Εικόνες σελίδας
PDF
Ηλεκτρ. έκδοση

was exhibited in the principal cities of the United States, but resulted in no useful or valuable application, though an attempt was made to drive a printing-press with it.

Of more recent attempts to apply this power, we may mention one made on the Edinburgh and Glasgow Railway, by Mr. Davidson, who constructed an electro-magnetic locomotive, 16 feet long, 6 feet broad, and weighing tons; and another, by Professor Jacobi, of St. Petersburg, at the expense of the Russian government. That of Mr. Davidson failed to propel the engine alone more rapidly than 4 miles an hour; but Professor Jacobi propelled a boat 28 feet by 7 feet, drawing 2 feet of water, at the rate of three miles an hour against the stream, with a party of about a dozen persons on

board.

It is evidently possible, when we possess a source of mechanical power, to accumulate it, either by employing one very large engine, or a number of smaller ones. The question whether electro-magnetic forces can be rendered practically useful, depends not, therefore, so much upon the amount of power required, as upon considerations of economy. Water and fuel are found everywhere in nature in the greatest abundance, while zine and acids are manufactured articles of comparatively high cost. An engine which consumes such substances in large quantity cannot, therefore, be employed with profit in competition with steam. And this conclusion, based upon theoretical reasons, is fully confirmed by experiwents, with such machines,

It should also be borne in mind, in estimating the effective

[blocks in formation]

port, K. The current of a Bunsen battery, with ten or eleven couples, enters at A, passes along to a cylinder, H, and thence to the bobbins, M, whence it goes along the wire g to H, descends along the wire i, again reaches the cylinder a, and goes out at B. The two soft iron cylinders, s and a, which form the axes of the bobbins, are pierced with cylindrical apertures, to allow the luminous rays to pass through. Lastly, at 6 and a are two Nicol prisms, the former serving as polariser, the latter as analyser. The latter is made to revolve at the centre of a graduated circle, P.

These two prisms being arranged in such a manner that their principal sections may be perpendicular to each other, the prism a completely extinguishes the light passed through the prism b. If you then put a piece of flint or glass with parallel surfaces at c on the axis of the two bobbins, the light is still extinguished as long as the current does not pass; but directly the communications are established, the light reappears, but coloured, and if the analyser a is turned to the right or the left, according to the direction of the current, the light exhibits the various colours of the spectrum, as is the case with plates of quartz cut perpendicularly to the axis. M. E. Becquerel has shown that a great number of solid and liquid substances are thus capable of deflecting the plane of polarisation under the influence of powerful magnets. Mr. Faraday considers that, in these experiments, the rotation of the plane of polarisation is owing to an action of the magnets upon the Juminous rays. Messrs. Birt and E. Becquerel think the phenomenon is owing to an action of the magnets upon the

Fig. 458.

[ocr errors]
[graphic]

working-power of an electro-magnetic engine, that the forces vary inversely as the square of the distance; and that, therefore, though a magnet, such as those of Professor Henry, acts with a cohesive attraction of more than a ton in contact, or at infinitely small distances, it exerts, when the contact is broken, or at appreciable distances, a force of only a few pounds.

The fact mentioned above, that the armature or lifter of a large electro-magnet adheres with a force of several hundred pounds from year to year, after connection with the battery is broken or the magnetising cause has ceased to act, shows that there is nothing improbable in the hypothesis of Ampère, that electrical currents circulate constantly round the axes of the magnetic molecules. And, indeed, the planetary motions, as well as correct mechanical ideas, have fully established the truth, that a state of motion, in the absence of resistance, will continue indefinitely. In the present state of our knowledge of the constitution of matter, we have reason to believe that no two atoms can possibly be brought into absolute contact; and that an ethereal fluid, such as electricity may be supposed to be, may revolve perpetually in their intervening spaces, is therefore neither an improbable hypothesis, nor without analogy in nature.

Optical Effects of powerful Electro-magnets.-Professor Faraday, in 1845, discovered that a powerful electro-magnet exercises such an influence upon several transparent substances, that if a polarised ray passes through them in the direction of the line of the magnetic poles, the plane of polarisation is deflected to the right or the left, according to the direction of the magnetisation.

transparent bodies brought under their influence-an hypothesis generally admitted.

