ACOUSTIC ARCHITECTURE. 455 



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 audito- 

 rium, 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 audito- 

 rium 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 con- 

 tains 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 tho observer, was mov- 

 ing a slight, hardly perceptible breeze. The air was optically very 

 clear, and appeared to be rather dry. The click of the mowing-ma- 

 chine 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 dis- 

 tinguishable. 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 



