It is called _reflection_ of sound. In what direction is sound reflected? Sound, like light, is reflected so that the angles of incidence and reflection are always equal. When the reflected sound comes back to the ear, what does it make? It makes an _echo_. When the sound comes back several times, what does it make? It makes several echoes. Where are echoes always to be heard? Echoes may be heard in the deep caves of the earth; because the walls of the caves reflect sounds striking against them. What curious echo may be mentioned? There is an echo in Fairfax County, Virginia, which sends back twenty notes played on a flute. Does every reflection of sound produce an echo? No; it does not. Why is there no echo from the walls of a small room? Because the walls are too close together to make an echo. How far away must the reflection of a sound be made, so as to produce a perfect echo? Sound must be made not less than one hundred and twelve feet distant to produce a perfect echo. How many syllables can be heard from an echo one hundred and twelve feet distant? Only one syllable can be heard. How far must the echo be to hear two syllables? It must be twice the one hundred and twelve feet, or two hundred and twenty-four feet, to hear two syllables; and three times that distance to hear three syllables, and so on. Why can a person when speaking, be heard better in a room than in the open air? Because the walls of the building reflect his voice to the hearers, so that more waves of sound reach their ears. Why are noises heard so distinctly in an unfurnished dwelling? Because there is then nothing but the walls to obstruct the waves of sound; but when the dwelling is furnished, each article of furniture helps to obstruct the sound and to make it less distinct. What may be said of the harmony of sounds? The babbling of the brook, the roaring of the cataract, the wailing of the wind, and the singing of the birds, everywhere show the Divine love of harmony; for where there is no human ear to listen, they still sing their heavenly anthems of praise to His ear alone. [Illustration: Horse-drawn trolley car.] [Illustration: Decoration.] CHAPTER VII. Matter. [Illustration: Illustrated Capital I] In what form is all matter found? In the form of a solid, a liquid, or a gas. What is true of all solids? All solids must have length, breadth, and thickness. Do solids vary much in length, breadth, and thickness? Yes; they vary so much that we can scarcely find two things exactly alike in shape and size. Can the shape of any substance be changed? Yes; its shape may be changed in many ways. What takes place in india-rubber when it is stretched? Its particles are drawn farther apart, and its shape is altered. When the force is removed, what does the india-rubber do? It springs back into its former shape.