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Galvanometer, Electro-Magnetic Multiplier. It is evident that, connected together by means of a copper wire, like those in fig. if a number of similar currents act in like manner upon a given 371, p. 457, and cannot be deflected separately, or one without current or magnet, the sum of the forces which they exert the other, cannot have exactly the same magnetic intensity will be directly proportional to their number. It also appears unless every current, whether strong or feeble, renders them from the four principles stated above, that any two currents always transverse to itself. flowing in opposite directions should act alike upon a magnet The bent branches K and I which are connected, below the placed between them. Upon these principles the electro-mag- apparatus, with the two ends of the circuit, are intended to netic multiplier, or galvanometer, of Schweigger, a German receive the conductors which transmit the current that we wish philosopher, is constructed. It is an extremely delicate appa- to observe. The foot screws C, serve to place the apparatus in ratus, serving to show the existence, direction and intensity a perfectly vertical position, so that the suspended thread of currents. It was invented by Schweigger shortly after the corresponds exactly to the centre of the dial. Lastly, a button discovery of Ersted.
E transmits the motion to the plate D and the dial—both of To understand the principle of it, let us consider the case of which are moveable about a vertical axis, in such a manner as a magnetised needle suspended by a silk untwisted thread, fig. to bring the wires of the circuit in the direction of the mage 433, and surrounded, in the plane of the magnetic meridian, with Fig. 433 Fig: 434.
a copper wire forming a complete circuit round the needle in the direction of its length. When a current passes along this wire, it follows, from the convention which we have already stated, that in every part of the circuit an observer lying along the wire in the direction of the arrows, and looking at the needle ab, would have his left turned towards the same point of the horizon, and, consequently, that the action of the current everywhere tends to deflect the needle in the same direction. In other words, the actions of the four branches of the circuit contribute to deflect the south pole in the same direction ; therefore, by winding the wire round the needle as the figure represents, we have multiplied the action of the current. If, instead of a single circuit, we have several, the action is multiplied still further, and the deflection of the needle is increased still more. However, we should not multiply the action of the current indefinitely by continuing the circumvolutions of the wire, for we shall soon see that the intensity of a current becomes feeble when the length of the circuit through which it passes increases.
As the directive influence of the earth tends incessantly to keep the needle in the magnetic meridian, and thus opposes the action of the current, the effect of the latter may be netic meridian without displacing the apparatus. The galvarendered more perceptible by making use of a system of two nometer represented in our illustration is the work of M. astatic needles, as seen in fig. 434. The action of the earth Billant, an ingenious manufacturer of such articles. upon the needles is then very feeble, and besides the actions When the galvanometer is intended for observing currents of the current on the two needles are added together. In fact, arising from chemical action, the wire of the circuit, as we shall the action of the complete circuit tends, according to the soon see, ought to be of a small diameter, and to make from direction of the current marked by the arrows, to deflect the four hundred to six hundred revolutions, or even more. On the south pole of the inner needle ab towards the west. With contrary, for thermo-electrical currents, of which we shall speak regard to the upper needle a' b', it is subject to the action of two on a future occasion, the wire ought to be of large diameter, contrary currents m n and pq, but the former being nearer, its and to make a much smaller number of revolutions. action prevails over the other. Now this current passing Thus constructed, the galvanometer gives no indication of below the
needle from the south to the north pole, evidently any current when we pass the electricity of an electrical tends to defect the pole a' towards the east, and consequently machine along the wire, by connecting one of the ends with the pole b' towards the west, that is to say, in the same direc- the conductors and the other with the earth. We can only tion as the pole a of the other needle.
