256. =Velocity of Sound.=--The velocity of sound varies in different media. Thus it passes through water four times as rapidly as it does through air. Dr. Franklin, with his head under water, heard distinctly the sound of two stones struck together in the water at the distance of more than half a mile. Sound passes through solids much more easily, and therefore more rapidly, than through liquids. Thus its velocity through copper is twelve times and through glass seventeen times greater than through air. If you place your ear against a long brick wall at one end, and let some one strike upon the other end, you will hear two reports, the first through the wall and the second through the air. Indians are in the habit of ascertaining the approach of their enemies by putting the ear to the ground. When the eruption of a volcano is heard at a great distance the sound comes through the solid earth rather than through the air. The ready transmission of sound through solids furnishes us with a very valuable means of examining diseases of the lungs and heart. The sounds occasioned by the movement of the air in the lungs and by the action of the heart are very distinctly heard through the solid walls of the chest. 257. =Measurement of Distances by Sound.=--It makes no difference with the velocity of sound whether it be loud or not. Thus the sounds of a band of music at a distance all reach your ear at the same time, the sounds of the instruments that can scarcely be heard keeping exact pace in the air with the sounds of the loudest. So, also, the velocity of sound is uniform throughout its whole course, being just as rapid when it is about to die away as it was when it began. It is from this uniformity in the velocity of sound that we can estimate the distance of the object by which any sound is made. We do it by a comparison between light and sound. Sound moves at the rate of 1120 feet in a second. Now light moves 192,000 miles a second, and therefore, for all ordinary distances on the earth, we need make no allowance of time for light in comparison with sound. If we see, then, the operation by which a sound is produced we can estimate its distance from us by the length of time which elapses between what we see and what we hear. In this way we can estimate very accurately the distance of a cannon that we see fired, or the distance of a flash of lightning.