ascend, and as the board cannot upset without raising the weight, G, the whole may be made to swing to and fro without falling.

A similar fact is more strikingly shewn by suspending a pail of water, as shewn in another part of fig. 1. The pail, G, is supported by a string or handle, H, which is secured to a board or stick, rather more than half of which rests upon the table. If the pail were allowed to hang with the handle upright, the whole assemblage would, of course, upset, since the greater part of the weight would be beyond the edge of the table, and the stick is not at all fixed to the table. But the whole acquires stability by merely placing a stick, F, in the position EG, The upper end is inserted into a notch in the stick at E, while the lower end presses against the pail, and forces the handle, H, out of the vertical position. Now, no motion can be given to the pail without raising the centre of gravity of the whole arrangement, and such an elevation being contrary to the laws of gravity, the position of the pail is one of stable equilibrium, which a slight disturbance is not sufficient to destroy.

Fig. 2.

Figs. 2 to 9 are additional illustrations of the truth that the centre of gravity always seeks the lowest part. They seem, at first

view, to be exceptions to the law; for a body does not naturally roll uphill, as in the following cases, but we

shall find that they

are as perfect illustrations of the law, as any that we have before given.

Fig. 2 is a double cone of wood, which rolls up the inclined plane ABCD, fig. 3.

is placed at C, and the although they appear to plane, they actually move or down a line slightly declined, as may be seen by inspecting fig. 4, where ce is the line along which the cone moves; ca is the upward inclination of the bars of the frame, which deceive the eye in the effect produced.

cones roll to A B: but move up the inclined along a horizontal line,

Fig. 4.

But ef is actually the path of the lowest part of the cone, and d a the path of the axis, both of which incline downwards.

In fig. 5, the cylinder of which A KI is a section, if placed on an inclined plane, C, will roll down, because the centre of gravity not being supported in the line of direction HID, it falls beyond the point of support, F, and the line FA does not coincide with the line of direction. But if the cylinder be not homogeneous; if it be formed partly of wood and partly of lead, as in figs. 6 and 7, where the shaded parts FF represent the lead, the centre of gravity is no longer the centre

Fig. 5.

of magnitude of the mass, but is on one side of it as at

E. Now, in fig. 7, the point of support is D,

and a perpendicular from the centre of gravity,

E, falls above

the point of sup

Fig. 6.

Fig. 7.

B

port, so that the cylinder rolls upwards until it falls to

the position shewn in fig. 6; such, that a perpendicular from the centre of gravity meets the point of contact D, when it will remain stationary, although on an inclined plane. Fig. 8 is a further illustration of this interesting ex

Fig. 8.

periment. The dotted line is the path of the centre of magnitude of the cylinder up hill; but the curved-line is the path of the centre of gravity, so that it will readily be seen that the cylinder has a tendency to roll a short distance upwards, in order that the centre of gravity may assume the lowest possible position whereby stability is acquired.

The same principle has been applied to make a watch shew time by rolling slowly down an inclined plane.

Fig. 9 is the section of a cylinder, which would roll down the inclined plane quickly but for a heavy body, P,

Fig. 9.

which is so adjusted that the cylinder turns round once in twelve hours, while the weight, P, maintains a constant direction with respect to the axis of the cylinder; so that the wheel to whose axis it is attached does not move round, but allows the cylinder to move round it. The

and act the usual parts of clock-work. On one end of the cylinder is a clock-face, the hands to which are attached to the axis of the central wheel.-S. M.

THE PHILOSOPHY OF A PEG-TOP.

We trust that our young readers will not be disposed to spin their tops with less zest when we assure them that this toy presents a very difficult problem to the natural philosopher; that the theory of its motions has engaged the attention of very eminent men; and that the questions arising therefrom are by no means satisfactorily answered. The boy who loves his pegtop because it is an ingenious toy, will, we hope, be taught by the present article to regard it with a higher degree of interest; and the man (if such there be) who despises the peg-top, because it is a toy, will have an opportunity of learning, that much philosophy may be gathered from childish things. The simple contrivance, whereby a top is set spinning, need not be particularly described. The string which is wound round the top, and suddenly uncoiled with a jerking kind of action, has the effect of imparting circular motion to the top. Now, circular motion is always the result of two forces, one of which attracts the body to the centre around which it moves, and hence is called the centripetal force; and the other impels it to move off in a right line from the centre, and this constitutes the centrifugal force. In all circular motion, these two forces constantly balance each other: if it were not so, the revolving body must evidently approach the centre of motion or recede from it, according as one or the other force prevailed. This is well illustrated by the action of a sling. When a stone is whirled round in the sling, a projectile force is imparted to the stone; but it

is prevented from flying off on account of the counteracting or centripetal force of the string; the moment, however, that the string is unloosed, the stone ceases to move in a circle, but darts off in a right line; because, being released from confinement to the fixed or central point, it is acted on by one force only, which always produces motion in a right line.

We need scarcely inform our young reader that it is impossible for him to set up his top, so that it shall stand steadily on its point without spinning it. He can never keep the line of direction within its narrow base: but when the rotating motion is once established, there is no difficulty in preserving it for a time in its position. Why is this? When a top is spinning, we have an example of circular motion round a central axis; and the more rapidly the top spins, the greater is the tendency of all its parts to recede from the axis; or, in other words, the greater is the centrifugal force: the parts which thus revolve may be regarded as so many powers acting in a direction perpendicular to the axis; but as these parts are all equal, and as they pass with great rapidity round the axis, the top is in equilibrio on the end of its axis, or point of support, and thus its erect position is maintained. But the top soon falls, on account of two great impediments to its motion,-viz., the friction of the peg on the ground, and the resistance of the air. If the top could be made to revolve on a point without friction, and in a vacuum, it would continue to revolve for ever, and always maintain the same position. But, as it is impossible to comply with these two conditions, let us see what results have followed the attempts to reduce the retarding forces as much as possible.

About the middle of the last century Mr. Serson contrived a top, which, instead of the usual pear-shape of the common peg-top, presented a horizontal surface similar to what we should obtain by piercing the centre of a disk of wood (or a trencher), with an axis or peg. The upper surface of this top was polished, and it presented, while spinning, a true horizontal plane. It