render the current which then passes along in the apparatus These principles having been premised, it is easy to explain perceptible, by making use of a very fine wire coiled round the theory of the multiplier. This apparatus, represented in two or three thousand times, and by completely isolating the fig. 435, is composed of a cylindrical copper case, round which circuits from one another by means of silk, varnish and gun is rolled a copper wire covered with silk along its whole length, lac. If these conditions are observed, the needles will be to isolate the circuits from one another. Above this case is a deflected by the electricity of an electrical machine, which horizontal dial-plate graduated, the zero point of which corres- shows the identity of statical and dynamical electricity. ponds to the diameter parallel to the direction of the copper Graduation of the Galvanometer. -The galvanometer which wire upon the dial. This dial has two graduations, the one we have just described is an extremely delicate apparatus on the right of the zero point, and the other on the left, but for establishing the fact of the presence of currents, but it does only as far as 90 degrees. By means of a support and an not make known their intensity. To make it serve this extremely fine silk fibre, an astatic system is suspended, con- purpose, it is necessary to construct tables, by means of which sisting of two sewing needles ab and A, the former above the the intensity of the current may be deduced from the deflection dial, and the latter within the circuit. These needles, which are l of the needle.
The simplest method of forming these tables is that of the in which the needle is magnetised, its length, its distance from double wire multiplier. Coil two copper wires at the same time the current, and, lastly, with the extent of the circuit. round the apparatus, each being covered with silk, and both of The double wire multiplier may also serve to establish the the same length and diameter. Then selecting a constant, difference of the intensities of two currents. This is effected but very feeble, source of dynamical electricity, pass the by passing a current along each wire at the same time, but in current along one of the wires, thus causing a certain deflec- contrary directions. The apparatus then bears the name of tion, say of five degrees. Then by means of an electric source the differential galvanometer. identical with the above, pass a current of the same intensity Uses of the the Galvanometer.-The galvanometer, on account along both wires at the same time, thus obtaining a deflection of its excreme sensibility, is one of the most valuable philoof ten degrees, owing to the simultaneous action of the two sophical instruments. It serves not only to establish the fact currents, or—which is the same thing-to a current of twice of the presence of the feeblest currents, but to make known the intensity of the first. If you then pass along one of the their direction and intensity. It was by means of this appawires a current capable of producing by itself a deflection of ratus that M. Becqueral was able to prove that a disengageten degrees, and along the other one of the currents which ment of electricity accompanies all chemical combinations, and gave a deflection of five degrees—which is evidently the same to determine the laws which regulate these combinations. thing as tripling the first current-you will obtain a deflection For example, if you fasten two platinum wires to the of fifteen degrees. Lastly, by passing along each of the wires extremities of the circuit of the galvanometer, and plunge at the same time a current capable of producing a deflection of them into a small vessel containing nitric acid, you discover no ten degrees, you may get a deflection of twenty degrees. In deflection of the needle, as might have been easily foreseen, other words, up to twenty degrees the deflections increase in pro- because the platinum is not acted upon by nitric acid. But if portion to the intensity of the current. Beyond that point they you pour a drop of hydro-chloric, chloro-hydric, or muriatic increase less rapidly; but by the same process you may acid, near one of the submerged wires, the needle of the gradually determine from point to point the deflections which galvanometer is immediately detected, which proves that there correspond to known intensities, and complete the table by the is a current in the circuit. In fact, we know that nitric and method of interpolations. Eacli galvanometer requires a table chloro-hydric acid by their mutual reaction produce chloroof its own, because the relation between the intensity of the nitric acid, which acts upon platinum. We gather also from current and the deflection of the needle varies with the degree the direction of the deflection, that the platinum is electrised.
negatively and the acid positively,
The series of rocks which contain coal are divided into three LESSONS IN GEOLOG Y.-No. LVIII. classes.
I. THE COAL MEASURES.
II. THE MILLSTONE GRIT.
III. THE MOUNTAIN LIMESTONE,
I. THE COAL MEASURES.
1. The rocks which are specially called Coal Measures, consist of deep beds of gritty sandstones, layers of soft and
pitchy clays called, on account of their being laminated, shales, ON THE COAL FORMATION.
seams of coal in an almost crystalline state, bands of clay
rocks containing iron-stone, and alternating with sand rocks. ♡ I. THE LITHOLOGICAL CHARACTER OF THE COAL The uppermost beds of the coal measures generally consist of ROCKS
gritty stones which give very little indication of coal. These
grits are sometimes three thousand feet thick, are for the most The rocks which embrace in one group mineral coal, iron ore part destitute of fossils, and are generally called “the upper and limestone, are the most important in the whole world. coal grits.”. They are represented in fig. 14. By opening up sources of industry and wealth, and by calling 2. Coal is a pitchy mass of vegetable matter mineralised. forth the energy and skill of a people, the carboniferous rocks The vegetable matter is the decomposed remains of cone-beartend to confer upon a country far greater power than it could ing trees, gigantic ferns and club mosses, with sometimes. derive from veins of silver or valleys of gold dust. Coal, iron the branches, and even the bodies, of entire trees of immense and limestone have made England what it is.
