Elements of Physics, or Natural Philosophy (Paperback)

This historic book may have numerous typos, missing text or index. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. 1829. Not illustrated. Excerpt: ... than in air. The blow of a hammer given to a wall by a person at one end, may be heard twice by a person at the other, viz. almost immediately by an ear applied to the wall, and a little after by an ear listening through the air. " Reflection of sound." (Read the Analysis.) As a wave of water turns back at a smooth wall or obstacle, so that at any distance after the reflection, it appears what it would have been at the same distance beyond the wall, only moving in an opposite direction; so the pulses or waves of sound are regularly reflected from flat surfaces, and produce what is called an Echo. Such flat surfaces of nature's works are found only among the rocks and hills; and hence the beautiful fiction of the ancient poets, that Echo was a nymph who dwelt concealed among the rocks. Science has now disclosed the secret of the viewless Echo; but who does not vividly recollect the wonder and delight with which he has listened, in the morning of his days, to his shrill call returned to him from some bold precipice, across the plain or river, or perhaps sent down to him again from the vaulted roof of ocean's caves The quickness with which an echo is returned to the spot where the sound originates, depends of course upon the distance of the reflecting surface; and, as sound travels 1,142 feet in a second, a rock at half that distance returns a sound exactly in one second. The number of syllables that can be pronounced in a second will, in such a case, be repeated distinctly, while the end of a longer story would mix with the commencement of the echo. The breadth of a river may easily be ascertained where there is an echoing rock on the farther shore. A perpendicular mountain's side, or sublime cliffs, such as in many parts skirt the British coasts, return an au...