Effects of Diamagnetism produced by powerful Electro-Magnets. -Professor Faraday's apparatus serves also to exhibit other very remarkable effects of powerful magnets. We will describe a few experiments which require soft iron armatures, s and a, of various forms (figs. 456, 457, 458) to be screwed on to the bobbins.

1. M. Bancalari first observed that on placing the flame of a candle between the two magnets, it is repelled, fig. 456. Professor Faraday having found that oxygen, which is magnetic at an ordinary temperature, becomes diamagnetic at the temperature of flame, the repulsion thus exhibited may be explained by the mere repulsion to which the air is subjected; an explanation analogous to that given of the repulsion of flame by electrical machines.

2. If a small cube of red copper be hung between two magnets by a twisted silk thread, fig. 457, and be allowed to revolve rapidly by untwisting itself, it will stop directly the current passes through the bobbins, and remain fixed in its position. If instead of a cube a small rectangular bar is substituted, it will come into a position at right angles with the axis of the bobbins, or in a line with it, according as it is made of a diamagnetic substance, as bismuth and antimony, or a magnetic substance, as iron, nickel, or cobalt. Professor Faraday, who first observed these phenomena, attributes them to currents of induction which are developed in bodies by the influence of magnets.

3. M. Plücker, having placed a solution of chloride of iron in a watch-glass, fig. 458, observed that, according to the The apparatus of Professor Faraday is represented in fig. distance of the bobbins, the solution formed one or two swell455. It is formed of two extremely powerful electro-magnets, ings, as represented at A and B. The curvature of the liquid M and N, attached to two iron slides, o and o', which are in this case evidently depends upon the magnetic action of the capable of being brought near each other by sliding on a sup-bobbins combined with that of gravity.

LESSONS IN GEOLOGY.-No. LX.

BY THOS, W. JENKYN, D.D., F.G.S., F.R.G.S., ETC.

CHAPTER V.

ON THE CLASSIFICATION OF ROCKS.

SECTION XI. (continued.)

ON THE COAL MEASURES.

§ III. GEOLOGICAL PHENOMENA OF THE COAL PERIOD.

You have already been told that the carboniferous strata or coal-bearing rocks consist of sandstones; clays, containing iron, and hence called iron-stone; a slaty clay called shale; beds of grit, limestone, and coal seams.

The number of these beds varies in different coal-fields, but the thickness of the entire series is almost invariably very great. In Durham and Northumberland the coal strata altogether form a mass of more than four thousand feet in depth, i. e. about three-quarters of a mile in thickness. This deposit consists of the following number of beds :

32 Beds of coal seams with their corresponding shales. 62 Beds of sandstone.

17 Beds of limestone towards the bottom. The enumeration of the strata may be different in the coalfields of Staffordshire, of South Wales, etc., but this specimen of the lithological structure and geological order of the series, is sufficient to indicate that the various beds must have been formed under different geological influences and agencies, which I will endeavour to explain.

I. THE VEGETABLE ORIGIN OF COAL.

The vegetable origin of coal is now almost universally admitted by all scientific geologists. The problem of its origin was rendered difficult by the fact that the vegetable matter, by bituminous fermentation and by the chemical and mechanical processes which converted it into a mineral, caused the original structure of the plants to be obliterated. Yet the evidences of its vegetable origin are not lost.

1. The experiments of Dr. Mc Culloch enabled him to trace successfully the gradual passage of vegetable matter from peat, brown coal, lignite and jet, to pitchy coal, anthracite, graphite, and plumbago or black-lead.

2. Professor Göppert, to demonstrate the vegetable origin of the impressions of leaves found in iron-stone, placed a common fern leaf in a lump of elay, which, after being dried, he exposed to red heat. When he broke this mass open, the inside presented an exact resemblance of fossil plants in iron-stone.

second class have thought that groves and forests were swept away by floods and inundations, and were thus drifted out to the ocean, where they were engulphed and became decomposed; while a third class think that the coal plants have grown and have been imbedded on the spot where the coal seams are now found.

