Because the particles of water are closer together than the particles of air. How can we prove that water is a conductor of sound? When a bell is rung under water, the sound may be heard by any person whose head is beneath the surface of the water. Does a solid substance conduct sound more rapidly than air? It does, much more rapidly. How can we prove this? When the ear is placed at one end of a log of wood and the other end is struck with a hammer, two sounds will be heard; first, the one coming through the log, and, afterwards, the one coming through the air. What use is made of this fact? The tramping of horses or the rumbling of cars and wagons, when at a distance, may be heard by placing the ear near the ground. Do all solid substances produce the same amount of sound when struck? No; soft substances, such as lead and wood, produce but little sound; while hard substances, such as copper, bell-metal, and glass, produce much more sound. What are hard bodies, producing much sound, called? They are called _sonorous_ bodies. Is a bell a very sonorous body? Yes; a bell is one of the most sonorous bodies that we have. Why does a bell ring when struck? Because the stroke causes the particles of the bell to vibrate. When we touch a bell while ringing, why does the sound stop? Because we stop its vibrations when we touch it. Why does a crack in a bell check its ringing? Because the crack stops the vibrations in the metal. What other substances ring when struck? All vessels made of glass, earth, stone, iron, etc., ring when struck. How may we detect a crack in any such vessel? By striking the vessel; if it is cracked there will be no ringing sound. Trumpets and Speaking-Tubes. How do the waves of sound move? The waves of sound move in every direction from the place where the noise is made. Why do we put both hands around the mouth when we call to a person at a distance from us? We do so to keep the waves of sound more together. Does it make the voice louder? It does make the voice louder. What instrument has been made so as to operate in the same way? The _speaking-trumpet_, as is shown in Fig. 24. [Illustration: Fig. 24.] Of what use are speaking-trumpets? They are of much use to firemen when giving orders at a fire, and to those on board of vessels when giving orders to the sailors. Why do we hold the hand behind the ear when we wish to hear more distinctly? Because the hand catches more of the waves of sound, and conducts them to the ear. What instrument has been made to serve the same purpose? The _ear-trumpet_, as is shown in Fig. 25. [Illustration: Fig. 25.] How does the ear-trumpet aid the hearing? The large end of the trumpet collects more of the waves of sound than the ear can; hence, more waves reach the ear. Why do such animals as the horse, the rabbit, and the deer, hear better than we do?