces, we | upon the quality of the mortar and the purpose to be so strongly featured as the last, but where the shall find it convenient to begin with rivers. If outlines of the surface give rise to a charming a river is left to itself it will inevitably cut a narvariety in the scenery, and are in the most unmis-row steep-sided trench. To test the truth of this takeable way dependent on geological structure. we must try to find a district in which rivers are The district I mean is the Weald of Kent and free from interference-that is from rain. Snch Sussex, and we may easily note all its main fea- a district is the country drained by the great tures in a railway journey across it, say from Colorado river of the West in North America. London to Brighton. The whole of this vast territory is a desolate

served by the brickwork. Some mortars swell (expand) in use; others shrink. The best samples of mortar in setting become hard and tough: poor samples remain soft, and crumble on exposure. A thin bed of the best mortar for such a work as 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 be in contact, even where in. thickness of bed may have been specified for, and this thickness of bed and joint may show on the face of the work.

With common

CHALK

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

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Good jecting portions of the bricks in contact. tough; and to secure the whole strength it is capable mortar, when set, is, as we have just said, hard and of giving, the entire bed and joint must be full, so that the whole area of beds and joints of bricks shall be cemented by intervening mortar.

plateau, traversed in every direction by profound | bricks, a bed of in. of mortar will leave rough pro-
gorges called canons, in which all the drainage of
the country runs off. It is hardly possible to
convey to the mind an adequate idea of the
enormous sizes of these gorges. The Great Canon
is 238 miles long, and from 2500 to 4000ft.
in vertical depth. I think I speak correctly
when I say that in some places they are only 200
or 300ft, across. The arrangement of the whole

THE GREASE TREE OF CHINA, mentioned p.

263, last number, is the Stillingia sebifera, Miclix., belonging to the Spurge order, or Euphorbiaceae. Particulars vide Technologist, vol. iv., p. 33.- BERNARDIN

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1, represents a plan of setting diamonds in an ani ular cutter, and will be easily understood. Fig. 2 represents wh t is termed a "No. 1 prospecting drill," so 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 swivel drillbead, gears and screw shaft, as shown in the engraving. The engine-an oscillator of from five to seven horse-power-is shown at A. B is the screw-shaft with drill passing through it. This shaft is made of hydraulic pipe from 5ft. to 7ft. in length, with a coarse thread cut on the outside. This thread, a portion of which is shown in the cut, runs the entire length of the shaft, which also carries a spline by which it is feathered to its upper sleeve-gear. This gear is double and connects by its lower teeth with the bevelled driving-gear, and by its upper 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 the fractional gear, whereby a different feed is produced. This fractional gear is attached to bottom of feed-shaft F by a friction-nut, thus producing a combined differential and fractional feed, which renders the drill perfectly sensitive to the character of the rock through which it is passing, and maintains an uniform pressure upon the same. The severe and sudden strain upon the entting-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 three rows of black diamonds in their natural rough state 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 outer and inner peripheries respectively. The diamonds of the first mentioned row cut the path of the drill in its forward progress, 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 channel is of course 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 are one-fourth of an inch thick, and the diamonds of the inner row project about one-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.

ROCK DRILLING MACHINE.

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no solid nucleus to condense it by its attraction.

core with it, is to the speed with which it pene-matter is necessarily very rare, because there is trates it 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.

As long as a mass of such matter remains by itself, it continues gaseous, non-luminous, 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 The small steam pump CC is connected by a-that is, about their common bounding surfaces; rubber hose with any convenient stream or reser- and the greater part of the matter will remain voir of water, and also with the outer end of the invisible. The apparent shape, then, of a nebula drill-pipe by a similar hose having a swivel joint, will by no means indicate the real shape of all as shown in the cut. Through this hose a fin. the matter composing it, but will rather reveal stream of water is forced by a pump into the drill the form of the common bounding surfaces of the from which it escapes between the diamond teeth different masses of gas. at 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 the manufacturers of Leschot's patent diamond pointed steam drills.

Inside the bit D is placed a self-adjusting wedge which allows the core to pass up into the A THEORY OF NEBULE AND COMETS.* drill without hindrance, but which impinges upon and holds it fast when the action of the drill is reversed-thus breaking it off at the bottom and bringing it to the surface when the drill is with

drawn.

In order to withdraw the drill it is only necessary to throw out the release gear E 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, but with a relocity sixty times greater-that is, the speed with which the drill leaves the rock, bringing the

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renders a theory regarding nebula and to lay before your ILL you allow me comets which I believe is in a great degree new, and which appears to me to afford a satisfactory explanation of the phenomena observed in those

remarkable bodies?

The theory may be stated as follows:-There exist in space large masses of matter in a gaseous,

non-luminous. and therefore invisible state.

This

By A. S. DAVIS. B. A., Mathematical Master, Leeds School, in the Philosophical Magazine.

The gases, so long as they remain separate, cannot condense into a liquid or solid state; but the compound formed by the chemical union may 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 they will also be produced by the loss of motion of the solid and liquid parts moving through the gases.

originally come into existence and assumed the This theory attempts to explain how nebula 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 phenomena observed in nebulæ.

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

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