460 THE POPULAR SCIENCE MONTHLY. 



While a plain blank wall opposite the speaker will throwback a strong 

 reverberation, if this wall be broken up by recesses, or spaced with 

 pilasters, or if a gallery be extended across it, the reverberation will 

 be much less. 



The lining of the walls should, if possible, be placed at a consid- 

 erable distance from the main wall, and supported by it so as to allow 

 as free vibration as possible. Thus when other considerations require 

 that the walls of a building shall be of stone or brick, the acoustic 

 qualities may be recovered by lining up with thin pine, corrugated 

 iron, or a thin coating of plaster on light laths, and susjoending this 

 lining as lightly as possible and at a considerable distance from the 

 solid wall. The arrangement of walls in panels is a very advanta- 

 geous one, as each panel may be so constructed as to be easily set in 

 vibration. Perhaps the ideal arrangement would be an auditorium 

 with brick walls, within which is a shell made up of thin wooden 

 panels, and placed at such a distance from the solid walls that the pas- 

 sage-ways to the entrances on the floor and various galleries maybe 

 placed between. 



In this connection it becomes necessary to discuss the proper shape 

 to be given to an auditorium. A good way to arrive at this is to con- 

 sider first an audience in the open air on a calm day. The open air 

 on a quiet day, when the atmosphere is not disturbed by convection- 

 currents, is probably the best possible place for speaking to a large 

 number of people. Wesley is said to have preached successfully to 

 twenty thousand people gathered together in a natural amphitheatre 

 formed by the hills, and on a day when the atmosphere was at rest. 



Let us take a small platform arranged for an open-air speaker, and 

 notice how an audience will form itself about it. If the audience is 

 large and each person anxious to hear, we shall find that the outline 

 of the crowd will be that of a section cut through an egg, with the 

 speaker placed at the focus of the smaller end. As in an auditorium 

 we trust to the natural diffusion of sound to absorb the stray rays, we 

 should evidently adopt this same ovate section for the outline of the 

 floor. 



But, when we come to consider further that it is desirable to place 

 each member of the audience so that he can see the speaker, and so 

 that the speaker's voice may come directly to him, we see that we con- 

 form still further to the egg-shape, for, in order that we may do this, 

 the floor must curve upward as it recedes from the speaker, and the 

 galleries form only a continuation of this curve. To that we may say 

 that the proper shape of an auditorium is in general that of an egg- 

 shell, the speaker being at the focus of the smaller end and the audi- 

 ence being seated over the lower half, while the upper half forms the 

 vaulted roof. Like an egg-shell, we have seen that the walls should be 

 thin and capable of absorbing, as fully as possible, all the stray rays 

 of sound. While the egg-shape is the ideal, other considerations fre- 



