5. How many miles away is lightning if the thunder is heard 12 seconds after the flash in seen, the temperature being 25°C.? 6. Four seconds after a flash of lightning is seen the thunder clap is heard. The temperature is 90°F. How far away was the discharge? 7. The report of a gun is heard 3 seconds after the puff of smoke is seen. How far away is the gun if the temperature is 20°C.? 8. An explosion takes place 10 miles away. How long will it take the sound to reach you, the temperature being 80°F?. How long at 0°F.? 9. How long after a whistle is sounded will it be heard if the distance away is 1/4 mile, the temperature being 90°F.? 10. The report of an explosion of dynamite is heard 2 minutes after the puff of smoke is seen. How far away is the explosion the temperature being 77°F.? (2) WAVES[N] AND WAVE MOTION [N] A wave is a disturbance in a substance or medium that is transmitted through it. =322. Visible Waves.=--It is best to begin the study of wave motion by considering some waves which are familiar to most persons. Take for example the waves that move over the surface of water (Fig. 316). These have an onward motion, yet boards or chips upon the surface simply rise and fall as the waves pass them. They are not carried onward by the waves. The water surface simply rises and falls as the waves pass by. Consider also the waves that may be seen to move across a field of tall grass or grain. Such waves are produced by the bending and rising of the stalks as the wind passes over them. Again, waves may be produced in a rope fastened at one end, by suddenly moving the other end up and down. These waves move to the end of the rope where they are _reflected_ and return. The three types of waves just mentioned are illustrations of _transverse_ waves, the ideal case being that in which the particles move at _right angles_ to the path or course of the wave. Such waves are therefore called _transverse_ waves. [Illustration: FIG. 316.--Water waves.] =323. Longitudinal waves.=--Another kind of wave is found in bodies that are elastic and compressible and have inertia, such as gases and coiled wire springs. Such waves may be studied by considering a wire spring as the medium through which the waves pass. (See Fig. 318.) [Illustration: FIG. 317.--The compression wave travels through the spring.]