direct opposition to each other, if they have equal quantities of motion, they will upon the stroke lose all their motion, and continue at rest; for, by the proposition, the bodies after impact will be carried with the difference of their motions before the stroke; which difference, in such a case, is nothing.

When two bodies meet with unequal quantities of motion, if the difference of their motions be divided by the sum of the bodies, the quotient will express their common velocity after the stroke; for, by the proposition, the difference of their motions before the stroke is equal to the sum of their motions after the stroke; consequently, that difference divided by the sum of the bodies must give the velocity.

Such are the principal laws which govern the collision of bodies devoid of elasticity. The motions of elastic bodies are determined by different rules: for when they are perfectly elastic, the velocity gained by the body struck, and the velocity lost by the striking body, will be twice as great as if the bodies were perfectly inelastic. In estimating therefore the motions of such bodies, we may first consider what they would have been after impact, had they been inelastic, and thence deduce the desired conclusion. See Helsham's Lectures, a work in which the subject appears to be very clearly treated.

Note 28, p. 185.-DRUIDICAL REMAINS.

Karn-brêh hill rises a little to the south-west of Redruth in Cornwall, to an elevation of 697 feet. Its principal interest is derived from the speculations of the antiquary, Dr. Borlase, who regarded it as having been once the grand centre of druidical worship: and he asserts, in his Antiquities of Cornwall,' that, at this very time, the remains of those monuments which were peculiar to that priesthood may be discovered, such as rock-basins, circles, rock-idols, cromlechs, karns, caves, religious enclosures, logan stones, a gorseddau, or place of elevation, whence the druids pronounced their decrees, and the traces of a grove of oaks. This is all very ingenious and imposing, and may be easily believed by those who have either not visited the spot, or, having visited it, not viewed the objects with geological eyes. There is no ground whatever for considering the druidical monuments of Dr. Borlase as the works of man: on the contrary, they are evidently the results of the operation of time and the elements, the usual agents employed by Nature in the decomposition of mountain masses. The "rock-basins” of antiquaries are rounded cavities on the surface of rocks, and are

occasionally as spheroidal internally as if they had been actually formed by a turning-lathe. It was this artificial appearance which first suggested the hypothesis concerning their origin, and induced the antiquary to regard them as pools of lustration. It may, however, be remarked, in the first place, that, supposing them to have been the works of the druids, these priests must have been indefatigable artists, for there is scarcely a block of granite on which one or more of such pools are not visible, although some are, undoubtedly, much more complete and imposing than others.

Dr. Macculloch, in an interesting paper, published in the Transactions of the Geological Society, on the decomposition of the granite of Cornwall, has justly observed that the true nature of these rock-basins may be easily traced by inspecting the rocks themselves. On examination, they will always be found to contain distinct grains of quartz, and fragments of the other constituent parts of the granite. A small force is sufficient to detach from the sides of these cavities additional fragments, showing that a process of decomposition is still going on under favourable circumstances. The principal of these circumstances is the presence of water, or rather the alternate action of air and water. If a drop of water can only make an effectual lodgment on a surface of this granite, a small cavity is sure to be sooner or later produced; this will insensibly enlarge as it becomes capable of holding more water; and the sides, as they continue to waste, will neces sarily retain an even and rounded cavity, on account of the uniform texture of the rock. This explanation is sufficiently satisfactory in addition to which, it may be stated, that these very basins not unfrequently occur on the perpendicular sides of rocks, as may be distinctly seen in the granite of Scilly, and in the gritstone rocks in the park of the late Sir Joseph Banks, in the parish of Ashover, in Derbyshire; a fact which at once excludes the idea of their artificial origin.

The other grotesque and whimsical appearances of rocky masses, such as rock idols, logan stones, &c., are to be explained by the tendency which granite possesses of wearing more rapidly on the angles and edges than on the sides; thus, then, upon simple and philosophical principles, are such appearances to be satisfactorily accounted for, and the phantasmagoria of Borlase vanishes as the light penetrates the theatre so long dedicated to its exhibition.

We shall conclude this note with a few observations upon the celebrated logan, or logging, stone, near the Land's End, Cornwall, of which we present our readers with a faithful sketch.

The foundation of this part of the coast is a stupendous group of granite rocks, which rise in pyramidal clusters to a great altitude, and overhang the sea. The celebrated logan stone here represented is an immense block weighing above sixty tons. The surface in contact with the under rock is of very small extent, and the whole mass is so nicely balanced, that, notwithstanding its magnitude, the strength of a single man applied to its under edge is sufficient to make it oscillate. It is the nature of granite to disintegrate into rhomboidal and tabular masses, which, by the further operation of air and moisture, gradually lose their solid angles, and approach the spheroidal form. The fact of the upper part of the cliff being more exposed to atmospheric agency than the parts beneath will sufficiently explain why these rounded masses so frequently rest on blocks which still preserve the tabular form; and since such spheroidal blocks must obviously rest in that position in which their lesser axes are perpendicular to the horizon, it is equally evident, that, whenever an adequate force is applied, they must vibrate on their point of support.

