184 SOUND surfaces of the prongs of the fork are oppo- site the ear, that sound will be perceived ; but when the edges of the fork formed by the meeting of those surfaces are opposite the ear, it will be found that no sound, but entire silence, .occurs. This phenomenon is readily explained. First, it is necessary to know that the prongs of a vibrating fork alternately ap- proach to and recede from each other, as is readily seen when we obtain on a piece of smoked glass the trace of two delicate wires attached to the ends of the prongs of the vi- brating fork. A trace thus made is accurate- ly shown in fig. 18. When the prongs recede from each other, condensations will be pro- duced in the air opposite the faces c c' (see fig. 19, which represents a plan of the ends of the prongs) ; but while these condensations are thus formed rarefactions are produced in the air op- posite the opening between the prongs at r r'. The reverse of these actions occurs when the prongs approach each other. The result of the actions will be evident from the figure, where the full lines show the centres of shells
FIG. 19. of condensed air, and the dotted lines the cen- tres of shells of rarefied air. These shells al- ternate, and meeting along the planes p, p, p, p, passing through the vertical edges of the fork, they neutralize each other's action. W. "Weber has shown that the points of quiescence in this case must lie in hyperbolic sheets. This must be so, for the difference in the distance of every point of quiescence from two fixed points must be a constant quantity, which in this ex- periment will be equal to the half of the wave 1. -Mirth given by the fork. The writer has used this experiment of Young to show the reflec- tion of sound from flames and from sheets of cold and heated gases, such as carbonic acid gas and hydrogen. Two resonators were placed as in fig. 20 with the planes of their mouths at FIG. 20. a right angle, and in this angle was firmly fixed the fork to whose note the resonator resound- ed. The broad face of one of its prongs faced the mouth of one resonator, while the space between the prongs faced the mouth of the other resonator. By trial the two planes of the fork are placed at such distances from the resonators that complete interference of the vi- brations issuing from their mouths is obtained, and the only sound that reaches the ear is the faint one given by the action of the fork on the air outside the angle included by the mouths of the resonators. If in these circumstances we place before the mouth of one of the resona- tors a flat coal-gas flame, we shall find that this flame reflects part of the sound which falls upon it, and thus partially screens the resona- tor, so that sonorous vibrations of diminished intensity now enter this resonator, and there- fore the balance of interference no longer exists, and a sound issues from the resonator which has not the gas flame opposite its mouth. But if a piece of French tracing paper be placed before the mouth of the latter resonator, the balance of interference will be restored, thus showing that the reflecting power of a gas flame is equal to that of tracing paper. In a similar manner the writer has shown and ap- proximately measured the reflecting power of sheets of cold carbonic acid and hydrogen gas- es. Change of Pitch caused by Translation of the Sounding Body. One of the most remark- able phenomena is the change in pitch caused by the motion of a sounding body to or from the ear; or, what is the same, by the motion of the ear to or from the source of sound. When the sounding body and the ear approach, we per- ceive a rise in the pitch ; when they recede from each other, a fall in pitch occurs. This is a fact known to all who have listened to the rapid change in pitch of a locomotive whistle which