size. The coal beds are found in seams, separated from each The whole series of roeks which contain sand rocks, clay other by layers of clayey shales, beds of hardened clay and shales, seams of coal, beds of grits and bands of limestone, thick masses of sand rock. The position of the seams of coal take their name from the coal only, and are therefore called in what are called Coal Basins is represented in the accomthe carboniferous system. This is somewhat remarkable, and panying engraving, fig. 15. must be taken as an indication of the great worth of coal, for 3. Every seam of coal has an under-bed of sandy shale and if all the seams of coal found in the group were measured, their a roof of slaty clay, or clay shale. united thickness would not be 150 feet, while the depth of The Floor, or the bed on which a seam rests, is called the the entire series, from the bottom of the Permian to the upper underclay, which is always co-extensive with every seam of surface of the Old Red Sandstone, is more than twelve coal. It consists of a sandy or gritty shale, which is somethousand feet.
times called firestone, or rather firo-clay, on account of its
capacity to be formed into bricks, which will stand the fiercest carbonised vegetable matter-occasionally in the proportion heat of the furnaces. These beds of underclay vary in thick- of ninety, per cent.--and also of a small proportion of earthy ness from a few inches to more than ten feet. Sometimes matter. It burns without either luminous flame or dense smoke. they are but very slightly coloured with coal matter, at others Anthracite is only common or black coal that lies in deeper Fig. 15. An Ideal Section of a Basin of Coal Measures.
seams in the earth, and that by its contiguity to some heated volcanic rocks has lost its pitch.
Culm, is coal lying nearer to volcanic rocks than Anthracite coal--80 near as not only to be charred by the heat, but also crushed and shivered by the pressure.
Sixthly. STEAM COAL.--Coal used to be divided into the bituminous and the anthracite, but lately a third kind has been discovered in the neighbourhood of Swansea, South Wales, an intermediate one called “Steam Coal," on account of its peculiar fitness for the engines of steam-packets. It is extremely compact, and hard, burns without any visible smoke, and contains so little pitch as not in the least to be
liable to spontaneous combustion. A. Seams of coal, shale, and sandstone.
Seventhly. BLACKLEAD.-Blacklead is more properly called B. The millstone grit.
Plumbago or Graphite. It ought never to have been called c. The mountain limestone.
blacklead, for though in drawing pencils, etc., it marks like they are completely blackened with it. Though this under- lead, it has not a particle of lead in its composition, clay always forms a poor for the coal seams, yet there are than culm or anthracite, by which processes it has become
Plumbago is coal that has undergone more chemical changes of this tough band are exposed to the atmosphere they become more mineralised. In many instances, when coal is found
contiguous to igneous rocks, it is converted into Plumbago, & gray friahle earth. This underclay bed contains, almost invariably, an abun- or Graphite, a name derived from the Greek ypaqw, grapho,
to write. dance of the fossil vegetable termed Stigmaria, which were
The manner, in which graphite is formed from coal may be once supposed to be aquatic plants, but are now found to be the roots of some of the plants, whose decomposition and in some measure understood from the following statement of mineralisation formed the seams of coal.
the different relations of gases. Hydrogen is the most abunThe Roop of the coal seams is generally a slaty clay called dant in fuid pitch, and pitch and carbon abound in coal. In shale. It seems to consist of the water worn detritus of other anthracite the pitch is wanting ; but in graphite both the pitch rocks. It abounds with the leaves, branches and fruits of and the hydrogen are absent, and nothing but the carton
remains, plants. It contains also layers and nodules of ironstone, which enclose leaves, insects and the remains of animals allied diamond is involved in great obscurity, but the prevalent
Eighthly. The DIAMOND.--At present the real origin of the to the crab.
