_Hearing_ is a telæsthetic sense. Through it we become aware of certain vibratory states of more or less distant objects. The vibrations of these bodies are transferred to the air or other medium surrounding the body, and are transmitted through the air or other medium to the ear. The sound-waves traverse the air at a rate of 337 metres (1106 feet) in a second; but they travel about four times as fast in water. If the vibration is periodic or regular, the sound is called a tone; non-periodic or irregular sounds are noises. The pitch of a tone is determined by the number of vibrations in a second. The lowest or gravest tone most of us can hear is that where there are about 30 vibrations in a second; twice this number give us a tone of an octave higher; twice this again, another octave; and so on. In musical composition, tones from about 40 to about 4000 vibrations per second are employed. This is a range of somewhat over six octaves. But many of us are capable of hearing sounds over a range of about ten octaves, that is to say, from 30 to 30,000 vibrations per second. The upper limit of hearing is, however, very variable. Some people are deaf to tones of more than 15,000 or 20,000 vibrations per second.[EW] Others may hear shrill tones of 40,000, or even in rare cases 50,000. I could as a boy hear the shrill squeak of a bat; now I am quite deaf to it. A friend of mine in South Africa was unable to hear the piping of the frogs in the pond, which was to me so loud as almost to drown the tones of his voice. Apart from the pitch of a note is its quality. The same note struck on different instruments or sung by different persons has a different ring. This is determined by the number and intensity of overtones, or partials, which are associated with the fundamental tone. Suppose the deep fundamental tone of 33 vibrations be sounded; with it there may be associated overtones, eight or nine in number, all of which are simple multiples (twice, thrice, four times, and so on) of the fundamental 33. The effects of these on the organ of hearing fuse or combine with the predominant effect of the fundamental tone. In harmonious chords, also, two or more fundamental tones, with their accompaniment of partials, blend in sensation so completely that it requires a keen musical ear and some training to analyze them into their component elements.