THE waves upon water are always objects of pleasing interest. From the ripples of the pond to the billows of the ocean, their beauty and their sublimity are sources of perennial inspiration to the poet and the painter. But there is an invisible realm of air-waves of a far subtler and more wonderful order. The water-waves belong to the sensuous eye and to art, but the aerial pulsations belong to the eye of the imagination and to science, the great revelator of the super-sensuous harmonies of the universe. Water-waves afford an agreeable spectacle, and have little further concern for us; but the waves of air take hold of our highest life, for the multitudinous sounds of Nature by which we are soothed and exhilarated, all the delights of music, the pleasures of speech, and the sweet experiences of social intercourse, are made possible only through their agency. Besides, air-waves form one link in the chain of agencies by which we pass from the material to the spiritual world. The first is the capacity by which matter may be thrown into vibration; second, the properties of air by which it can take up the impulses of vibration in the form of waves; third, those properties of the mechanism of hearing by which it can take up the motion of air-pulses; and, fourth, those properties of nerves by which they can take up the tympanic vibrations and translate them into feeling or consciousness. How the last step is effected we do not know, but many of the preliminary conditions to it are understood, and to some of these we ask the reader's attention.
All sound begins in those collisions and attritions among material things by which their parts are thrown into tremors. These are almost as various in quality as the properties of material substances. The sounds we hear are but indices to the vibrations of bodies from which they proceed, and the multitude of such terms as splash, roar, ring, thud, crack, whiz, squeak, crash, illustrate the marvellous diversity of characters which material vibrations may take. In the pro-