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sion of sound. Nearly 200 degrees of heat are thus developed by the mechanical action of the sound-waves.
Other gases than atmospheric air conduct sound with greater or less facility. Solids and liquids conduct it with much greater velocity than air. According to the experiments of M. Colladon, in 1826, sound travels through water at the rate of 4707 feet in a second, which is only ten feet more than the calculated velocity; so that but little heat is developed by water in transmitting sound. In his experiments M. Colladon used the bell as a sounding body, which was struck about three feet below the surface of the water, and he was enabled to hear it across the width of the Lake of Geneva, a distance of about nine miles.* The soundwave passes through iron at the rate of about 17,000 feet in a second, and the various species of wood range from 11,000 to 17,000 feet. If an iron rod connected the moon to the earth it would require about two days and a half for the effect of a force applied at one end to reach the other.
The successive transmission of sound gives rise to some curious phenomena. Two persons placed at unequal distances from the source of sound cannot hear it at the same time. It is on account of this principle that in long ranks of soldiers where two bands of music are placed at a considerable distance from each other, it is impossible for the two bands to agree in keeping time. It is often noticed “that if from an eminence we look upon a long column which is marching to a band of music in front, the various ranks do not step exactly together. Those in the rear are in each step a little later than those before them. This produces a sort of undulation in the whole column, which it is difficult to describe, but which all who have noticed it well understand.”+ This is owing to the fact that all do not hear the music when it is made, but after the lapse of a time sufficient for sound to pass the interval between the music and the soldier, at the rate of 1125 a second, the velocity of sound at the ordinary temperature.
Two sounds may be so related to each other that they
*Bartlett's Acoustics, p. 42. Chamber's Information for tho People, Vol. I, p. 210.
produce silence. This is caused by the interference of the .sound-waves. If we throw two stones into the water near each other, there will be seen a line where the waves meet in such a way as to destroy one another, along which the water will be calm. Sonnd-waves may produce similar effects, and at such points no sound can be heard. Sound-waves may partly destroy one another, and very much weaken the sound; or they may operate together and increase the sound. Mixed gases, owing to the different lengths of the waves in each, may be bad conductors of sound. The gases, however, which compose atmospheric air, are so related as to produce no such effect.
An echo is one of the most interesting phenomena in acoustics. It is produced by the reflection of the soundwaves, as waves in water are reflected from smooth surfaces. A rough surface is a bad reflector of sound, as of light, because the irregularities reflect it in so many different directions, that the various waves interfere with one another. The velocity of reflected sound-waves is the same as that of direct, so that we can easily determine the time required for an echo to reach us, when we originate the sound, by considering that the sound has to pass over twice the distance between us and the reflecting surface at the rate of 1,125 feet a second. The number of syllables that we can hear distinctly echoed will depend on the rapidity with which we pronounce them, and the distance of the reflecting surface. If the reflecting surface be near, the echo and the direct sound will reach us so nearly together that the former will strengthen the latter. There is a limit to this, however, and it is evident that a speaking hall should be constructed in accordance with this principle. In large rooms used for speaking purposes, all echoes which can accompany the voice of the speaker, syllable by syllable, are useful for increassing the volume of sound; but all that reach the hearers sensibly later only tend to produce confusion. Professor Henry found by experiment that if a sound and its echo reach the ear within from one-fifteenth to one-twentieth of a second, depending upon the nature of the sound, they seem as one. This is called the limit of perceptibility. This gives us for the distance of the reflecting surface from thirty to forty feet. *
If we take the mean of these numbers as the usual limit, we see that if a lecture hall have its walls farther from the speaker than thirty-five feet, some arrangement must be made to prevent the echo from interfering with the distinctness of the direct sound. Those who are within thirty-five feet of the echoing surface will hear the speaker more distinctly than without its effect; but others will find it a disadvantage. Distant walls should be broken up into small portions, presenting surfaces in different directions. This may be effected in various ways. There may be a gallery with the seats and the floor rising rapidly behind one another, so that much of the sound which would otherwise reach the remote wall, will be caught directly by the hearers. “Especially should no large and distant surfaces be parallel to nearer ones, since it is between parallel walls. that prolonged reverberation occurs.”
