Thus in the case of ordinary speech or song, the waves are from 2 to 8 feet in length, that is, from one compressed region to the next. In the case of a whistle, the wave-length may be 1 or 2 inches, whilst the deepest note of an organ produces a sound of which the wave-length is about 32 feet. As in every other instance of wave-motion, air waves may differ from each other in three respects. First, in _wave-length_; secondly, in _amplitude_; and thirdly, in _wave-form_. The first determines what we call the tone, _i.e._ whether the sound is high or low, treble or bass; the second determines the intensity of the sound, whether faint or loud; and the third determines its _quality_, or, as the Germans expressively call it, the sound-colour (_Klangfarbe_). We recognize at once a difference between the sound of a vowel, say _ah_, sung by different persons to the same note of the piano and with the same loudness. There is a personal element, an individuality, about voices which at once arrests our attention, apart altogether from the tone or loudness. This _sound-quality_ is determined by the form of the wave-motion, that is, by the nature of the movement of the air-particle during its little excursion to and fro in which it takes part in producing a zone of compression or rarefaction in the air and so forms a sound wave. We have next to discuss the speed with which this air-compression is propagated through the air. Every one knows that it is not instantaneous. We see the flash of a gun at a distance, and a second or so afterwards we hear the bang. We notice that the thunder is heard often long after the lightning flash is seen. It would take too long to describe the experiments which have been made to determine precisely the speed of sound waves. Suffice it to say that all the best experiments show that the velocity of a sound wave in air, at the temperature of melting ice, or at 0° C. = 32° Fahr., is very nearly 1087 feet per second, or 33,136 centimetres per second. This is equivalent to 741 miles per hour, or more than ten times the speed of an express train. At this rate a sound wave would take 4 hours to cross the Atlantic Ocean, 16 hours to go half round the world or to the antipodes, and some 2 minutes to cross from Dover to Calais.