In some places, its beds contain fresh-water shells, and in opinion is that it is of vegetable origin. It forms the hardest others, sea shells, and these beds are interstratified with pitchy substance
known to chemists. It seems to be but pure vegetable shales and fine slaty clays, micacious sands, and grits and matter perfectly, carbonised, or pure mineral carbon crystals pebbles of limestone and sandstone.
lised. Diamonds are generally found in the detritus of beds Hence you learn, First, that the underclay was the natural that have been contiguous to igneous rocks, but they are beds soil in which the Stigmaria, the roots of the coal forest, grew. Bundel Kund in India, the diamond is found imbedded in the
that are associated with the carboniferous system. Thus, at Secondly, that the coal is the foliage, branches and trunks of such forest, decomposed during milleniums of ages, and then New Red Sandstone ; for 400 feet of that rock underlay the mineralised. Thirdly, that the Roof or the shaly stratum was lowest diamond beds, and beneath this rock are clear indicaformed by the detritus worn from other rocks, and either tions of coal measures. That diamond is pure coal is evident transported by a flood, or brought by the sea after a submers from the fact that its combustion produces the same results gence that overwhelmed the decomposed forest.
and yields the same elementary residuum as charcoal. Llene, 4. The decomposed vegetable matter which forms coal is however, thinks that diamonds owe their origin, not to it capable of various modifications.
but to decay, and that they have been formed in the bumid First. Peat. When vegetable matter is exposed to a certain way, that is, as pure carbon in a liquid state. amount of moisture, combined with a certain degree of tempera
II, THE MILLSTONE GRIT.
Secondly. LIGNITE. --Lignite seems nohting but a bed of Peat coal measures, are thick strata of sandstones more or lex
divisions of the pebbles of quartz, limestone, and granite, of various site
, of, in some instances, by a finale of the same vegetable or woody matter as lignite, and differs paste. Such a piece, when tooled as a hand specimen
, ba from it only by having been buried in the earth a longer time, the following appearance, fig. 16. during which it has undergone different and greater chemical
Fragments of these beds appear sometimes in large mere modifications, by which it has been mineralised. This is sometimes called "Black Coal," to distinguish it which the rock derives its name of millstone grit.
some of which are dressed and tooled into millstones, free from the lignite, or brown coal. It is more or less combined with earthy particles, which accordingly, after
burning, yield a composed of the fine detritus of other and older sandsted
3. There are also associated with this grit beds of sandstras proportionable quantity of ashes or cinder. The more earthy rocks. In such beds of sandstone are found water-worn there and the remains of such coal, after its distillation, form the which bear the marks of having been transported and deposit mass called coke. Fourthly. ANTHRACITE.-Anthracite coal is called, on account
by currents. of its compactness, “Stone Coal," and on account of its brile system, the general rule is that it is under the coal measure
4. As to the real position of this rock in the carboniera liant hues, Glance Coal." "It consists, like pitch coal, of but yet, in some coal fields, the seams of coal are found imao
stratified in the beds of this grit. In other districts the lowest | imperfect coal measure called culm takes the place of the
been originally a coral reef, for it abounds with bands of
shelly and encrinital remains, and also with fragments of Fig. 16. A small Fragment of Conglomerated Millstone Grit, fishes, tooled on the surface.
Some of its beds are indeed destitute of fossils, but the rock itself is, for the most part, made up of the remains of coral, shells, encrinites, etc., which often form three-fourths of the mass, as may be seen in the different marbles, especially those of Derbyshire.
3. This group of limestone is remarkable for the variety of substances found in it. In some districts it is interstratified with shales, and grits, and amygdaloidal rocks, or volcanic rocks which have cells in them, and which cells are filled with a different substance. In other districts these limestone beds have in them layers and nodules of chert, just as beds of chalk have those of flints.
These rocks are also distinguished for their spars. The fluor spar, which is the celebrated Derbyshire spar, is a fluate of lime; and it occurs among these limestones both in crystals and nodules. The blue John spar is very celebrated, as the best material for making vases and other ornaments. This is found to occur both as veins and also as large and irregular masses.
4. Most of the caverns of the earth occur in this limestone, Wherever it occurs it abounds in deep chasms and fissures, by which it is traversed in all directions, and also in deep subterranean caverns like those of Somersetshire, Derbyshire, and Yorkshire.