Walls intended to aid a speaker by their echoes should be: smooth, but not too solid. It is found that plaster on lath. is better than plaster on brick or stone; the first echo is louder and the reverberations less. Drapery behind a speaker deprives him of just so much echoing surface. A lecturing hall is improved by causing the wall behind the speaker to change its direction to the right and left of the speaker, at a very obtuse angle, so as to exclude the rectangular corners from the room. The voice is in this way reinforced by reflection, and the resonance arising from parallel walls is in a measure avoided. The ceiling should not be too high ; and concave surfaces, generally, should be avoided. “An equal diffusion of sound throughout the apartment, not concentration of it to particular points, is the object to be sought in the arrangement of its parts.”.
In Chamber's "Information for the People,” it is said that “the best known form of apartment for the proper distribution of sound, is that in which the length is from
*An. Sci. Dis. for 1852, pp. 153-4.
+ See a paper on “ Acoustics applied to Public Buildings, by Professor Joseph Henry, in the Proceedings of the An. Assoc., Albany, 1856.
Vol. i., p. 210.
a third to a half more than the breadth, the height somewhat greater than the breadth, and having a roof bevelled off all round the sides. This species of ceiling, called, technically, a coved or coach roof, from its being lower at the sides than centre, is in all cases best suited for conveying sounds clearly to the ears of auditors.”
The principles of acoustics are well understood, but they are too seldom applied to the construction of speaking rooms. In many instances costly assembly halls and churches are very defective in regard to public speaking. The fancy of the architect seems rather to be consulted in their construction, than scientific principles. “The subject urgently demands consideration in connection with architecture."*
Reflecting surfaces may be so related to one another as to cause multiple echoes. At Srevley-Fels, on the Rhine, is a position in which a sound is repeated by echo, seventeen times. At the Villa Sinsonetta, near Milan, is another where it is repeated thirty times.t In Woodstock Park there is an echo which repeats seventeen syllables by day and twentyeight by night. Sir John Herschel mentions an echo in the Manfroni palace at Venice, where a person standing in the centre of a square room, about twenty-five feet high, with a concave roof, hears the stamp of his feet repeated a great many times, but as his position deviates from the centre the echoes become feebler and at a short distance they entirely cease.
If a sound be made in one focus of an ellipsoidal surface, it will be reflected to the other focus. Whispering galleries are constructed upon this principle. The ear of Dionysius is celebrated in ancient history. It was a grotto cut out of the solid rock at Syracuse, in which a person placed at one point could hear every word, however faintly uttered, in the grotto.
Lastly, we may refer to the moral effect of sounds, especially very loud sounds. The red man hears the voice of the Great Spirit speaking from out the cloud when it thunders; and a similar feeling, mingled with some fear, has a tendency to prevent man from wrong doing when the thunder roars, or when he hears such subterraneous sounds as accompany earthquakes.
Brewster's Natural Magic, p. 205.
* Chamber's Encyclopædia, vol. I, p. 32. 1 Bartlett's Acoustics, p. 89.
ART. V.-1. Report : Orange Lodges, Associations or Societies in Ire
land. Ordered by the House of Commons to be printed, · July, 1835. 2. Report : Orange Institutions in Great Britain and the Colonies.
Ordered by the House of Commons to be printed, Septem
ber, 1835. 3. Reports from the Select Committees appointed to inquire into the
nature, character, extent, and tendency of Orange Lodges, Associations, or Societies in Ireland, with the Minutes of evidence, Appendix and Inder. Ordered by the House of Commons to
be printed, 1835. 4. Mr. Hume's Speech in the House of Commons, Feb. 23, 1836. 5. Battle of Magheramayo. Newry : James Henderson, 1849. 6. Mr. Berwick's Report to the Lord Lieutenant of Ireland, on the
occurrences which took place in the District of Castlewellen, on the occasion of an Orange Procession, on the 12th of July, 1849. Dublin Evening Post, 17th October, 1849.
A LARGE proportion of those who inflict the most serious injuries on their neighbors do so under the impression that they are doing right; hundreds are guilty of conduct as members of a faction or society from which they would shrink with horror as individuals. The moralist as well as the philosopher should take this into account in estimating the amount of violence or depravity manifested in'any particular community. Even those who are injured are bound to take into consideration the motives of their assailants. It is true that if one's leg or arm is broken, for his house burned, his pain or loss is not diminished by learning that his assailant was prompted by erroneous motives. Nevertheless no