such, and are thereby induced to pursue the investigation of a subject which has hitherto excited far too little attention, we beg to submit the following remarks:

If a top could be made to revolve on a point without friction, and in a vacuum, in the case of its velocity being infinite, it would continue to revolve for ever, in the same position, without gyration. If the velocity were finite, it would for ever remain unchanged in position, in the event of the centre of gravity being directly over the point of rotation. In any other position (supposing its velocity very great, although not infinite) there would arise a continued uniform gyration; the line which passes through the point of rotation, and the centre of gravity, always making the same angle with the horizon, or describing the same circle round the zenith. But in all artificial experiments the circumstances are very remarkably changed; if, indeed, the centre of gravity happens to be situated perpendicularly over the point of rotation, the top will continue quite steady, or sleeping, as it is termed, till nearly the whole of its velocity of rotation is expended. In any other position the top begins to gyrate, but, reclining at all times on the outside of its physical point of gyration, the top is uniformly impelled inwards: and this (when the velocity is considerable, and the point broad) acts with a force sufficient for carrying the top towards its quiescent or sleeping point; but when the velocity is much diminished, this power becomes feeble, the gyrations increase in diameter, and the top ultimately falls.

Note 27, p. 124. The Cycloid.

A cycloid is a peculiar curved line, and is described by any one point of a circle as it rolls along a plane, and turns round its centre; thus, for instance, the nail on the felly of a cart-wheel traces a cycloid in the air as the wheel proceeds. This curve is distinguished by some remarkable properties, the most important of which is that mentioned in the text, viz., that any body moving in such a curve, by its own weight, or swing, will pass through all distances of it in exactly the same time; and it is for such a reason that penduInms are made to swing in cycloids, in

order that they may move in equal times, whether they go through a long or a short part of the same curve. Where the arc described is small, a portion of the circle will be sufficiently accurate, because it will be seen that such an arc will not deviate much from an equal portion of a cycloidal curve.

The cycloid is remarkable as being that path, with the exception of the perpendicular, through which a body will move with the greatest velocity: suppose, for example, a body is to descend from any one point to any other, by means of some force acting on it, together with its weight: a person unacquainted with mechanics would say at once that a straight line is the path it must take to effect this in the shortest possible time, since that is the shortest of all lines that can be drawn between two points. Undoubtedly it is the shortest; notwithstanding which, however, the body would be longer in traversing it than in moving through a cycloid. If a body were to move through a space of fifty or a hundred yards, by its weight and some other force acting together, the way it must take to do this in the shortest possible time is by moving in a cycloid. It is supposed that birds which build in rocks possess an instinctive knowledge of this fact, and drop or fly down from height to height in this course. There is certainly a general resemblance between the curved path they describe on such occasions and the cycloid, but it would be difficult to establish the fact by experiment. Man, however, has founded upon this principle some applications of great value in practical mechanics. Switzerland, and in several parts of Germany, for example, slides have been constructed along the sides of mountains, by which the timber felled near their summits is conducted with extreme rapidity to the distant valleys.

NOTE 28, p. 127. Billiards.

In

This interesting game is of French origin (billiard, of bile, and from the Latin pila, a ball.) It was hailed as a favourite diversion at the court of Henry III. of France; and was thence communicated to all the courts of modern Europe. To the novice it may appear as a game of accidents and chances, but experience has enabled us to determine the effects of the stroke given to a

ball with wonderful precision; and it is quite extraordinary to observe the accuracy with which an accomplished player can effect his object, by measuring with his eye the angle at which he should make the stroke, the position of the ball with respect to the cushion, and the distance of the point of the ball from its centre, at which it should be struck. By such skilful management the ball may be made to take directions which would, at first view, be regarded as contrary to all the known laws of motion, such, for instance, as passing round an object, such as a hat placed on the table, and to strike a ball behind it into a pocket.

Upon this subject the reader should consult a work by M. Mingaud, translated and published by John Thurston, the late celebrated billiard-table maker of Catherine Street, Strand, to whom we are indebted for mechanical improvements in the table, which have brought it as near to perfection as the nature of the materials will probably ever allow. His plane surface, composed of slate, is no longer liable to be warped by meteoric changes, which rendered the old wooden table so uncertain; then, again, he has substituted India-rubber for the cushions, and which, by a late peculiar process, termed "vulcanizing," is enabled to retain its elasticity under all the vicissitudes of temperature.

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. The author, who has repeatedly examined this rocky eminence, does not hesitate to state that there is not the slightest pretence 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 lodgement 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 necessarily 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.

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 touchstone of truth, acquitted or condemned the accused by its motions. poetically alludes to this supposed property in the following lines:

"Behold yon huge

And unknown sphere of living adamant,

Mason

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 even a giant's prowess nerved his arm,
It stands as fix'd as Snowdon"

NOTE 31, p. 138.

Contact, and its necessary consequer.ce, 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.

and to walk steadily along a surface: whereas, could we insure 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 surfaces 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 the late 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 32, p. 139.

Animal Suction.

His

We are indebted to Sir Everard Home for a description of that peculiar structure by which several species of animals are enabled to sustain their bodies in opposition to the force of gravity. 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, as to admit of examination, until Sir Joseph Banks mentioned that the lacerta gecko, a specics 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 stand

from the lizard, with a long flattened pole, which, being made suddenly to