Although we are thus led to deny the druidical origin of this stone, for which so many zealous antiquaries have contended, still we by no means intend to deny that the druids employed it as an engine of superstition; it is possible that,

having observed so curious a property, they dexterously contrived to make it answer the purposes of an ordeal, and, by regarding it as the touch-stone of truth, acquitted or condemned the accused by its motions. Mason poetically alludes to this supposed property in the following lines:—

"Behold yon huge

And unknown sphere of living adamant,

Which, poised by magic, rests its central weight
On yonder pointed rock; firm as it seems,
Such is its strange and virtuous property,
It moves obsequious to the gentlest touch
Of him whose heart is pure; but to a traitor,
Though ev'n a giant's prowess nerv'd his arm,
It stands as fix'd as Snowdon."

Note 29, p. 187.-CONTACT, AND ITS NECESSARY CONSEQUENCE, COHESION.

After all, there may be no such thing in nature as absolute contact; the term is purely conventional, denoting such a proximity as may be necessary for the production of certain phenomena; and in all philosophical discussions such is the interpretation of the term. As long as pores exist in bodies it is obvious that their particles cannot be said to be in contact, and the existence of such porosity in every species of matter which can be subjected to our senses is sufficiently proved by its compressibility. There is no known substance, however dense, that may not be made, either by pressure or reduction of temperature, to occupy less space; and were it possible to bring the ultimate atoms into absolute contact, the globe itself might be compressed into such an extremely narrow compass as to be too small even as a foot-ball for the Pigmies.

When we speak therefore of contact, we only mean that proximity necessary for educing the power of cohesive attraction. It has been shown that by friction we are enabled to retain a firm footing, and to walk steadily along a surface; whereas, could we ensure absolute contact, we should not be able to move at all; suppose, for instance, the soles of our feet came into actual contact with the ground, a cohesive attraction would take place between the sui fa es which no muscular effort could overcome. We may, to a certain extent, have experienced this difficulty in walking over a muddy road, when the water, by filling up the inequalities of the surfaces, converts our shoes into a kind of " Sucker."

The cohesive power imparted to particles of matter, by bringing them into the closest possible contact, receives a pleasing illustration from the ingenious invention of Mr.

Brockedon, for forming lead pencils, and for which he received an honorary medal from the Commissioners of the Great Exhibition. His process consists in consolidating finely-powdered and sifted black-lead into a mass as dense, and of the same specific gravity, as native Cumberland lead, which is effected by packing the powder in air-tight cases, and pumping out the air, which always offers a great obstacle to contact, and then subjecting the contents thus prepared to a pressure of five thousand tons, inflicted by two blows.

Note 30, p. 189.-ANIMAL SUCTION.

We are indebted to Sir Everard Home for description of that peculiar structure by which several species of animals are enabled to sustain their bodies in opposition to the force of gravity. His first paper upon this subject is published in the 106th volume of the Philosophical Transactions, in which he says he was not aware that any animal larger than the house-fly was endowed by nature with such a power, so as to admit of examination, until Sir Joseph Banks mentioned that the lacerta gecko, a species of lizard, which is a native of the island of Java, comes out of an evening from the roofs of the houses, and walks down the smooth, hard, and polished chinam walls, in search of the flies which settle upon them, and which are its natural food, and then runs up again to the roof of the house. Sir Joseph, while at Batavia, amused himself with catching this animal, by standing close to the wall, at some distance from the lizard, with a long flattened pole, which, being made suddenly to scrape the surface of the wall, knocked the animal down. He presented Sir Everard with a specimen weighing five ounces and three quarters avoirdupois, which enabled him to ascertain the peculiar mechanism by which the feet of this animal can keep their hold of a smooth, hard, perpendicular wall, and carry up so large a weight as that of its body.

The foot has five toes, at the end of each of which, except that of the thumb, is a very sharp and much-curved claw; on the under surface of each toe are sixteen transverse slits, leading to so many cavities or pockets, the depth of which is nearly equal to the length of the slit that forms the orifice; they all open forward, and the external edge of each opening is serrated, like the teeth of a small-toothed comb. The cavities, or pockets, are lined with a cuticle, and the serrated edges are also covered with it. The structure just described is supplied with various muscles, whose action is to draw down the claw, open the orifices of the pockets, and turn down the serrated edges upon the surface on which the animal