others are sea shells, such as Nautilus, Spirifer, Orthoceras, etc. 3. The sea tribes of shells are in a great measure confined to the Mountain Limestone, which forms the basis of the coal formation. This rock, as has been intimated, is frequently crammed with corals, often of a large size, and allied to the class which are forming coral reefs in our present seas. Among the bivalve sea shells, the greatest portion are referred to the two extinct genera called the Spirifer and the Productus. The Pecten, or scallop, appears for the first time in great numbers, presenting about seventy different species. It contains also many fossil univalves related to existing genera, such as the Turritella Buccinum; but the most common univalve in it is the extinct genus Euomphalus, that coils its shell around itself like the ammonite. One remarkable fossil in this rock is the Bellerophon, a shell that is not found in any group that is newer than the coal formation. III. COAL REPTILES. 1. Geologists had examined the different beds of the coal system, for about half a century, without discovering that any animal higher than a fish, or any creature that could live in the air, had existed at the coal epoch. In all that time no animal with a backbone, except a fish-no creature bringing forth its young alive-no frogs, no tortoises, no snakes, no lizards-nothing that could breathe air, except a few insects and two species of beetles, had been discovered in rocks so ancient as those of the coal. The first clue to the probable existence of air-breathing animals was furnished by a singular tooth, found in the cannel coal of the Fifeshire coal-fields. The animal to whom this tooth belonged seemed to have been a true fish, but its tooth indicated that some parts of its organisation were higher than those of a mere fish. 2. In 1844, the upper part of the skeleton of a true reptile was discovered in the coal-fields of Rhenish Bavaria, which was called Apateon pedestris. In 1847, the coal-works of Saarbrück, near Strasburg, furnished the skeletons of three distinct species of reptiles, which were classified under one genus called Archegosaurus. There can be no doubt that these reptiles belonged to the coal period, for the plants and the fish found in the same strata were those of the true carboniferous epoch. Fig. 20. The upright Stems of Equiseta, or Calamites, in the Coal Measures at Treuil, by St. Etienne, near Lyons. A, Horizontal beds of sandstone, traversed by erect trunks of tree-like bamboos or Equiseta. B, Coal shales with impressions of plants. c, Shales with nodules of clay-iron-stone. D, A layer of sandstone. ON PHYSICS, OR NATURAL PHILOSOPHY. No. LXVI. (Continued from page 603.) ELECTRO-DYNAMICS. Mutual Action of Electric Currents.-In a previous lesson we gave a brief summary of the principles established by Ampère in reference to electro-dynamical attraction and repulsion. We now propose to enter into a more detailed consideration of some of them. When two metallic wires near together are traversed by one electric current at the same time, certain attractions and repulsions between the wires are produced, analogous to those in operation between the poles of two magnets. These phenomena, which were discovered by Ampère soon after Ersted's discovery already referred to, constitute a branch of dynamical electricity known by the name of electro-dynamics. laws which regulate them present different cases, according as the currents are parallel or angular, rectilinear or curvilinear. Laws of parallel currents.-1. Two parallel currents in the same direction attract each other. The 2. Two parallel currents in contrary directions repel each other. To demonstrate these laws, divide the circuit through which the current passes into two parts, the one fixed and the From this point commences the moveable part of the circuit, which consists of a red copper wire, one end of which rests by means of a pivot upon the capsule B, and the other dips into a second capsule o, from which the current rises in the column on the right, which is connected at the top with the negative electrode of the pile. The arrangement of the arrows shows that the current proceeds in contrary directions in the columns and in the moveable circuit. Now this latter, which is carefully placed in the plane of the axes of the columns before the passage of the current, recedes from it by turning on its pivot B, as soon as the current passes, which proves the second law. To demonstrate the first, remove the moveable circuit in fig. 440, and substitute in its stead that represented in fig. 441. Fig. 441. of the curved portion m n is in equilibrium with the rectilinear portion no. When we come to speak of electro-magnetism, we shall see that the earth has a directive action upon currents; but we shall also see that, by their form, the currents represented in figs. 440, 441 and 442 are withdrawn from this action. Directive Action of an Indefinite Current upon a Finite one.A B C D being a rectangular current, moveable about a vertical axis K H, fig. 443, and rQ a horizontal current that is fixed Fig. 444. Fig. 443. B The current being then in the same direction in the columns and in the moveable part of the circuit, attraction is proved to exist, for the moveable circuit always returns to the plane of the axes of the columns directly after it is removed from it. Laws of angular currents.