In the north-western parts of Yorkshire the mountain limestone is one thousand eight hundred feet thick, and may be divided into two groups. The upper forms the Yoredale Rocks, and the lower is what is called the Scar Limestone. Both of these abound in large natural caverns. Though these two groups of limestone are true mountain limestone, they differ from the mountain limestone of the south of England, by having occasionally thin layers of coal among them, and by being divided into several thin beds by partitions of grit and shale.
5. Many lead mines have been opened in the mountain
limestone, especially in the counties of Somerset, Derby,
III. THE MOUNTAIN LIMESTONE.
By J. R. BEARD, D.D.
his mother, a woman of a tender heart, encouraged the pro2. The mountain limestone is the basis of the coal formation, pensity. In Edinburgh, whither he had gone in order to as it lies between the coal beds and the more ancient rocks, continue his studies, he did not distinguish himself beyond the old red sandstone-or where this group is absent, the other students. Nay, under the impulse of a natural call to Silurians. In the south-western extremity of England an the poetic office, he seems to have undervalued learned pur
of sandstrass it. There web atum is best ck, Out of your
ed of the mic insists of tomt and granite
, is vle, all of website
in some inte soled us lais .6, pear sometimes nd tooled in a se of milkstone with this en 18 of other stone are , and eren con ng been transport
of this rock # dat it is under
THE POPULAR EDUCATOR. suits. However, in compliance with the wishes of his friends, poet; but the unsuitableness was compensated for by an he applied to theological studies. When under examination income of £300 a-year. He might now have lived at his for the pulpit he was required to expound one of the Pealms. ease, but a cold which he took soon ended his days (Aug. The stiff and rigid Scotch theologian pronounced the version 27th, 1748). A tragedy, named "Coriolanus," was published too poetical. The condemnation engendered in Thomson so after his decease. great an aversion to theology, that from that moment he resolved to devote himself to the art for which he felt nature the best. Thomson drew his pictures immediately from nature.
Among Thomson's works, "The Seasons" may be pronounced had created him. At the time there existed in the University Hence their truth, their lively colouring and their varied beauty. a Belles Lettres Club, which called itself “The Athenian He says of himself in his “ Autumn," Society.” This society published a collection of poetry under the title of “The Edinburgh Miscellany.”. To this work
** I solitary court Thomson appears to have contributed, though his pieces have
Th’inspiring breeze, and meditate the book not been identified. Convinced that his talents could find due
Of Nature, ever open ; aiming thence recognition only in London, and in the hope of obtaining some
Warm from the heart to pour the moral song." assistance from a family known to him that lived there, he betook himself to the metropolis in the autumn of 1725. In Seasons,” but his works of that class lack the concentrated
In his dramatic works you recognise the author of "The London,
among other of his college mates, he found David thought and rapid action essential to perfection. His poem Mallet, who filled the post of tutor to a son of the Duke of on "Liberty” has some fine passages, yet, as a whole, must Montrose. To him he showed his “Winter," which then be pronounced heavy. More attractive is “ The Castle of consisted simply of fragments. At Mallet's suggestion he Indolence." finished and published the poem. It appeared in 1726, and had a favourable reception. It is still accounted Thomson's rest.
“The Seasonis" have been often printed apart from the master-piece. The dedication of the poem to Sir Spencer Thomson's collected works appeared first in 1730. See also Compton procured the author only twenty guineas. A larger “Essays on the Life and Writings of Saltoun and the poet reward, however, he found in the reputation
it brought him. Thomson, by the Earl of Buchan," London 1792 ; aikin's Among other advantages it gained him an acquaintance " Essay on the Plan and Character of Thomson's Seasons ;" with Pope. In 1727 he published his * Summer,” dedicated Lives of the poet by Johnson, Anderson, Nicola, etc. We to Lord Meloombe. In the same year he put forth a “Poem present as specimens of this poet passages from Sacred to the Memory of Sir Isaac Newton," and another under the title of “Britannia,” being an invective against the government. Next appeared (1728) "Spring," with a dedica
A PANEGYRIC ON GREAT BRITAIN. tion to the Countess of Hertford, afterwards Duchess of Heavens! what a goodly prospect spreads around, Somerset. This compliment brought him an invitation to Of bulls, and dales, and woods, and lawns, and spires, Lord Hertford's mansion, where he remained some months. And glittering towns, and gliding streams, till all The “ Autumn" did not appear until our poet published an
The stretching landscape into smoke decays ! edition of his works in 1730. The same year Thomson placed
Happy Britannia! Where, the queen of arts, on the stage his tragedy of " Sophonisba.” Having been read
Inspiring vigour, Liberty abroad in private cireles, the play had excited great expectations.
