But perhaps the strongest illustration of the differing densities of these two media is furnished by an attempt to drive a nail under water. By an absolute law such an effort, if guided by sight independent of calculation, must fail. Habit and experience, tested in atmospheric light, will control the muscles, and direct the blow at the very point where the nail-head is not. For this reason the ingenious expedient of a voltaic lantern under water has proved to be impracticable. It is not the light alone which is wanted, but that sweet familiar atmosphere through which we are habituated to look. The submarine diver learns to rely wholly on the truer sense of touch, and guided by that he engages in tasks requiring labor and skill with the easy assurance of a blind man in the crowded street. The conveyance of sound through the inelastic medium of water is so difficult that it has been called the world of silence. This is only comparatively true. The fish has an auditory cavity, which, though simple in itself, certifies the ordinary conviction of sound, but it is dull and imperfect; and perhaps all marine creatures have other means of communication. There is an instance, however, of musical sounds produced by marine animals, which seems to show an appreciation of harmony. In one of the lakes of Ceylon, Sir Emerson Tennent heard soft musical sounds, like the first faint notes of the aeolian harp or the faint vibrations of a wineglass when its rim is rubbed by a wet finger. This curious harmony is supposed to be produced by a species of testaceous mollusk. A similar intonation is heard at times along the Florida coast. Interesting as this may be, as indicating an appreciation of that systematic order in arrangement which in music is harmony, it does not alter the fact that to the ears of the diver, save the cascade of the air through the life-hose, it is a sea of silence. No shout or spoken word reaches him. Even a cannon-shot comes to him dull and muffled, or if distant it is unheard. But a sharp, quick sound, that appears to break the air, like ice, into sharp radii, can be heard, especially if struck against anything on the water. The sound of driving a nail on the ship above, for example, or a sharp tap on the diving-bell below, is distinctly and reciprocally audible. Conversation below the surface by ordinary methods is out of the question, but it can be sustained by placing the metal helmets of the interlocutors together, thus providing a medium of conveyance.