The recent explosion of a powder-laden barge in the Regent’s Park produced effects similar to those mentioned in § 7. The sound-wave bent round houses and broke the windows at the back, the coalescence of different portions of the wave at special points being marked by intensified local action. Close to the place where the explosion occurred the unconsumed gunpowder was in the wave, and, as a consequence, the dismantled gatekeeper’s lodge was girdled all round by a black belt of carbon. SUMMARY OF CHAPTER I The sound of an explosion is propagated as a wave or pulse through the air. This wave impinging upon the tympanic membrane causes it to shiver, its tremors are transmitted to the auditory nerve, and along the auditory nerve to the brain, where it announces itself as sound. A sonorous wave consists of two parts, in one of which the air is condensed, and in the other rarefied. The motion of the sonorous wave must not be confounded with the motion of the particles which at any moment form the wave. During the passage of the wave every particle concerned in its transmission makes only a small excursion to and fro. The length of this excursion is called the _amplitude_ of the vibration. Sound cannot pass through a vacuum. A certain sharpness of shock, or rapidity of vibration, is needed for the production of sonorous waves in air. It is still more necessary in hydrogen, because the greater mobility of this light gas tends to prevent the formation of condensations and rarefactions. Sound is in all respects reflected like light; it is also refracted like light; and it may, like light, be condensed by suitable lenses. Sound is also diffracted, the sonorous wave bending round obstacles; such obstacles, however, in part shade off the sound. Echoes are produced by the reflected waves of sound. In regard to sound and the medium through which it passes, four distinct things are to be borne in mind—intensity, velocity, elasticity, and density. The intensity is proportional to the square of the amplitude as above defined. It is also proportional to the square of the maximum velocity of the vibrating air-particles. When sound issues from a small body in free air, the intensity diminishes as the square of the distance from the body increases. If the wave of sound be confined in a tube with a smooth interior surface, it may be conveyed to great distances without sensible loss of intensity. The velocity of sound in air depends on the elasticity of the air in relation to its density. The greater the elasticity the swifter is the propagation; the greater the density the slower is the propagation. The velocity is directly proportional to the square root of the elasticity; it is inversely proportional to the square root of the density. Hence, if elasticity and density vary in the same proportion, the one will neutralize the other as regards the velocity of sound.