It is possible that coal may have been formed under each of these three circumstances, and that in some places each of these causes may have been in operation in producing a coal seam. Some of the coal deposits in Yorkshire have every appearance of having been formed at the bottom of fresh-water lakes, as the associated beds have fresh-water shells. Seams in the coal field to the south and west of Shrewsbury, seem formed in the bed of a river or estuary. Other seams were no doubt formed at the bottom of the ocean, for the associated grits and limestones contain marine shells.

On these theories, Sir RODERICK MURCHISON makes the following remarks in his "SILURIA," p. 279: "The supposition of many and successive subsidences of vast swampy jungles beneath the level of the waters, best explains how the different vegetable masses became covered by beds of sand and mud, so as to form the sandstone and shales of such coal fields. But this theory of oscillation, or of the subsidence en masse of ancient marshes, and their re-elevation, with occasional sand-drifts, though good in such examples as those of the South Wales and Newcastle coal fields in England, as also of the large coal fields in British North America,-can have little application to those other seams of coal, which are interstratified with beds containing marine shells, the animals of which, such as Producti and Spirifers, must have lived in comparatively deep sea-water. In such examples, and nearly all the older coal-beds come into the category, we may, on the contrary, endeavour to explain the facts by the supposition, that the ancient streams, like the present Mississippi and other large rivers, which flowed through groves or low lands and mud banks, transported great quantities of trees, leaves and roots entangled in earth, and deposited them at the bottom of adjacent estuaries, or that they were carried on masse into the broad, open sea.'

[ocr errors]

Notwithstanding this high authority for the formation of coal on the theory of plants, groves, and forests being drifted by rivers into estuaries, lakes, or seas, the structure of the coal itself supplies us with many arguments to prove that the coal grew and decayed upon the spot. seams are the decomposed remains of plants and trees that All these arguments subvert the theory of its having been drifted, and show that the instances in which plants and trees have been drifted, are nothing but the accidental results of the inundation by which the submersion of the forest was effected.

1. THE UNIFORM THICKNESS OF EACH SEAM OF COAL.-A seam of coal is of equal thickness, and without any inequality throughout the whole of its extent, for many miles wide and some scores of miles long. Over such areas, some of them hundreds of square miles, no stream, flood, or inundation would have formed a deposit of equal thickness everywhere. The

By frequent experiments he found that, according to the degree of heat in which such clay was placed, the plant became brown, or shining black or the plant became entirely lost, in which case only the impression remained. The disappear-only exceptions are the cases in which they thin out. ance of the plant had caused the whole of the surrounding clay to be stained black-a fact which indicates that the black colour of the coal shales is derived from the carbon of the plants.

3. The microscope has satisfactorily revealed in chips or slices of coal, duly prepared, traces of the fibrous tissue and woody structure of the plants. In the same manner traces of the spiral vessels and the cells of plants are found in the white

ashes after the coal has been burnt.

Sometimes the mineral coal appears to be entirely composed of very small leaves, bruised and decayed, but all matted together.

2. THE PRESERVATION OF THE COAL PLANTS.-The coal plants are found in the most perfect state of preservation, with their soft leaves, the delicate texture of ferns, and the sharp angles of stems, all well defined. These plants were once soft, and a little rolling would have crushed and disintegrated them; and yet all the lines, streaks, dottings and flutings of the Sigillaria, etc., are preserved in all their delicate textures. The fruits also of some of the coal plants are found in clusters and heaps. If these plants had been drifted, their structure would have been destroyed, and their fruits would have been dispersed by

currents.

3. THE CHEMICAL ELEMENTS OF COAL.-Coal consists of carbon combined with the gases hydrogen, oxygen, and nitrogen. These three gases are very volatile, and easily escape. If successive masses of vegetable matter were drifted and deposited by floods or tides, such an agency, by allowing the gases to escape during decomposition, would be inadequate to produce coal; for without the gases being detained, there would be no coal.

