VELOCITY OF SOUND.

sound, as a burning stick whirled round before the eye forms a circle of light. That the sound may be a single one, nearly sixteen separate sounds must follow

one another every second, and when these are exactly similar, and recur at equal intervals, they form a musical sound. Of no consequence is it to the production of a tone in what way the pulses of the air are caused, provided they follow with sufficient regularity.

VELOCITY OF SOUND.

The sounds of a stick pulled along a grating, are not tones, only because the pulses follow too slowly. And whenever a continued sound is produced by impulses which do not follow in regular succession, like those of an elastic body, the effect is called a noise.

The velocity of sound is far inferior to that of light. The axe of the woodman working on the hill, is seen to fall a considerable time before the sound of the stroke is heard. In like manner the flash of a gun precedes the report. Sound travels at the rate of 1142 feet per second, or a mile in about four seconds and a half, varying little with the density or temperature of the air. The pulse at the wrist of a healthy man is a convenient measure of time for ascertaining distances by the progress of sound. Each beat marks nearly a second, and therefore indicates a distance of nearly a quarter of a mile.

A wave of water turns back at any obstacle, so that it appears at any distance after the reflexion what it would have been at the same distance beyond the wall, only moving in an opposite direction. In like manner, the pulses or waves of sound are regularly reflected from flat surfaces, and produce what is called an echo. The rapidity with which it is returned to the spot where the sound originates, depends on the

EXTRAORDINARY ECHO.

distance of the reflecting surface; and the latter may therefore be determined by the former.

Some echoes are very remarkable. One of these is at the palace of a nobleman near Milan. About one hundred paces before the mansion a small brook glides gently; and over this is a bridge, forming a communication between the palace and the garden. On firing a pistol from this bridge, fifty-six vibrations of the report have been heard. The first twenty were distinct, but in proportion as the sound died away, and was answered at a greater distance, the repetitions were so doubled that they could scarcely be counted, the principal sound seeming to be saluted in its passage by reports on either side at the same time.

In concluding these remarks, the importance of the sense of hearing, as a means of improvement, demands particular attention. How much valuable knowledge have we thus received! It is only for us to visit those who are deaf, especially those who were born so, to have a lesson, which should excite our gratitude to God for granting to us, and continuing in exercise, so precious a power. And well may our sense of obligation be increased, as we observe the words of the apostle Paul: "Whosoever shall call upon the name of the Lord shall be saved. How, then, shall

FAITH COMETH BY HEARING.

they call on him in whom they have not believed? and how shall they believe in him of whom they have not heard? and how shall they hear without a preacher ? and how shall they preach, except they be sent ? As it is written, How beautiful are the feet of them that preach the gospel of peace, and bring glad tidings of good things! But they have not all obeyed the gospel. For Esaias saith, Lord, who hath believed our report? So, then, faith cometh by hearing, and hearing by the word of God." Rom. x. 13—17.

Reader! have you so heard the gospel, as to have this faith? It is not yours, unless, under a conviction that as a sinner you are lost, you have fled to the only Saviour to put your whole trust in Him. Should you have delayed the exercise of this confidence till now, defer it no longer, for all must perish who have not an interest in Christ; and our life is but "a vapour, that appeareth for a little time, and then vanisheth away."