your ball must come to the ground by that cause alone; but besides the attraction of gravitation, there is a resistance which the air, through which the ball moves, makes to its passage. Emma. That cannot be much, I think.

Father. Perhaps, with regard to the ball struck from your brother's trap, it is of no great consideration, because the velocity is but small; but in all great velocities, as that of a ball from a musket or cannon, there will be a material difference between the theory and practice, if it be neglected in the calculation. Move your mamma's riding-whip through the air slowly, and you observe nothing to remind you that there is this resisting medium; but if you swing it with considerable swiftness, the noise which it occasions will inform you of the resistance it meets with from something, which is the atmosphere.

Charles. If I now understand you, the force which compels a body in motion to stop, is of three kinds; (1.) the attraction of gravitation; -(2.) the resistance of the air; and (3.) the resistance it meets with from friction.

Father. You are quite right.

Charles. I have no difficulty of conceiving, that a body in motion, will not come to a state of rest, till it is brought to it by an external force, acting upon it in some way or other. I have seen a gentleman, when skating on very slippery ice, go a great way without any exertion to

himself, but where the ice was rough, he could not go half the distance without making fresh efforts.

Father. I will mention another instance or two on this law of motion. Put a basin of water into your little sister's wagon, and when the water is perfectly still, move the wagon, and the water, resisting the motion of the vessel, will at first rise up in the direction contrary to that in which the vessel moves. If, when the motion of the vessel is communicated to the water, you suddenly stop the wagon, the water, in endeavouring to continue the state of motion, rises up on the opposite side.

In like manner, if while you are sitting quietly on your horse, the animal starts forward, you will be in danger of falling off backward; but if while you are galloping along, the animal stops on a sudden, you will be liable to be thrown forward.

Charles. This I know by experience, but I was not aware of the reason of it till to-day.

Father. One of the first, and not least important uses of the principles of natural philosophy is, that they may be applied to, and will explain many of the common concerns of life.

We now come to the second law of motion, which is ;-"that the change of motion is proportional to the force impressed, and in the direction of that force."

Charles. There is no difficulty in this, for if

while my cricket-ball is rolling along after Henry has struck it, I strike it again, it goes on with increased velocity, and that in proportion to the strength which I exert on the occasion; whereas, if while it is rolling, I strike it back again, or give it a side blow, I change the direction of its course.

Father. In the same way, gravity, and the resistance of the atmosphere, change the direction of a cannon-ball from its course in a straight line, and bring it to the ground; and the ball goes to a further or less distance in proportion to the quantity of powder used.

The third law of motion is;" that to every action of one body upon another, there is an equal and contrary re-action." If I strike this table, I communicate to it (which you perceive by the shaking of the glasses) the motion of my hand; and the table re-acts against my hand just as much as my hand acts against the table.

If you press with your finger one scale of a balance to keep it in equilibrio with a pound weight in the other scale, you will perceive, that the scale pressed by the finger, acts against it with a force equal to a pound, with which the other scale endeavours to descend.

A horse drawing a heavy load, is as much drawn back by the load as he draws it forward. Emma. I do not comprehend how the cart draws the horse.

Father. But the progress of the horse is im

VOL. I.F

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 incumbrance 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.

1440 9་ས་

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?