70. When waves of sound meet any fixed surface tolerably smooth, they are reflected according to the law of equal angles of incidence and reflexion. In this way echoes are produced. Between two parallel surfaces a loud sound is reflected backwards and forwards, and several echoes are audible. When the parallel surfaces are much nearer together, as the walls of a room, although a larger number of echoes are produced, yet they follow each other too rapidly to be distinguished, and, as they arrive at the ear after equal intervals, they produce a musical note, however unmusical the original noise may have been. Hence all the phenomena of reverberation. pitch of the note depends solely on the distance of the two walls that produce it, and may be calculated therefrom.

The

A noise may also produce a musical echo by being reflected from a large number of equidistant surfaces receding from the ear, so that the sound reflected from each surface may arrive successively at equal intervals. Thus a shrill ringing will be heard on stamping near a long row of palisades. A fine instance of the same kind is said to occur on the steps of the great pyramid. If the distance from edge to edge of each step were two feet one inch, the note yielded would be the tenor C, beause each echo having to go and return would be four feet two inches later than the previous one, which

second of time than if he were at rest, in the proportion which his velocity bears to the velocity of sound. But, if he move from the sounding body, he will meet a smaller number in that proportion. In the former case he will hear the sound a semitone higher, and in the latter a semitone lower than the observer at rest. In the case of two trains meeting at this velocity, the one containing the sounding body and the other the observer, the effect is doubled in amount. Before the trains meet the sound is heard two semitones too high, and, after they pass, two semitones too low, being a difference of a major third.

SYMPATHETIC VIBRATION.

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is the length of the waves of that note. But as the steps gradually diminish in size upwards, the echo, if produced and heard at the bottom, must gradually rise in pitch.

These facts explain the principle of those windinstruments in which there is no vibrating solid. The vibrating body is not the pipe, but the air contained in it. An agitation produced in the air at one end of the pipe is communicated to the other end, and reflected backwards and forwards from end to end, producing isochronous (or equal-timed) impulses, the frequency of which depends entirely on the length of the pipe. Hence all organ pipes of the same length yield the same note as to pitch, its quality only being affected by the form or material of the pipe. It will thus be at once perceived that the lowest C (the waves of which are 64 feet long) requires a pipe of 32 feet to produce it; and that all notes from this, to the shrillest whistle, are easily calculated, by dividing 550 feet by the length of the pipe.

71. Some very remarkable effects of sound are produced in the phenomena of sympathetic vibration, numerous instances of which must be familiar to the reader. If a flute be sounded in the same room with a piano, the notes of the flute will cause some of the wires to vibrate. The waves of sound set in motion by the flute produce motion in those very wires which yield the same notes as are being played. If the voice be pitched to the same note as that yielded by a glass goblet, the former will set the latter ringing. And to take a still more remarkable instance, if two pendulum clocks standing against the same wall be both wound up, and one set going while the other is at rest, the small vibrating impulses of the going clock will gradually communicate motion to the pendulum at rest,

and in the course of some hours the latter will be found in full swing.

72. It will be seen from the foregoing survey, that the atmosphere, like a vast and complicated machine, has a variety of movements, the result of omniscient design. Some of the latest and most valuable discoveries respecting the motions of the aerial currents beautifully illustrate a passage in Scripture: "The wind goeth towards the south, and turneth about unto the north; it whirleth about continually, and the wind returneth again according to his circuits."* Amidst all the mutations of science it is cheering to the student to notice, that no sooner is a natural law brought to light and fairly established than we immediately perceive its harmony with sacred truth. Whatever discrepancies may seem at present to exist arise from the imperfection of our knowledge of natural laws, and will, as science advances, gradually be cleared

away.

* Eccles. i. 6.