Emma. I have sometimes trembled to pass by the hop-wagons which we have met on the Kent road.

Father. And without any impeachment of your courage, for they are loaded to such an enormous height, that they totter every inch of the road. It would indeed be impossible for one of these to pass with tolerable security along a road much inclined; the centre of gravity being removed so high above the body of the carriage, a small declination on one side or other would throw the line of direction out of the base.

Emma. When brother James falls about, is it because he cannot keep the centre of gravity between his feet?

Father. That is the precise reason why any person, whether old or young, falls. And hence you learn that a man stands much firmer with his feet a little apart than if they were quite close, for by separating them he increases the base. Hence also the difficulty of sustaining a tall body, as a walking cane, upon a narrow foundation.

Emma. How do rope and wire dancers, whom I have seen at the Circus, manage to balance themselves?

Father. They generally hold a long pole, with weights at each end, across the rope on which they dance, keeping their eyes fixed on some object, parallel to the rope, by which

means they know when their centre of gravity declines to one side of the rope or the other, and thus by the help of the pole, they are enabled to keep the centre of gravity over the base, narrow as it is. It is not however rope-dancers only that pay attention to this principle, but the most common actions of the people in general are regulated by it.

Charles. In what respects?

Father. We bend forward, when we go up stairs, or rise from our chair, for when we are sitting, our centre of gravity is on the seat, and the line of direction falls behind our base; we therefore lean forwards to bring the line of direction towards our feet. For the same reason a man carrying a burden on his back leans forward: and backward if he carries it on his breast. If the load be placed on one shoulder he leans to the other. If we slip or stumble with one foot, we naturally extend the opposite arm, making the same use of it as the ropedancer does of his pole.

This property of the centre of gravity always endeavouring to descend, will account for appearances, which are sometimes exhibited to excite the surprise of spectators. Emma. What are those, papa ?

Father. One is, that of a double cone, appearing to roll up two inclined planes, forming an angle with each other, for as it rolls it sinks

between them, and by that means the centre of gravity is actually descending.

Let a body FE (Plate 11. Fig. 13.) consisting of two equal cones uniting at their bases, be placed upon the edges of two straight smooth rulers, AB and CD, which at one end meet in an angle at A, and rest on a horizontal plane; and at the other are raised a little above the plane; the body will roll towards the elevated end of the rulers, and appear to ascend; the parts of the cone that rest on the rulers growing smaller as they go over a large opening, and thus letting it down, the centre of gravity descends. But you must remember that the height of the planes must be less than the radius of the base of the cone.

Charles. Is it upon this principle that a cylinder is made to roll up hill?

Father. Yes, it is; but this can be effected only to a small distance. If a cylinder of pasteboard, or very light wood AB, (Plate 11. Fig. 11.) having its centre of gravity at c, be placed on the inclined plane CD, it will roll down the inclined plane, because a line of direction from that centre lies out of the base. If I now fill the little hole o above with a plug of lead, it will roll up the inclined plane, till the lead gets near the base, where it will lie still: because the centre of gravity, by means of the lead, is removed from c towards the plug, and therefore

is descending, though the cylinder is ascending.

Before I put an end to this subject, I will show you another experiment, which without understanding the principle of the centre of gravity cannot be explained. Upon this stick A, (Plate II. Fig. 12.) which, of itself, would fall, because its centre of gravity hangs over the table EF, I suspend a bucket B, fixing another stick a, one end in a notch between A and k, and the other against the inside of the pail at the bottom. Now you will see that the bucket will, in this position, be supported, though filled with water. For the bucket being pushed a little out of the perpendicular, by the stick a, the centre of gravity of the whole is brought under the table, and is consequently supported by it.

The knowledge of the principle of the centre of gravity in bodies, will enable you to explain the structure of a variety of toys which are put into the hands of children, such as he tlittle sawyer rope-dancer; tumbler, &c.

CONVERSATION XI.

On the Laws of Motion.

Charles. Are you now going, papa, to describe those machines, which you call, mechanical powers?

Father. We must, I believe, defer that a day or two longer, as I have a few more general principles with which I wish you previously to be acquainted.

Emma. What are these, papa ?

Father. In the first place, you must well understand what are denominated the three general laws of motion: the first of which is, "that every body will continue in its state of rest, or of uniform motion, until it is compelled by some force to change its state."*

Charles. There is no difficulty of conceiving that a body, as this inkstand, in a state of rest,

*The author is aware that this Law of Motion is not admitted by some modern philosophers of high name; to him, however, their reasonings appear inconclusive. At any rate, in a work intended for very young minds, he thinks it a duty to avoid metaphysical distinctions: preferring, at all times, rather to guide them by matters of fact than to load their tender memories with curious and subtile theories.