“In all longitudinal vibrations,” says the same writer, “the tones depend merely on the length of the sonorous body, and on the quality of the substance, the thickness and form being of no consideration; yet the tones are not varied by the specific gravity of the vibrating substance; for fir-wood, glass, and iron, give almost the same tone as brass, oak, and the shanks of tobacco-pipes.” He also mentions several kinds of longitudinal vibration; in one, to use his own words, “there is a certain point in the middle at which the vibration of each half-stops; in the next there are two, each at the distance of a fourth part from the end; and, in the following, there are three, or more. The tones correspond with the natural series of the numbers 1, 2, 3, 4, etc. If a rod be fastened at one end, during the first kind of longitudinal vibration, the alternate expansion and contraction of the whole rod will take place in such a manner, that they stop at the fixed end; in the next tone there is a resting-point at the distance of one-third from the free end; and in the following there are two. The tones correspond with the numbers 1, 3, 5, 7, and the first of these tones is an octave lower than the first tone of the same rod when perfectly free.” When examining the nature of sonorous bodies, Dr. Chladni imagined the possibility of producing musical sounds by rubbing glass tubes longitudinally. It, however, became a difficult question to determine in what way an instrument of this kind should be constructed. After much and long-continued unsuccessful thought, he returned home one evening exhausted with walking, and he had scarcely closed his eyes to fall asleep in his chair, when the arrangement he had so long been seeking, occurred to his mind. He soon after completed an instrument, which in every respect answered his expectations. The euphone, signifying an instrument having a pleasant sound, consists of forty-one fixed and parallel cylinders of glass, equal in length and thickness. In its external appearance it resembles a small writing-desk, which, when opened, presents a series of glass tubes about sixteen inches long, and the thickness of a quill. They are fixed in a perpendicular sounding-board, at the back of the instrument. When used, the tubes are wetted with a sponge, and stroked in the direction of their length with wet fingers; the intensity of the tone being varied by greater or less pressure.