II. THE DEPOSITION OF THE COAL SEAMS. -Though all naturalists admit the vegetable origin of coal, the manner in which that vegetable matter was deposited is a question of considerable dispute among geologists. From the various circumstances in which the coal fields are found, sorue geologists have thought that the present coal seams were at 4. THE PURITY OF THE COAE.-With the exception of the first mere peat bogs, and that their different layers were pro- chemical production called Iron Pyrites, every seam of coal is duced by successive subsidence and elevation. of the land. Alan unmixed mineral-without a pebble, without gravel, with

out sand. If the seams of coal had been formed by drifted wood, the current or flood that drifted the wood would have drifted also some gravel or sand. But of this there is not a vestige in mineral coal.

5. TREES STANDING ERECT IN THE COAL MEASURES.-The instances are very numerous, in which the workings of the coal measures expose the stems, trunks and roots of trees, which seem to have grown on the identical banks of sand or mud where we now discover them, and whose decomposed leaves and associated plants formed the coal.

in the Silurian beds; but they bear no proportion, either in extent or in thickness, to the coralline limestones of the coal period.

The bottom rock, then, of the coal strata consists, in most countries, of solid compact limestone, the whole of which, in some places, is the production of the coral insect. This bed exists everywhere as a cover over the old red sandstone, except in Ireland, where it is either covered up by other rocks, or, most likely, replaced by the millstone grit.

The existence of this mountain limestone indicates that in our latitudes, at the time of its formation, there was a shallow sea, the bottom of which formed a fit foundation for coral such as we have now in the Pacific. On banks in this bottom the coral insects began to work, and as such banks belonged to a land that was gradually sinking, the insects continued to work upwards until they produced those enormous masses of limestone, which now form high mountains in both the Old and the New Worlds.

Imagine that you had a garden of about a quarter of an acre of ground. You remove the soil, several feet deep,-say for brick-making. As the earth is wheeled away, you find tree-reefs, trunk after tree-trunk exposed to view, standing some feet high, and with their roots fixed. Would you doubt that those trees grew there? would you infer that some flood placed them there all in an upright position? The evidence would be demonstrative that the trees grew on the spot. Precisely such a quarter of an acre is found at Parkfield, near Wolverhampton. The workmen cleared away all the rock that covered the coal, and then they found throughout that area seventy-two upright trees with their roots attached to the bottom soil. Some of these trees were more than eight feet in circumference. Some of the trunks were lying prostrate in different directions, for they had been broken off close to the root by some powerful agent. This was undoubtedly the remains of a submerged forest. But below this bed, there was another forest of the same character, and five feet below that there was a third forest with large stumps of trees.

These seams of coal, therefore, are the vegetable matter of decomposed plants and trees that grew on the spot. The spot was liable to be inundated from time to time, and was consequently raised higher and higher by the accumulation of sediments, as is now the case in jungles and swamps near large rivers like the Ganges or the Mississippi. As one growth of trees perished, and as the soil rose higher, other trees grew up from the new soil, now several feet higher above the level of the first morass, as is represented in our engraving of the coal measures at St. Etienne, fig. 20.

I might quote instances of a similar character from the coal works of Capel Coelbren, near Swansea, of Balgray, near Glasgow, and from a railway cutting, near Salford, on the Manchester and Bolton line; but one other, from Cape Breton, in Nova Scotia, will be sufficient. In the Sydney coal field of that country, Sir CHARLES LYELL saw erect trees occurring at different levels successively. In that section there are fortyone different beds of clay, with roots of Stigmaria in their natural position, and eighteen layers having upright trees at levels one above the other. The whole section, he says, furnishes clear evidence that at least FIFTY-ONE forests, now fossil, had grown and decayed on that spot, one after another.

III. THE PROGRESSIVE FORMATION OF THE COAL

STRATA.

1. At the commencement of the coal epoch, or rather at the close of the age of the old red sandstone, much of the northern part of the globe appears to have been covered with the ocean. But now, in that ocean, a very great change took place as to its elementary constituents. Carbonate of lime seems to have predominated, and to have extended itself over much of its area from the arctic regions to the equator. The gradual diminution of the size of the pebbles, in the upper portions of the old red sandstone, implies that the alternation with fine submarine gravel was not very sudden.

There are, indeed, limestones, and those produced by corals,

2. During the formation of this limestone in the seas that then c ered England, there was, in what is now called the Atlanti a large tract of land, extending from the north and south of England for some hundreds of miles westward to the ocean. In the meantime, the building up of such extensive coral reefs, and of such great thickness, would necessarily raise the level of the sea bottom. Nevertheless, the land is still supposed to be sinking.