Walks unconfined, even to thy farthest cots,
And scatters plenty with unsparing hand. Disappointment ensued. The line
Rich is thy soul, and merciful thy clime; “Oh, Sophonisba ! Sophonisba, oh!"
Thy streams unfailing in the summer's drought :
Unmatch'd thy guardian-oaks; thy valleys float was parodied thus,
With golden waves, and on thy mountains flocks
Bleat numberless; while, roving round their sides, “Oh, Jemmy Thomson ! Jemmy Thomson, oh!"
Bellow the blackening herds in lusty droves.
Beneath thy meadows glow, and rise unquell'd Not long after, the poet accompanied Charles Talbot, eldest
Against the mower's scythe. On every hand son of the Lord Chancellor, in a tour on the continent, whence
Thy villas shine. Thy country teems with wealth ; he returned home enriched with varied knowledge. He was And property assures it to the swain, rewarded for his care of the young man with an office, which Pleased, and unwearied, in his guarded toil. secured him an income without requiring much of his time.
Full are thy cities with the sons of art; While on his travels he formed the first idea of his poem on And trade with joy, in every busy street, "Liberty," to the composition of which he devoted two years. Mingling are heard; even Drudgery himself, During the time his pupil died. The loss occasioned the dedi- As at the car he sweats, or dusty hews cation to the young man's memory of a few lines at the com
The palace-stone, looks gay. Thy crowded ports, mencement. Thomson set a great value on this production,
Where rising masts an endless prospect yield,
With labour burn and echo to the shouts and expected it would secure unusual acceptance. The hope
Of hurried sailor, as he hearty waves was but partially fulfilled. It consists of five books or cantos, His last adieu, and, loosening every sheet, having these titles : 1, Ancient and Modern Italy compared, Resigns the spreading vessel to the wind. 2. Greece, 3. Rome, 4. Britain, 5. The Prospect. A short time Bold, firm, and graceful, are thy generous youth ; after its publication his patron died, and Thomson lost his By hardship sinew'd and by danger fired, place. In consequence he was thrown on literature as a source Scattering the nations where they go; and first of support. In 1738 his tragedy of “ Agamemnon" was sub- Or on the listed plain, or stormy seas. mitted to the public in Drury-lane Theatre. It obtained no Mild are thy glories too, as o'er the plans very marked success. About this time Thomson became
of thriving peace thy thoughtful sıres preside; acquainted with the Prince of Wales, who, affecting popularity,
In genius, and substantial learning high ; had become a patron of poets and scholars. He gave our poet
For every virtue, every worth renown'd,
Sincere, plain-hearted, hospitable, kind; a pension of £100. In 1731 the prince desired that Thomson's
Yet like the muttering thunder, when provoked; tragedy, “ Edward and Elenora,” should be played, but the
The dread of tyrants, and the sole resource Lord Chamberlain forbad the representation, because the Of those that under grim oppression groan. author was of the party in opposition to the Court. In 1740
Thy sons of glory many! Alfred thine,
And his own muses love; the best of kings ! popularity than any other of his plays. The last of his publi
With him thy Edwards and thy Henrys shine,
Names dear to fame; the first who deep impress'd cations, and perhaps the loveliest, his "Castle of Indolence,”
On haughty Gaul the terror of thy arms, appeared in 1746.. About this time Thomson, through the influence of his friend Mr. Lyttleton, obtained the post of
That awes her genius still. In statesmen thou,
And patriots, fertile. 'l hine a steady More, "Surveyor of the Leeward Islands," a strange office for a Who, with a generous, though mistaken real.