-1. Two rectilinear currents, the directions of which form an angle, attract each other when they both approach or recede from the vertex. 2. They repel each other, if one approaches the vertex while the other recedes from it. These laws are demonstrated by passing the current simultaneously along two horizontal wires near to each other, the one fixed and the other moveable, and the two being inclined at an angle. This angle then decreases or increases, according as the two currents have the same or a contrary direction in relation to the vertex or point of intersection, that is to say, the two currents always tend to become parallel and in the same direction. Ampère inferred from the second of the above laws that an angular current tends to become straight, and that in a rectilinear current, each element of the current repels the next one and is repelled by it. The demonstration of this principle is usually attempted by showing that, when the current passes from a mercury bath along a thin copper wire which rests upon the surface of the mercury, this wire is repelled; but the resistance resulting from the change of the conductor may suffice to produce the phenomenon. Law of curvilinear currents.—The action of a curvilinear current is the same as that of a rectilinear current equal in length to the Fig. 442. and indefinite, the fixed current tends to bring the moveable current into a position parallel to PQ, and such that, along the wires D and PQ, the direction is the same. This principle, which may be demonstrated by experiment, is a consequence of the two principles stated above in reference to angular currents. It is easy to see, in fact, from the direction of the currents along the wires PQ and AD CB, that the portion PH of the fixed current acts by attraction upon the parts A D and D H of the moveable current, for both the currents move so as to approach each other. The portion HQ also acts in the same way upon the parts BC and Hc of the moveable currents, for both the currents move so as to separate from each other. The moveable current, therefore, tends to come into a plane parallel to PQ, in such a manner that in CD and PQ the currents may be in the same direction. If, when the moveable current went always in the direction of the arrows, the fixed current proceeded from a to P, all the above attractions would be changed into repulsions, and the moveable current, after having performed a semi-revolution, would still come into a plane parallel to PQ, so that in the part CD the current would be in the same direction as the current Q P. The same principles apply to the circular current EF, fig. 444. Rotation of Currents by means of Currents.-The attraction and repulsion going on between angular currents may easily be transformed into a circular movement. be placed in a horizontal plane, the one ABC, fig. 445, red Fig. 445. B Let two currents projection of the curve.-This principle is demonstrated by arranging a current mno, half curved and half rectilinear, near a moveable current ABCD, fig. 442. Neither attraction nor repulsion is then observable, which shows that the action and circular, the other mn rectilinear and moveable about the centre n. These currents, being in the same direction as the arrows, attract each other in the angle nc, for they both go towards the vertex. In the angle na B, on the contrary, they repel each other, for the one goes towards the vertex and the other from it. The two effects, therefore, concur to turn the wire nm constantly in the direction AC B. Several apparatus have been devised for producing the rotation of a current by means of a current. That represented in fig. 446, consists of a glass vessel, round which is coiled a copper wire covered with silk, and traversed by a fixed current. In the centre of this vessel is a copper column B, terminated by a small capsule containing mercury. Into this mercury is plunged an iron pivot, which supports a red copper wire DC, bent vertically in the direction of the branches DK and oн. A and B of the apparatus represented in fig. 449; the electro-magnets with electro-dynamical cylinders, as Ampère has done dynamical linder is then perfectly moveable about a vertical axis, and we direct a rectilinear current below it, parallel to its axis, and which passes at the same time through the cylinder, we shall see the cylinder turn and come into a position at right angles with the current, that is to say, in a position such that its circuits are parallel to the fixed current, and further, in the lower part of each of the circuits the current is in the same direction as in the rectilinear wire. the south pole always being on the left. To demonstrate this principle, arrange the experiment as shown in fig. 449. The circuit through which the current passes is moveable, and underneath its lower branch a strongly magnetised bar is brought near. The circuit immediately begins to turn, and stops after some oscillations in a plane perpendicular to the magnet, so that the south pole is on the left of the current in the lower part of the circuit. The rotatory motion of a moveable current about a magnet is easily and beautifully shown by means of mercury. If the conducting wire from one of the poles of a battery terminate in a circular copper ring immersed in a shallow basin of mercury, and the end of the wire from the other pole be dipped into the mercury at a point near the centre of the ring; then the convergent or divergent currents between the ring and the centre will give rise to rapid rotation of the mercury, whenever the pole of the magnet is brought near to that centre, either above or below the basin. BIOGRAPHY.