THE EAR, AND HEARING—PHYSIOLOGY OF HEARING AND SOUND—SOUND AS COMPARED WITH LIGHT—WHAT IS SOUND?—VELOCITY OF SOUND—CONDUCTIBILITY—THE HARMONOGRAPH. Before entering upon the science of ACOUSTICS, a short description of the ear, and the mode in which sound is conveyed to our brain, will be no doubt acceptable to our readers. The study of the organs of hearing is not an easy one; although we can see the exterior portion, the interior and delicate membranes are hidden from us in the very hardest bone of the body—the _petrous_ bone, the temporal and rock-like bone of the head. [Illustration: Fig. 170.—1. Temple bone. 2. Outer surface of temple. 3. Upper wall of bony part of hearing canal. 4. Ligature holding “hammer” bone to roof of drum cavity. 5. Roof to drum cavity. 6. Semicircular canals. 7. Anvil bone. 8. Hammer bone. 9. Stirrup bone. 10. Cochlea. 11. Drum-head cut across. 12. Isthmus of Eustachian tube. 13. Mouth of tube in the throat. 14. Auditory canal. 15. Lower wall of canal. 16. Lower wall of cartilaginous part of canal. 17. Wax glands. 18. Lobule. 19. Upper wall of cartilaginous portion of canal. 20. Mouth of auditory canal. 21. Anti-tragus.] The ear (external) is composed of the auricle, the visible ear, the auditory canal, and the drum-head, or _membra tympani_. The tympanum, or “drum,” is situated between the external and the internal portions of the ear. This part is the “middle ear,” and is an air cavity, and through it pass two nerves, one to the face, and the other to the tongue. The internal ear is called the “labyrinth,” from its intricate structure. We give an illustration of the auditory apparatus of man (fig. 170). The auricle, or exterior ear, is also represented, but we need not go into any minute description of the parts. We will just name them (fig. 171). Sound is the motion imparted to the auditory nerve, and we shall see in a moment how sound is produced. The undulations enter the auditory canal, having been taken up by the auricle; the waves or vibrations move at the rate of 1,100 feet a second, and reach the drum-head, which has motion imparted to it. This motion or oscillation is imparted to other portions, and through the liquid in the labyrinth. The impressions of the sound wave are conveyed to the nerve, and this perception of the movement in the water of the labyrinth by the nerve threads and the brain causes what we term “hearing.” [Illustration: Fig. 171.—1. Pit of anti-helix. 2, 6, 10. Curved edge of the auricle. 3. Mouth of auditory canal. 4. Tragus. 5. Lobe. 7. Anti-helix. 8. Concha. 9. Anti-tragus.] Let us now endeavour to explain what sound is, and how it arises. There are some curious parallels between sound and light. When speaking of light we mentioned some of the analogies between sound and light, and as we proceed to consider sound, we will not lose sight of the light we have just passed by.