Page:Popular Science Monthly Volume 21.djvu/469

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upon its acoustic qualities, and it will be shown, as would naturally be expected, that the condition of the medium which conveys the sound exercises a very considerable influence on the facility and accuracy with which the sound is conveyed. In the second part is considered the effect of the arrangement of the walls which inclose the auditorium, and of the materials of which these walls are composed. In the third and last part there are discussed several minor points, attention to which will aid in the securing of a building which shall be good and not bad for sound. These three headings, it is believed, will cover the whole ground.

It seems almost self-evident that the condition of the air, which is the medium by which sound is conveyed from one part of an auditorium to another, will exercise a considerable effect upon its acoustic properties. An experimental inquiry shows us that such is the case. What peculiar condition of the air is it that affects the transmission of sound? Whether the air is hot or cold, wet or dry, whether it contains a larger or smaller percentage of oxygen, nitrogen, or carbonic acid, seems to have no effect on its acoustic properties. But whether the air is quiet and mechanically homogeneous, or whether there are mingled draughts of hot and cold air moving in various directions, does seem to have a considerable effect. In other words, the motion of the air within an auditorium does have a very perceptible effect on its acoustic qualities.

Probably most readers have noticed, or in any event they have seen recorded, instances in which sounds of very ordinary intensity have been heard, and heard distinctly, at a very considerable distance from the source. In particular, the author remembers an instance which came to his notice one summer afternoon, while resting half-way up the side of one of the hills near the Green Mountains. The hill sloped gently to a meadow at the foot, and its sides curved somewhat like the walls of an amphitheatre, of which the meadow was the floor. Nearly a mile away, across the meadow, a man was mowing grass with a mowing-machine drawn by horses. The day was slightly cloudy, and from the mower, up the side of the hill to the observer, was moving a slight, hardly perceptible breeze. The air was optically very clear, and appeared to be rather dry. The click of the mowing-machine was heard with wonderful distinctness, but the author was not a little surprised when, the machine having stopped for the moment, the "Go along!" of the driver was plainly heard as he urged his horses on. Not only were the words plainly intelligible, but the provincial twang peculiar to the country-folk of that region was distinctly distinguishable. Here the human voice, raised probably very little above the ordinary tone, and not at all above that of a preacher in his pulpit or an actor on the stage, was distinctly heard, and with all its peculiarities of quality, nearly a mile away.

Another instance is that mentioned by Sir John Ross in his account