3. After the land continued to sink gradually for some time, a change took place, when it began to rise again. In the course of its elevation, the limestone work of the corals became covered with a very extensive and thick bed of sandstone and grit, now called the Millstone Grit, consisting of the detritus, probably, of the large tract of land which we have supposed to be extending to the Atlantic.

4. The deposition of this grit was succeeded by the formation of muddy and sandy beds, on which a most luxuriant vegetation began to grow. These extensive beds appear to have been first formed, especially in South Wales, in water of a moderate depth, during a slow and perhaps intermittent subsidence of the ground, in a region to which rivers, from the supposed Atlantic land, would be bringing down a constant supply of sand and muddy sediment.

As these muddy beds rose above the waves, the whole area became covered with forests, such as we see now in the deltas of large American rivers in warm climates-deltas which are liable to be submerged beneath the sea, or covered by inundations of fresh water, should the district sink but a very few feet.

5. The peculiar character of this coal forest has been represented in one of our late engravings, fig. 17. This forest is depicted as growing in a broad sheet of water, or in a shallow lagoon, which received at intervals deposits of mud and silt, the detritus of neighbouring lands. Such lakes or sheets of water would be speedily filled up by a growth of profusion of plants and trees, until, by the accumulation of muddy sediment and the mixture of decayed vegetables, it would be converted into a swamp or morass.

On the surface of this morass or boggy ground, a fresh growth of plants and trees would now take place of plants and trees somewhat different from the preceding, and consisting of reed-like plants, called Equiseta and Calamites, with here and there a large tree. As these plants in their turn decayed, the decomposed matter furnished beds of peat, etc.

6. The succession of beds of coals, clays, sandstones, shales and limestones, implies that our lagoon or morass forest may, by a repetition of subsidences and elevations, have sunk beneath the level of the sea, and have rendered its basin the

Fig. 21. The North-east Portion of the Coal Measures in the Plauen Grund, near Dresden.

Rothliegender

L

receptacle of deposits of sands and clays, which produced the strata of sandstone and shale, and occasionally limestone, between the seams of coal.

The alternation of the coal measures with marine deposits abounding in sea shells is explained by supposing that the area of the vegetation must have subsided beneath the level of a neighbouring or adjacent sea, in consequence of which depression, the submerged soil would become covered with marine sediments embedding the remains of sea animals.

In time, either by the deposition of sand, or mud or clay drifted from land, or by the elevation of the bed or bottom of the sea, the swamp or morass would be again and again filled, and become the area of a fresh growth of plants and trees, while the repetition of depressions and elevations accounts for the alternations of sandstone and shales.

IV. DISTURBANCES OF THE COAL BEDS.

After the completion of the coal strata, and before the formation of the Permian or New Red Sandstone beds, very great disturbances were produced, by volcanic agency, in the stratification.

1. In some cases the earthquake agency would curve the greatest portion of the beds, and tilt them upward at one of their extremities, as represented in fig 21.

In this illustration, the perpendicular black lines represent shafts sunk into the coal measures; the curved lines, the contorting influence of volcanic agency on the overlying beds; and the inclined lines on the extreme right, the dislocations of the coal seams by the upward pressure of porphyry and greenstone.

2. Such dislocations in the strata are called Faults, probably because when a miner is working in one of these beds, and comes to a fissure or crack, he finds himself at fault. In some cases the volcanic agency has dislocated a seam of coal in so many directions, that the mining engineer becomes greatly puzzled where and how to find the continuation of the bed. The engraving, fig. 22, will show how this occurs in coal fields. The letters a, b, c, d represent shafts sunk in search of coal. At a the miner has reached the seam, which he may work right and left till he loses it. At b he is misled to think that he has reached a thicker seam of coal. At e he may think that he has found two beds. At d his shaft fails of the coal altogether.

[merged small][merged small][graphic]
[blocks in formation]

A very remarkable instance of such dislocation is represented Instances of these zigzag flexures are found at Mons, in in fig. 23.

Fig. 23. Various Dislocations of a Coal Bed at Vieille-Pompe, France.

