Page:Popular Science Monthly Volume 12.djvu/643
they yield; plants, for example, being poor witnesses in such a case, and the higher vertebrates the best. Geologists will be interested in the researches of the author, the results of which are also given, into the origin and succession of the Tertiary fresh-water lakes of the Rocky Mountain region; a subject upon which but little accurate information has been hitherto attainable. The portion of the paper printed in the present Monthly includes the fishes, amphibians, reptiles, and birds; and the second part will be devoted to the mammals, which will be discussed more in detail. Prof. Marsh has kindly prepared for us a geological section of the earth's crust, revised so as to illustrate the present aspect of the subject of the introduction and succession of vertebrate life on this continent, and which the reader will find valuable for reference in studying the address.
We commend to those who persist in ventilating the ancient prejudice against the material medium of which all things around us are constituted, as "dead matter," "gross matter," "brute matter," etc., a little meditation on the remarkable powers manifested by the telephone.
In another part of the Monthly will be found an excellent explanation of the mechanism and mode of working of this remarkable contrivance. But, to appreciate its deeper meaning, we must recall some of the characteristics of sound and the conditions of the production of voice. The brain, the spinal centres, the nerves, and the muscles, have all to be coördinated in that expulsion of air through the apparatus of speech which results in the utterance of words. The vocal cords are thrown into vibration by the air-current which sets up wave-motions that are transmitted in all directions. To appreciate what here goes on in this light, invisible medium, we must strive to keep in mind the behavior of the air-particles. In the propagation of sound, a stream of thrills is shot out from its source, at the rate of about eleven hundred feet per second, and the series of air-waves is simply a succession of condensations and rarefactions of the elastic medium, in which the aërial particles successively take up and pass on the motions of the original impulse. According to the extent and complexity of these molecular motions is the intensity and quality of the noise. The size of the waves varies with the pitch of the sound; the first A of the bass in a piano producing air-waves about forty feet in length, while the waves of the last A of the treble are not quite four inches long. But these sound-waves are far more complex than at first seems, so that the motion of the air-particles involves something else than a mere backward-and-forward movement. A stretched string, vibrating its whole length, gives its fundamental tone, but while it thus swings as a whole, its different parts are thrown into separate and quicker vibrations, which are executed in harmonic ratio—2, 3, 4, 5, 6, 7, or 8 times— during the vibration of the whole string. These over-vibrations produce what are called overtones, which are, so to speak, drowned in the fundamental note, but which, nevertheless, serve to give it a peculiar character. It is thus that sounds from all sources acquire distinguishing marks, by which they are identified. It is by the effects of these frills, or fringes, of the larger waves, that different musical instruments, and different human voices, are distinguished from each other. The infinite varieties of sound are thus due to the subtile capacity of complex motion possessed by the air-particles. They always move exactly in the same way in the production of the same effects, and differently in yielding different effects. If we could see the dance of the air-particles when