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distance of a ship that has fired her watch-guns. Suppose you were in a vessel, and saw the flash of a gun, and between that and the report 24 seconds elapsed, what would be the distance of one vessel from another?
Emma. I should multiply 1142 by 24, and then bring the product into miles, which in this instance is equal to something more than five miles.
Father. The mischief occasioned by lightning is supposed to depend much on the distance at which the storm is from the spot from whence it is seen.
By counting the number of seconds elapsed between the flash of lightning and the clap of thunder, you may ascertain how far distant you are from the storm.
Charles. I should like to have a stop-watch to be able to calculate
this for myself. Father.
As it will, probably, be some time before you become possessed of this expensive instrument, I will tell you of something which you have always about you, and which ill answer the purpose.
Emma. What is that, papa? Father. The pulse at your wrist, which, in healthy people, generally beats about 75 times in a minute* in the same space of time sound flies 13 miles: therefore, in one pulsation sound passes over 13 miles divided by 75, that is about 915 feet, or the 4th part of a mile, consequently in six pulsations it will pass over a mile. Emma. If I see a flash of light
* In children the pulse is more rapid.
ning, and between that and the thunder I count at my wrist 36 or 60 pulsations, I say the distance in one case is equal to six miles, in the other ten because, if sound travel ath of a mile in the interval between two pulsations at the wrist, it will travel 6 miles, during 36 pulsations; and 10 miles, during
Father. You are right; and this method will, for the present, be sufficiently accurate for all your pur poses.
Of the Speaking-Trumpet.
CHARLES. I have been thinking about the nature of sound, and am ready to ask what it is; I can conceive of particles of light issuing from the sun, or other luminous bodies, but I know not what sound is.
Father. It would be but of little use to give you a definition of sound, but I will endeavour to illustrate the subject. Sound is not a body like light, but it depends on the concussion or striking together of other
bodies that are elastic, which being put into a tremulous motion, excite a wave in the surrounding air.
Emma. Is it such a wave as we see in the pond when it is ruffled by the wind?
Father. Rather such a one as is produced by throwing a pebble into still water.
Charles. I have often observed this; the surface of the water forms itself into circular waves.
Father. It is probable that the tremulous motion of the parts of a sonorous body communicate undulations in the air in a similar manner. Two obvious circumstances must strike every observer with regard to the undulations in water. (1.) The waves, the farther they proceed from the striking body, become less and