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66                      HAND-BOOK OF ACOUSTICS.
serted in the ear. Helmholtz caused this nipple to fit closely into the aural passage, by surrounding it with sealing wax, softening the latter by heat, and then gently pressing it into the ear. The resonator when thus used, has practically only one opening. In using these instruments, one ear should be closed, and the nipple of the resonator inserted in the other. On listening thus to simul­taneous sounds of various pitches, most of them will be damped; but whenever a sound occurs of that particular pitch to which the resonator is tuned, it will be wonderfully reinforced by the co-vibration of the air in the resonator. In this way, anyone, even though unpractised in music, will readily be able to pick out that particular sound from a number of others. When, from the faint-ness of the sound to be detected, or from some other cause, any difficulty in hearing it is experienced, it is of advantage to alternately apply the resonator to, and withdraw it from the ear
A resonator, which is capable of being tuned to any pitch within the compass of rather more than an octave, has been used for some years by the writer. It is composed of three tubes of brass, sliding closely within one another. The innermost, fig. 38a, which is
c                 i                 «
Fig. 38.
about four inches in length, and an inch or more in diameter, is closed at one end by a cap which is screwed on to the tube. In the centre of this cap is an aperture, about half an inch in diameter, which is closed by a perforated cork, through which passes a short piece of glass tube, the end of which is fitted to the ear. The resonator is thus a closed one, and its length can be increased by means of the sliding tubes, b and c (which are each about 4 inches ]:ng), from about 4 inches to 12 inches. It is best tuned approxi­mately, by first calculating the length of stopped tube corresponding to a certain note, according to the method explained in the present