On striking a tuning-fork briskly on a bard substance, other notea besides the fundamental note may sometimes be heard. This arises from nodal divisions in each limb, after the manner of strings and tubes [PIPE], but not following quite the same laws. Where there is ono node it is at a distance from the fixed point, a little greater than two thirds the length of the prong; the note then given by this, which is called the second mode, is much sharper than when there is no node (or by the first mode), and the relative number of vibrations is 7. A still higher note corresponds to two nodes (third mode), and one still higher to three nodes (fourth mode). These subdivisions of vibrating rods may also be produced in tubes. The squares of the odd numbers 3, 5, 7, 5:e., represent with sufficient exactness the relations of the number of vibrations corresponding to the second mode, third mode, fourth mode, &e. Under all these circumstances it may be stated: 1. That the number of vibrations of similar springs of different lengths is in the inverse ratio of the squares of the lengths ; 2. That the number of vibrations of springs of the same length, but of different thicknesses, is proportional to their thicknesses; 3. That the width of a spring has no influence on the number of its vibrations, provided it be small with respect to Its length, and that it vibrates after the first mode ; 4. That springs of equal size, but of different material, such as wood, glass, steel, &c., do not give the same note, because the number of the vibrations depends upon the density and rigidity of matter.
A tuning-fork may be made to furnish several beautiful illustrations of interference. [1:vreneensercs.] If a vibrating tuning-fork be held
to the ear and turned gradually round, the sound will increase and diminish in a remarkable manner; when the prongs are presented at an angle of 45* the sound is scarcely if at all perceptible. In such eases, when the prongs coincide, or are equally distant from the ear, the waves of eound combine their effects, whilst in intermediate positions they reach the ear in different phases, and interfere and produce total or partial silence. A similar effect is produced by fixing the fork to the mandril of a lathe, the length of the fork coinciding with the axis of motion; if the fork be vibrated no sound will be beard while it is rotating. If a vibrating tuning-fork ho held with its handle obliquely in contact with the table, the resonance of the table is beard while the fork is at rest; but if the fork be moved parallel to itself along the surface of the table, the resonance ceases, from the interference of the planes of vibration with each other. The moment the handle is brought to rest, the resonance is again heard. If the tuning-fork be held vertically, the resonance Is not interrupted by moving it about, since the planes of vibration coincide. If a vibrating tuning-fork be held over a cylindrical glass vessel of suitable length, the air in the glass will be made to vibrate and produce a tone. If a second glass cylinder be held at right angles to the first, so that the respective openings of the two vessela form a right angle, the musical tone pre viously heard will cease, but will sound again on removing the second versed, an effect which arises from the interference of the vibrations of the air in the two vessels.