_Caroline._ Now I understand it; the effects of preceding impulses continue, whilst gravity constantly adds new ones, and thus the velocity is perpetually increased. _Mrs. B._ Yes; it has been ascertained, both by experiment, and calculations which it would be too difficult for us to enter into, that heavy bodies near the surface of the earth, descending from a height by the force of gravity, fall sixteen feet the first second of time, three times that distance in the next, five times in the third second, seven times in the fourth, and so on, regularly increasing their velocities in the proportion of the odd numbers 1, 3, 5, 7, 9, &c. according to the number of seconds during which the body has been falling. _Emily._ If you throw a stone perpendicularly upwards, is it not the same length of time in ascending, that it is in descending? _Mrs. B._ Exactly; in ascending, the velocity is diminished by the force of gravity; in descending, it is accelerated by it. _Caroline._ I should then imagine that it would fall, quicker than it rose? _Mrs. B._ You must recollect that the force with which it is projected, must be taken into the account; and that this force is overcome and destroyed by gravity, before the body begins to fall. _Caroline._ But the force of projection given to a stone in throwing it upwards, cannot always be equal to the force of gravity in bringing it down again; for the force of gravity is always the same, whilst the degree of impulse given to the stone is optional; I may throw it up gently, or with violence. _Mrs. B._ If you throw it gently, it will not rise high; perhaps only sixteen feet, in which case it will fall in one second of time. Now it is proved by experiment, that an impulse requisite to project a body sixteen feet upwards, will make it ascend that height in one second; here then the times of the ascent and descent are equal. But supposing it be required to throw a stone twice that height, the force must be proportionally greater. You see then, that the impulse of projection in throwing a body upwards, is always equal to the action of the force of gravity during its descent; and that whether the body rises to a greater or less distance, these two forces balance each other. I must now explain to you what is meant by the _momentum_ of bodies. It is the force, or power, with which a body in motion, strikes against another body. The momentum of a body is the product of its _quantity of matter_, multiplied by its _quantity of motion_; in other words, its weight multiplied by its velocity. _Caroline._ The quicker a body moves, the greater, no doubt, must be the force which it would strike against another body. _Emily._ Therefore a light body may have a greater momentum than a heavier one, provided its velocity be sufficiently increased; for instance, the momentum of an arrow shot from a bow, must be greater than that of a stone thrown by the hand.