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nfinitely worse than any thunder that I ever heard.

Father. This was because you were ear to them: gunpowder, so tremendous s it is in air, when inflamed in a vacuum makes no more sound than the bell in like ircumstances,

Mr. Cotes mentions a very curious expe iment, which was contrived to show that ound cannot penetrate through a vacuum. A strong receiver filled with common atospheric air, in which a bell was sus ended, was screwed down to a brass plate

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tight that no air could escape, and this as included in a much larger receiver. When the air between the two receivers as exhausted, the sound of the bell could ot be heard.

Emma.

Could it be heard before the

r was taken away?

Father. Yes: and also the moment it as re-admitted.

Charles. What is the reason that some odies sound so much better than others?

Bell-metal is more musical than copper or brass, and these sound much better than many other substances.

Father. All sonorous bodies are elastic, the parts of which by percussion are made to vibrate and as long as the vibrations continue, corresponding vibrations are com municated to the air, and these produce sound. Musical chords and bells are instances that will illustrate this.

Emma. The vibrations of the bell are not visible; and musical chords will vibrate after the sound has vanished.

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Father. If light particles of dust be on the outside of a bell when it is struck, you will, by their motion, have no doubt but that the particles of the metal move too, though not sufficiently to be visible to the naked eye and though the motion of a musical string continues after the sound ceases to be heard, yet it does not follow that sound is not still produced, but only that it is not sufficiently strong to produce a sensation in the ear. You see in a dark

night the flash of a gun, but, being at a considerable distance from it, you hear no. report. If, however, you knew that the light was occasioned by the inflammation of gun-powder, in a musket or pistol, you would conclude that it was attended with sound, though it was not sufficiently strong to reach the place where you are.

Charles. Is it known how far sound can be heard?

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Father. We are assured upon good authority, than the unassisted human voice has been heard at the distance of ten or twelve miles; namely, from New to Old Gibraltar. And in the famous sea-fight between the English and Dutch in 1672, the sound of cannon was heard at the dis tance of two hundred miles from the place of action.-In both these cases the sound passed over water; and it is well known that sound may be always conveyed much farther along a smooth than an uneven sur face.

Experiments have been instituted to as

certain how much water, as a conductor of sound, was better than land; and a person was heard to read very distinctly at the distance of 140 feet on the Thames, and on land he could not be heard farther than 76 feet.

Emma. Might not there be interruptions in the latter case?

Father. No noise whatever intervened by land, but on the Thames there was some occasioned by the flowing of the

water.

Charles. As we were walking last summer towards Hampstead, we saw a party of soldiers firing at a mark near Chalk Farm, and you desired Emma and me to take notice, as we approached the spot, how much sooner the report was heard after we saw the flash than it was when we first got into the fields.

Father. My intention was, that you should know from actual experiment that sound is not conveyed instantaneously, but

takes a certain time to travel over a given space.

When you stood close to the place, did you not observe the smoke and hear the report at the same instant?

Emma. Yes, we did.

Father. Then you are satisfied that the light of the flash, and the report, are always produced together. The former comes to the eye with the velocity of light, the latter reaches the ear with the velocity with which sound travels: if then light travels faster than sound, you will at any considerable distance from a gun that is fired, see the flash before you hear the report. Do you know with what velocity light travels. Charles. At the rate of 12 million of miles in a minute*.

Father. With regard then to several hundred yards, or even a few miles, the motion of light may be considered as in

See Vol. I. Of Astronomy. Conversation XLVII.

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