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.

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 c, 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. 140, 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.ABCD being a rectangular current, moveable about a vertical axis K H, fig. 443, and PQ a horizontal current that is fixed Fig. 444.

Fig. 443.

Ki

B

The current being then in the same direction in the columns nd in the moveable part of the circuit, attraction is proved o exist, for the moveable circuit always returns to the plane f the axes of the columns directly after it is removed from it. Laws of angular currents.-1. Two rectilinear currents, the "irections of which form an angle, attract each other when they oth approach or recede from the vertex.

2. They repel each other, if one approaches the vertex while the ther recedes from it.

ame direction.

These laws are demonstrated by passing the current simulaneously along two horizontal wires near to each other, the ne fixed and the other moveable, and the two being inclined t an angle. This angle then decreases or increases, according s the two currents have the same or a contrary direction in elation to the vertex or point of intersection, that is to say, he two currents always tend to become parallel and in the Ampère inferred from the second of the above laws that an ngular current tends to become straight, and that in a rectiliear current, each element of the current repels the next one and is pelled by it. The demonstration of this principle is usually ttempted by showing that, when the current passes from a ercury bath along a thin copper wire which rests upon the rface of the mercury, this wire is repelled; but the resistnce resulting from the change of the conductor may suffice to roduce the phenomenon.

Law of curvilinear currents.-The action of a curvilinear current the same as that of a rectilinear current equal in length to the

C

and indefinite, the fixed current tends to bring the moveable current into a position parallel to P Q, 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 A DCB, that the portion PH of the fixed current acts by attraction upon the parts AD 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 C D the current would be in the same direction as the current a 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. Let two currents be placed in a horizontal plane, the one ABC, fig. 445, fixed

Fig. 445. B

ojection of the curve. This principle is demon ranging a current mno, half curved

ear a move

or repul

and circular, the other mn rectilinear and m the centre n. These currents, being in the s ach other in the singe

the arrows

go towar

they rep the oth the w

x. In the se

for the me m he tw

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 concep tion 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.

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 to General Sinclair on his expedition to the French coast and his embassy to Vienna and Turin, after having been foiled in his attempt to gain the chair of Moral Philosophy in the Edinburgh University. In 1751, after his return to Scotland, Hume put forth his "Inquiry concerning the Principles of Morals." There also appeared from his pen, "Political Discourses" (1752); a collection of "Essays and Treatises on several Subjects" (1755); and the "Natural History of Religion," written in that sceptical spirit which marks too much of what fell from his pen, and which characterises the age. His appointment, in 1752, as superintendent of the Advocates' Library in Edinburgh, led to those investigations and studies which issued in his great historical compositions. He first wrote (1754-6) the "History of England under the Stuarts,' in which he unconsciously gave a proof how a well-read man may misunderstand and misinterpret facts, and by his colourings and perversions there affords a warning against undue reliance on the statements and views which he puts forward on religious and moral topics. Following up this portion with

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 (!) 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 inhabitants 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 inhabitants 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.

"The religion of the Britons was one of the most considerable parts of their government; and the Druids, who were their priests, possessed great authority among them, Besides ministering at the altar, and directing all religious duties, they presided over the education of youth; they enjoyed an immunity from wars and taxes; they possessed both the civil and criminal jurisdiction; they decided all controversies among states as well as among private persons, and whoever refused to submit to their decree was exposed to the most severe penalties. The sentence of excommunication was pronounced against him; he was forbidden access to the sacrifices or public worship; he was debarred all intercourse with his fellow, citizens, even in the common affairs of life; his company was universally shunned, as profane and dangerous; he was refused the protection of law; and death itself became an acceptable relief from the misery and infamy to which he was exposed. Thus the bands of government, which were naturally loose among that rude and turbulent people, were happily corroborated by the terrors of their superstition.

"No species of superstition was ever more terrible than that