-No. XVII. DAVID HUME. BY J. R. BEARD, D.D. How much style may do to make an author immortal, is seen in the history of David Hume. The chief monument of his fame, his "History of England," is now known to labour under serious defects and numerous inaccuracies; his conception of the nature of his task falls far short of perfection, and the execution of his plan is rather popular than philosophical. Yet does his work retain its place on our shelves, and, what is more important, it is often found open on our tables. To live, an author must be read. Hume is read notwithstanding his shortcomings. There is a charm in his narratives which makes his volumes favourites. You can hardly open one of them but you fall on some passage which detains your eye and repays your attention. Far less of a scholar than Johnson, and with a lower position on the ladder of literature, Hume is far more read and has a far better prospect of a living immortality. a "History of the House of Tudor" (1759), he published a view of our history (1761) in its earlier periods. In 1763 the entire work appeared as a "History of England from the Invasion of Julius Cæsar to the Revolution of 1688." In the year 1763, Hume, now a great name, accompanied the Earl of Hertford as Secretary of Embassy to Paris. On his return he was honoured (1) with the appointment of Under-Secretary of State (1767). This office he soon laid down: the shadows of evening were gathering around him. He died in Edinburgh, Aug. 25th, 1776. After his death his interesting autobiography made its appearance (1777), as well as "Dialogues concerning Natural Religion" (1779). See John Hill Burton's "Life and Correspondence of David Hume" (Edinburgh, 1842, 2 vols. 8vo). The following will afford some idea of Hume's ability and manner. THE ANCIENT BRITONS. "All ancient writers agree in representing the first inhabi tants of Britain as a tribe of the Gauls or Celta, who peopled that island from the neighbouring continent. Their language was the same; their manners, their government, their superstition, varied only by those small differences, which time or a communication with the bordering nations must necessarily introduce. The inhabitants of Gaul, especially in those parts which lie contiguous to Italy, had acquired, from a commerce with their southern neighbours, some refinement in the arts which gradually diffused themselves northwards, and spread but a very faint light over this island. The Greek and Roman navigators or merchants (for there were scarcely any other travellers in those ages) brought back the most shocking accounts of the ferocity of the people, which they magnified as usual, in order to excite the admiration of their countrymen. The south-east parts, however, of Britain, had already, before the age of Caesar, made the first and most requsite step towards a civil settlement; and the Britons, by tillage and agriculture, had there increased to a great multitude. The other inhabi tants of the island still maintained themselves by pasture: they were clothed with skins of beasts. They dwelt in huts, which they reared in the forests and marshes, with which the country was covered; they shifted easily their habitation, when actuated either by the hopes of plunder, or the fear of an enemy; the convenience of feeding their cattle was even a sufficient motive for removing their seats; and as they were ignorant of all the refinements of life, their wants and their possessions were equally scanty and limited. "The Britons were divided into many small nations or tribes; and being a military people, whose sole property was their arms and their cattle, it was impossible, after they had acquired a relish for liberty, for their princes or chieftains to establish any despotic authority over them. Their governments though monarchical, were free, as well as those of all the Celtic nations; and the common people seem even to have enjoyed more liberty among them than among the nations of Gaul, from whom they were descended. Each state was divided into factions within itself: it was agitated with jealousy or animosity against the neighbouring states: and while the arts of peace were yet unknown, wars were the chief occupation and formed the chief object of ambition among the people. David Hume, born in Edinburgh the 26th of April, 1711, was the younger son of a Scottish laird, belonging to the family of Home. At an early period he felt himself drawn to classical literature and philosophy, yet his health, injured by severe study, and the narrow means of his family, occasioned his becoming a learner in "the art and mystery" of commercial life, which he studied