NATURAL PHILOSOPHY. 211 



all his hearers than for himself, as twice the path of the voice and echo for 

 the speaker is considerably longer than the difference between the paths 

 of the direct and reflected sounds of one of the auditors. The direct echo 

 from the ceiling then becomes an advantage, by strengthening, without 

 confusing, the sound. But echo acts in still another way by being 

 repeated between opposite surfaces. The effect is like the multiplication 

 of the image of a candle between two opposite and parallel mirrors. I 

 have noticed it in the long, unfinished room of the Smithsonian, where 

 the sound produced by clapping the hands is repeated so as to resemble a 

 lauo-h ha ! ha ! ha ! In this case, the distance between the end walls is 



O 



such as to separate the successive echoes ; but when the walls are nearer 

 the sound becomes contiguous, and is the ringing sound often produced in 

 speaking in empty rooms, and called reverberation. This might trouble 

 us between the ceiling and floor of our room ; but a thick carpet, absorb- 

 ing sound, and not reflecting it, will remove this difficulty. The great 

 size of the room needed for the meeting of the House of Representatives 

 makes it impossible to bring the walls within the limit of perceptibility. 

 Professor Reid proposed, in the House of Parliament, to make the ceiling 

 high in the centre, declining towards the sides, with floors and galleries 

 rising from the centre towards the walls thus reducing the height and 

 surface of the walls so as to diminish the quantity of sound reflected 

 from them as much as possible. All regard for architectural beauty forbids 

 the adoption of this construction, which seems to have been modelled upon 

 an empty tortoise-shell. Breaking the walls into deep panels has also 

 been proposed. But when we recur to the limit of perceptibility, we shall 

 perceive that a panel or recess must be over thirty feet deep to separate the 

 echoes from the bottom of the recess and from the face of the wall. The 

 surfaces of mountains covered with trees and rocks return echoes. Xo 

 wall of an inhabited apartment can be made rougher than these natural 

 reflectors. A simpler and more effectual method of controlling the echoes 

 from the walls will be, to cover them with drapery absorbent of sound. 

 The echoes from the ceiling are thus turned to account, and those from the 

 floor and walls guarded against ; but the echoes from small objects and 

 surfaces may still be troublesome unless provided for. The trunks of 

 trees in the edge of a forest return together a distinct echo. The beams 

 under the flooring of the Menai Suspension Bridge are instanced by Her- 

 schel as giving a curious echo ; and even such small objects as the vertical 

 iron rods composing the fence in front of the President's House will be 

 found to return a singular whistling echo to the sound made by striking a 

 smart blow upon the pavement on a still night. To guard against this, it 

 will be sufficient to cushion the chairs and cover the desks with some 

 material which will not reflect sound. This may seem, and rnayVpossibly 

 be, an unnecessary precaution ; but I wish to leave no possible cause of 

 confusion unnoticed, and to point out what I consider the means to obtain, 

 for the first time, a room perfect in its acoustic arrangements. 

 Having thus disposed of the question of echoes and reverberations, the 