Belgium, and at Charleroi, in France. In these cases, the shaft sunk in the coal measures cuts through the same bed of coal several times, in the manner represented in fig. 24.

4. Instead of bending the seams in an angular direction, as in the last illustration, sometimes the disturbing agency seemed as if in a freak, for it curves and twists them in the wildest manner. The next diagram, fig. 25, represents these dislocations and contortions of the coal seams. The vertical and the horizontal lines represent the shafts and the adits by which the miner seeks to find the coal.

5. The flexures, curvatures and contortions of the coal seams, represented in the preceding diagrams, are confined to comparatively narrow limits; but they are developed on an enormous scale in the Alleghanies, or Appalachian [Ap-pal

[graphic]

átch-yan] Mountains, in the United States of America, of which you have an outline in fig. 26.

In this diagram, A to B is part of the Atlantic plain; в to σ the Atlantic slope; c to D the Appalachian Mountains; D to E part of the Appalachian coal field. 1. Chalk group. 2. Gneiss. 3. New Red Sandstone. 4. Silurian Rocks. 5. Old Red Sandstone. 6. The Coal Measures. 6'. Anthracite or Stone Coal. 7, 8 and 9. The parallel folds of the Appalachians becoming successively more open and flat in going from east to west-7 and 8 representing portions of the curved rocks removed by denudation.

The illustration shows, that on the eastern side the dips predominate to the south-east, in consequence of the beds having been folded back upon themselves, as under 7, while those on the north-west side of the arch have been inverted. The coal strata are horizontal to the westward of D, and become more and more curved as we come eastward towards c. It is invariably found that the coal is more pitchy towards the west, where it is horizontal. It becomes gradually less pitchy as we come to the south-eastward of D, from 9 to 6, where the beds are most disturbed. At length we come to a completely insulated coal field, 6', associated with the boldest flexure

[merged small][graphic]

The Appalachian strata consist of Silurian, Devonian and Carboniferous rocks, extend from Vermont to Alabama, and are about 1,000 miles long, from 50 to 100 miles broad, and from 2,000 to 6,000 feet thick. These strata are all folded and bent into a succession of curves, convex and concave, parts of which, as at 7, 8, 9, have been subsequently exposed by denudation. The bendings and the breakings of the beds are greatest on the south-eastern side of the Atlantic districts; and as we go westward the strata become less and less distorted, until at length they regain at E their original horizontality.

where the strata are actually turned over, and where the coal has become pure anthracite.

This section teaches three lessons:-First, the flexure of the beds has been produced by the intrusion of igneous rocks, represented by 2. Secondly, the disturbance must have taken place before the bed 3, or New Red Sandstone, was deposi ed; for otherwise it also would have been tilted up. Thirdly, the peak in the New Red Sandstone, and the dotted curves in 7 and 8, show that enormous quantities of rock have been removed by denudation.

Fig. 26. The Geological Structure of the American Districts between the Atlantic and the Mississippi, as described by Sir

[blocks in formation]

BIOGRAPHY.-No. XIX.

ROBERTSON.

BY J. R. BEARD, D.D.

ROBERTSON forms one of the trio-Hume, Robertson and Gibbon-who together constitute the pedestal of English Historical Writing. Before them there was, strictly speaking, no history in the vernacular tongue of Great Britain. By profound research, by minute scrutiny, by accurate investigation, by careful comparison and exact representation, as well as by tracing events to their causes and exhibiting facts in their consequences,-in a word, by introducing science and

system into the subject, and by spreading over it the soft and attractive light of a correct and elegant style, they converted the Chronicle into History, and carried History so far forward to perfection as to recall the ancient models offered in Tacitus, Thucydides and Xenophon, thereby forming a school the pupils in which may never surpass their masters, but must ever remain very largely in their debt.

William Robertson was born in the year 1721, at Borthwick, County of Mid-Lothian, Scotland, where his father was the parish minister. His first educational discipline he received from Leslie, a then celebrated teacher at Dalkeith. When his father in 1733 was promoted to a church in Edinburgh, he took his son with him, and sent him to the Metropolitan High School, where the boy found eminent instructors. On

« ΠροηγούμενηΣυνέχεια »