in the then flourishing city of Bristol. Soon weary of a pursuit for which he had as little taste as aptitude, he returned to his native city, where he resumed his academical studies; on the termination of which he went to France. There, while pursuing his self-culture, he wrote a critical and psychological essay, the "Treatise upon Human Nature" (3 vols. London, 1738-40), the publication of which was followed by his "Essays, Moral, Political, and Literary" (Edinburgh, 1742). In 1745 he took on himself the delicate office of tutor to the young Marquis of Annandale, whose mind was none of the strongest, and became secretary "The religion of the Britons was one of the most considerable to General Sinclair on his expedition to the French coast and parts of their government; and the Druids, who were their his embassy to Vienna and Turin, after having been foiled in priests, possessed great authority among them. Besides minis his attempt to gain the chair of Moral Philosophy in the Edin- tering at the altar, and directing all religious duties, they burgh University. In 1751, after his return to Scotland, presided over the education of youth; they enjoyed an immuHume put forth his "Inquiry concerning the Principles of nity from wars and taxes; they possessed both the civil Morals. There also appeared from his pen, "Political and criminal jurisdiction; they decided all controversies Discourses" (1752); a collection of "Essays and Treatises on among states as well as among private persons, and whoever several Subjects" (1755); and the "Natural History of Reli- refused to submit to their decree was exposed to the most severe gion," written in that sceptical spirit which marks too much penalties. The sentence of excommunication was pronounced of what fell from his pen, and which characterises the age. against him; he was forbidden access to the sacrifices or public His appointment, in 1752, as superintendent of the Advocates' worship; he was debarred all intercourse with his fellowLibrary in Edinburgh, led to those investigations and studies citizens, even in the common affairs of life; his company was which issued in his great historical compositions. He first universally shunned, as profane and dangerous; he was refused wrote (1754-6) the "History of England under the Stuarts," the protection of law; and death itself became an acceptable in which he unconsciously gave a proof how a well-read man relief from the misery and infamy to which he was exposed. may misunderstand and misinterpret facts, and by his colour-Thus the bands of government, which were naturally loose ings and perversions there affords a warning against undue among that rude and turbulent people, were happily corroboreliance on the statements and views which he puts forward rated by the terrors of their superstition. on religious and moral topics. Following up this portion with "No species of superstition was ever more terrible than that of the Druids. Besides the severe penalities which it was in the power of the ecclesiastics to inflict in this world, they inculcated the eternal transmigration of souls, and thereby extended their authority as far as the fears of their timorous votaries. They practised their rites in dark groves or other secret recesses; and in order to throw a greater mystery over their religion, they communicated their doctrines only to the initiated, and strictly forbade the committing of them to writing, lest they should at any time be exposed to the examination of the profane vulgar. Human sacrifices were practised among them; the spoils of war were often devoted to their divinities; and they punished with the severest tortures whoever dared to secrete any part of the consecrated offering. These treasures they kept in woods and forests, secured by no other guard than the terrors of their religion; and this steady conquest over human avidity may be regarded as more signal than their prompting men to the most extraordinary and most violent efforts. No idolatrous worship ever attained such an ascendant over mankind as that of the ancient Gauls and Britons; and the Romans, after their conquest, finding it impossible to reconcile those nations to the laws and institutions of their masters, while it maintained its authority, were at last obliged to abolish it by penal statutes; a violence which had never, in any other instance, been practised by those tolerating conquerors." CHARACTER OF ELIZABETH. "There are few great personages in history who have been more exposed to the calumny of enemies, and the adulation of friends, than Queen Elizabeth; and yet there is scarce any whose reputation has been more certainly determined by the unanimous consent of posterity. The unusual length of her administration, and the strong features of her character, were able to overcome all prejudices; and, obliging her detractors to abate much of their invectives, and her admirers somewhat of their panegyrics, have at last, in spite of political factions, and, what is more, of religious animosities, produced an uniform judgment with regard to her conduct. Her vigour, her constancy, her magnanimity, her penetration and vigilance, are allowed to merit the highest praise, and appear not to have been surpassed