matter which they contain, that is, four times a greater force of gravity is exerted on a weight of four pounds, than upon one of a single pound. The consequence of this principle is, that all bodies at equal distances from the earth fall with equal velocity.)

Emma. What do you mean, papa, by velocity?

Father I will explain it by an example or two; if you and Charles set out together, and you walk a mile in half an hour, but he walk and run two miles in the same time, how much swifter will he go than you?

Emma. Twice as swift.

Father. He does, because, in the same time, he passes over twice as much space; therefore we say his velocity was twice as great as yours. Suppose a ball, fired from a cannon, pass through 800 feet in a second of time; and in the same time your brother's arrow pass through 100 feet only, how much swifter does the cannon ball fly than the arrow?

Emma. Eight times swifter.

Father. Then it has eight times the velocity of the arrow; and hence you understand that swiftness and velocity are synonymous terms; and that the velocity of a body is measured by the space it passes over in a given time, as a second, a minute, an hour, &c.

Emma. If I let a piece of metal, as a penny piece, and a feather fall from my hand at the

same time, the penny will reach the ground much sooner than the feather. Now how do you account for this if all bodies are equally affected by gravitation, and descend with equal velocities, when at the same distance from the earth?

Father. (Though the penny and feather will not, in the open air, fall with equal velocity, yet if the air be taken away, which is easily done, by a little apparatus connected with the airpump, they will descend in the same time. Therefore the true reason why light and heavy bodies do not fall with equal velocities, is, that the former, in proportion to its weight, meets with a much greater resistance from the air than the latter.

Charles. It is then, I imagine, from the same cause, that if I drop the penny and a piece of light wood into a vessel of water, the penny shall reach the bottom, but the wood, after descending a small way, rises to the surface.

Father. In this case the resisting medium is water instead of air, and the copper being about nine times heavier than its bulk of water, falls to the bottom without apparent resistance. But the wood, being much lighter than water, cannot sink in it, therefore, though by its momentum*, it sinks a small distance, yet as soon

* The explanation of this term will be found in the next Conversation.

as that is overcome by the resisting medium, it rises to the surface, being the lighter substance.

CONVERSATION VI.

Of the Attraction of Gravitation.

Emma. The term momentum which you made use of yesterday, is another word which I do not understand.

Father. If you have understood what I have said respecting the velocity of moving bodies, you will easily comprehend what is meant by the word momentum.

The momentum, or moving force of a body, is its weight multiplied into its velocity. You may, for instance, place this pound weight upon a china plate without any danger of breaking, but if you let it fall from the height of only a few inches, it will dash the china to pieces. In the first case, the plate has only the pound weight to sustain, in the other, the weight must be multiplied into the velocity, or, to speak in a popular manner, into the distance of the height from which it fell.

If a ball a (Plate 1. Fig. 6.) lean against the obstacle b, it will not be able to overturn it, but if it be taken up to c and suffered to roll down the inclined plane AB against b, it will certainly overthrow it;-in the former case, b would only have to resist the weight of the ball a, in the latter it has to resist the weight multiplied into its motion or velocity.)

Charles. Then the momentum of a small body whose velocity is very great, may be equal to that of a very large body with a slow velocity.

Father. It may, and hence you see the reason why immense battering rams, used by the ancients, in the art of war, have given place to cannon balls of but a few pounds weight.

Charles. I do, for what is wanting in weight, is made up by velocity.

Father. Can you tell me what velocity a cannon ball of 28 pounds must have to effect the same purposes, as would be produced by a battering ram of 15,000 pounds weight, and which, by manual strength, could be moved at the rate of only two feet in a second of time?

Charles. I think I can ;-the momentum of the battering ram must be estimated by its weight, multiplied into the space passed over in a second, which is 15,000 multiplied by two feet, equal to 30,000; now if this momentum, which must also be that of the cannon ball, be divided by the weight of the ball, it will give the velocity required; and 30,000 divided by 28, will

give for the quotient 1072 nearly, which is the number of feet which the cannon ball must pass over in a second of time, in order that the momenta of the battering ram and the ball may be equal, or in other words, that they may have the same effect in beating down an enemy's wall./ Emma I now fully comprehend what the momentum of a body is, for if I let a common trapball accidentally fall from my hand, upon my foot, it occasions more pain than the pressure of a weight several times heavier./

Charles. If the attraction of gravitation be a power by which bodies in general tend towards each other, why do all bodies tend to the earth as a centre?

Father. I have already told you that by the great law of gravitation, the attraction of all bodies is in proportion to the quantity of matter which they contain. Now the earth, being so immensely large in comparison of all other substances in its vicinity, destroys the effect of this attraction between smaller bodies, by bringing them all to itself.-If two balls are let fall from a high tower at a small distance apart; though they have an attraction for one another, yet it will be as nothing when compared with the attraction by which they are both impelled to the earth, and consequently the tendency which they mutually have of approaching one another will not be perceived in the fall. If, however, any two bodies were placed in free space, and