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 cylin der 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 the little 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.

must always remain so, if no external force be impressed upon it to give it motion. But I know of no example which will lead me to suppose, that a body once put into motion would of itself continue so.

Father. You will, I think, presently admit the latter part of the assertion, as well as the former, although it cannot be established by experiment.

Emma. I shall be glad to hear how this is.

Father. You will not deny that the ball which you strike from the trap, has no more power either to destroy its motion, or cause any change in its velocity, than it has to change its shape.

Charles. Certainly; nevertheless, in a few seconds after I have struck the ball with all my force, it falls to the ground, and then stops.

Father. Do you find no difference in the time that is taken up before it comes to rest, even supposing your blow the same?

Charles. Yes, if I am playing on the grass, it rolls to a less distance, than when I play on the smooth gravel.

Father. You find a like difference when you are playing at marbles, if you play in the gravel court, or on the even pavement in the arcade.

Charles. The marbles run so easily on the smooth stones in the arcade, that we can scarcely shoot with a force small enough.

Emma. And I remember Charles and my cousin were, last winter, trying how far they could shoot their marbles along the ice in the canal; and they went a prodigious distance, in comparison of that which they would have gone on the gravel, or even on the pavement in the arcade.

Father. Now these instances properly applied will convince you, that a body once put into motion, would go on for ever, if it were not compelled by some external force to change its

state.

Charles. I perceive what you are going to say it is the rubbing or friction of the marbles against the ground which does the business. For on the pavement there are fewer obstacles than on the gravel, and, fewer on the ice than on the pavement; and hence you would lead us to conclude, that if all obstacles were removed, they might proceed on for ever. But what are we to say of the ball, what stops that?

Father. Besides friction, there is another and still more important circumstance to be taken into consideration, which affects the ball, marbles, and every body in motion.

Charles. I understand you: that is the attraction of gravitation.

Father. It is for from what we said when we conversed on that subject, it appeared that gravity has a tendency to bring every body in motion to the earth; consequently, in a few seconds.