Because their large ears act like ear-trumpets to collect the waves of sound, and thus increase their faculty of hearing. How can these animals still farther increase their sense of hearing? By being able to turn their ears, just as we do the ear-trumpet, in the direction whence the sound comes. What are speaking-tubes? They are metal pipes extending from one part of a building to another part. Of what use are speaking-tubes? They are used to convey the voice to the more distant parts of a building. Do they require loud talking? No; even a whisper has been heard through a metal pipe over three thousand feet long. Velocity of Sound. How fast do the waves of sound move through the air? They move at the rate of ten hundred and ninety feet in a second of time, when the temperature of the air is thirty-two degrees. What effect does warm air have on sound? Sound travels more rapidly in warm air. How much does heat increase the velocity of sound? The velocity of sound is increased one foot in a second of time, for every degree of heat added to the air; so that sound travels eleven hundred and twenty feet in a second, when the temperature is at sixty-two degrees, which is nearly _one mile_ in _five seconds_. Does light travel more rapidly than sound? Yes; light comes from the moon to the earth, a distance of two hundred and forty thousand miles, while sound moves eleven hundred and twenty feet. What familiar examples of this difference may be given? A wood-chopper’s axe is seen to descend before we hear the stroke, and the smoke from a gun is seen before we hear the report. How can we tell the distance of the wood-chopper from us? If we count the number of seconds between the stroke of the axe and the time when the sound reaches our ear, and multiply eleven hundred and twenty by this number, it will give us the distance in feet; because sound travels eleven hundred and twenty feet in a second. How can we tell the distance of lightning from us? If we count the number of seconds between the lightning and the thunder, and divide this number by five, it will give the distance in miles; because sound travels one mile in every five seconds. How rapidly does sound travel through water? Sound travels about _four_ times as rapidly through _water_ as it does through air. How rapidly does sound travel through a solid? Sound travels _ten_ times as rapidly through _wood_, and _sixteen_ times as rapidly through _glass_, as it does through air. Does a loud sound travel more rapidly than a faint one? No; it does not. How do we know this? Because the notes from a band of music come to us in the order in which they are played, whether they are loud or faint. Reflection of Sound. What becomes of the waves of sound when they strike a solid substance? They are thrown back, as light from a looking-glass, or a ball from a stone wall. What is this called?