Tuning-Fork is a rod of metal bent into two prongs and supported on a handle, fixed at the bend. The fork can be thrown into a state of vibration by hitting it sharply against something hard or by drawing a violin bow across its prongs. The vibration can be seen if the fork be carefully observed, and the motion it sets up in the air can be detected by the hand. A sound is produced as the fork vibrates, and, the vibration becoming less and less in amplitude, the sound gradually dies away, the same note being, however, maintained since the number of vibrations per second remains unchanged, the fork merely moving a shorter distance to and fro as time goes on. The motion of the fork is beautifully shown by Lissajon's figures (q.v.), and the number of vibrations it makes per second can be experimentally determined by attaching a style to one prong and letting the style just touch a piece of smoked glass, which is allowed to move onwards with a certain known velocity. The number of little undulations made on a given length of the smoked glass [SOUND] measures the vibrations of the fork, and therefore gives the pitch of the note. [PITCH.] When the fork gives its lowest or fundamental note, there are two nodes (q.v.) in the curved portion, and the free ends oscillate about these fixed points. When the fork gives its first overtone there are two other nodes - one in each prong - as well as the two in the bend. [SOUND.] The position of the nodes can be determined by Chladmi's method of placing fine sand on the fork and noticing the points at which it accumulates when the fork is made to vibrate. It is a curious fact that two tuning-forks may be in perfect unison when they produce their fundamental note, but their first overtones may be audibly discordant, and vice versa. Helmholtz discovered that the number of vibrations of the first overtone was not a constant multiple of the number given by the fundamental. The vibrations of the overtone may be rather more or rather less than six times those of the fundamental. Tuning-forks are used to regulate the pitch of the notes of musical instruments. One fork, usually giving the note [??image??] or [??image??], is used to test the accuracy of the corresponding note on an instrument; a musician generally adjusts the rest by ear.