by any person who ever filled a throne. A conduct less vigorous, less imperious, more sincere, more indulgent to her people, would have been requisite to form a perfect character. By the force of her mind she controlled all her more active and stronger qualities, and prevented them from running into excess. Her heroism was exempt from all temerity, her frugality from avarice, her friendship from partiality, her active spirit from turbulency and a vain ambition. She guarded not herself with equal care, or equal success, from lesser infirmities; the rivalship of beauty, the desire of admiration, the jealousy of love, and the sallies of anger. Her singular talents for government were founded equally on her temper and on her capacity. Endowed with a great command of herself, she obtained an uncontrolled ascendant over her people; and, while she merited all their esteem by her real virtues, she also engaged their affections by her pretended ones. Few sovereigns of England succeeded to the throne in more difficult circumstances; and none ever conducted the government with such uniform success and felicity. Though unacquainted with the practice of toleration (the true secret for managing religious factions), she preserved her people, by her superior prudence, from those confusions in which theological controversy had involved all the neighbouring nations; and though her enemies were the most powerful princes in Europe, the most active, the most enterprising, the least scrupulous, she was able by her vigour to make deep impressions on their state; her own greatness meanwhile remaining untouched and unimpaired. "The wise ministers and brave warriors who flourished during her reign share the praise of her success; but instead of lessening the applause due to her, they make great addition to it. They owed, all of them, their advancement to her choice; they were supported by her constancy; and, with all their ability, they were never able to acquire any undue ascendant over her. In her family, in her court, in her kingdom, she remained equally mistress. The force of the tender passions was great over her, but the force of her mind was still superior; and the combat, which her victory visibly cost her, serves only to display the firmness of her resolution and the loftiness of her ambitious sentiments. "The fame of this princess, though it has surmounted the prejudices both of faction and bigotry, yet lies still exposed to another prejudice which is more durable, because more natural, and which, according to the different views in which we survey her, is capable of either exalting beyond measure or diminishing the lustre of her character. This prejudice is founded in consideration of her sex. When we contemplate her as a woman, we are apt to be struck with the highest admiration of her great qualities and extensive capacity; but we are apt also to require some more soitness of disposition, some greater lenity of temper, some of those amiable weaknesses by which her sex are distinguished. But the true measure of estimating her merit is, to lay aside all these considerations, and consider her merely as a rational being, placed in authority, and entrusted with the government of mankind. We may find it difficult to reconcile our fancy to her as a wife or a mistress; but her qualities as a sovereign, though with some considerable exceptions, are the object of undisputed applause and approbation." LESSONS IN ALGEBRA.-No. XXVIII. A VARIETY of other formulæ may be deduced from the equations in our last lesson, the investigation of which will afford the student a pleasing and profitable exercise. arithmetical means, between two given numbers. For the By the third formula, e.g., may be found any number of whole number of terms consists of the two extremes and all the intermediate terms. If then m = number of means, m+2=n, the whole number of terms. Substituting m + 2 for n in the third equation, we have, Prob 5. Find 6 arithmetical means, between 1 and 43. Ans. {The series, 1, 7, 13, 19, 25, 31, 37, 43. It is obvious from the mode in which we obtained an expression for the sum of an arithmetical series, that the sum of the extremes is equal to the sum of any other two terms equally distant from the extremes. Thus, in the series 3, 5, 7, 9, 11, the sum of the first and last terms, of the first but one and last but one, etc., is the same in each case, viz. 14. The same is true of every series. Prob. 6. If the first term of an increasing arithmetical series is 3, the common difference 2, and the number of terms 20, what is the sum of the series? Prob. 7. If 100 stones are placed in a straight line, at the distance of a yard from each other, how far must a person travel, to bring them one by one to a box placed at the distance of a yard from the first stone? Prob. 8. What is the sum of 150 terms of the series Prob. 12. A gentleman bought 47 books, and gave 10 shillings for the first, 30 shillings for the second, 50 shillings for the third, etc.: what did he give for the whole? Prob. 13. A person put into a charity-box a shilling the first day of the year, two shillings the second day, three shillings the third day, etc., to the end of the year: what was the whole sum for 365 days? |