peded by the load, which is the same thing: for the force which the horse exerts would carry him to a greater distance in the same time, were he freed from the encumbrance of the load, and therefore, as much as his progress falls short of that distance, so much is he, in effect, drawn back by the re-action of the loaded cart.

Again, if you and your brother were in a boat, and if, by means of a rope, you were to attempt to draw another to you, the boat in which you were would be as much pulled towards the empty boat as that would be moved to you; and if the weight of the two boats were equal, they would meet in a point half way between the two.

If you strike a glass bottle with an iron hammer, the blow will be received by the hammer and the glass; and it is immaterial whether the hammer be moved against the bottle at rest, or the bottle be moved against the hammer at rest, yet the bottle will be broken, though the hammer be not injured, because the same blow, which is sufficient to break glass, is not sufficient to break or injure a mass of iron.

From this law of motion you may learn in what manner a bird, by the stroke of its wings, is able to support the weight of its body.

Charles. Pray explain this, papa.

Father. If the force with which it strikes the air below it, is equal to the weight of its body, then the re-action of the air upwards is likewise equal to it; and the bird being acted upon by

two equal forces in contrary directions, will rest between them. If the force of the stroke is greater than its weight, the bird will rise with the difference of these two forces; and if the stroke be less than its weight, then it will sink with the difference.

CONVERSATION XII.

On the Laws of Motion.

Charles. Are those laws of motion which you explained yesterday of great importance in natural philosophy?

Father. Yes, they are, and should be carefully committed to memory. They were assumed by Sir Isaac Newton, as the fundamental principles of mechanics, and you will find them at the head of all books written on these subjects. From these also, we are naturally led to some other branches of science, which, though we can but slightly mention, should not be wholly neglected. They are, in fact, but corollaries to the laws of motion.

Emma. What is a corollary, papa?

Father. It is nothing more than some truth clearly deducible from some other truth before demonstrated or admitted. Thus by the first law of motion, every body must endeavour to continue in the state into which it is put, whether it be of rest, or uniform motion in a straight line: from which it follows as a corollary, "that when we see a body move in a curve line, it must be acted upon by at least two forces."

Charles. When I whirl a stone round in a sling, what are the two forces which act upon the stone ?

Father. There is the force, by which, if you let go the string, the stone will fly off in a right line; and there is the force of the hand, which keeps it in a circular motion.

Emma. Are there any of these circular motions in nature?

Father. The moon, and all the planets move by this law;-to take the moon as an instance. It has a constant tendency to the earth, by the attraction of gravitation, and it has also a tendency to proceed in a right line, by that projectile force impressed upon it by the Creator, in the same manner as the stone flies from your hand ; now, by the joint action of these two forces it describes a circular motion.

Emma. And what would be the consequence, supposing the projectile force to cease?

Father. The moon must fall to the earth; and if the force of gravity were to cease acting upon

the moon, it would fly off into infinite space. Now the projectile force, when applied to the planets, is called the centrifugal force, as having a tendency to recede or fly from the centre; and the other force is termed the centripetal force, from its tendency to some point as a centre.

Charles. And all this is in consequence of the inactivity of matter, by which bodies have a tendency to continue in the same state they are in, whether of rest or motion?

Father. You are right; and this principle which Sir Isaac Newton assumed to be in all bodies, he called their vis inertiæ.

Charles. A few mornings ago, you showed us that the attraction of the earth upon the moon* is 3600 times less than it is upon heavy bodies near the earth's surface. Now as this attraction is measured by the space fallen through in a given time, I have endeavoured to calculate the space which the moon would fall through in a minute, were the projectile force to cease.

Father. Well, and how have you brought it

out?

Charles. A body falls here 16 feet in the first second, consequently in a minute, or 60 seconds it would fall 60 times 60 feet, multiplied by 16, that is 3600 feet, which is to be multiplied by 16; and as the moon would fall through 3600 times less space in a given time than a body here, it would fall only 16 feet in the first minute.

* See Conversation IV.

Father. Your calculation is accurate. I will recall to your mind the second law, by which it appears, that every motion or change of motion produced in a body, must be proportional to, and in the direction of, the force impressed. Therefore, if a moving body receives an impulse in the direction of its motion, its velocity will be increased; if in the contrary direction, its velocity will be diminished;-but if the force be impressed in a direction oblique to that in which it moves, then its direction will be between that of its former motion, and that of the new force impressed.

Charles. This I know from the observations I have made with my cricket-ball.

Father. By this second law of motion, you will easily understand, that if a body at rest receives two impulses, at the same time, from forces whose directions do not coincide, it will, by their joint action, be made to move in a line that lies between the direction of the forces impressed.

Emma. Have you any machine to prove this satisfactorily to the senses?

Father. There are many such invented by different persons, descriptions of which you will hereafter find in various books on these subjects. But it is easily understood by a figure. If on the ball A, (Plate 11. Fig. 14.) a force be impressed, sufficient to make it move with a uniform velocity to the point B, in a second of