234 ACOUSTICS APPLIED TO PUBLIC BUILDINGS. 



The above illustration exhibits a perspective view of the lecture- 

 room from the west side under the gallery. 



The speaker's platform is placed between two oblique walls. The 

 corners of the room which are cut off by these walls afford recesses 

 for the stairs into the galleries. The opposite corners are also -par- 

 titioned off, so as to afford recesses for the same purpose. The ceiling 

 is twenty-five feet high, and, therefore, within the limit of percepti- 

 bility. It is perfectly smooth and unbroken, with the exception of an 

 oval opening nearly over the speaker's platform, through which light 

 is admitted. The seats are arranged in curves, and were intended to 

 rise in accordance with the 2^anoptic curve, originally proposed by Pro- 

 fessor Bache, which enables each individual to see over the head of 

 the person immediately in front of him. The original form of the 

 room, however, did not allow of this intention being fully realized, 

 and therefore the rise is a little less than the curve would indicate. 



The walls behind the speaker are composed of lath and plaster, and 

 therefore have a tendency to give a more intense, though less pro- 

 longed sound than if of solid masonry. They are also arranged for 

 exhibiting drawings to the best advantage. 



The general appearance of the room is somewhat fan-shaped, and 

 the speaker is placed as it were in the mouth of an immense trumpet. 

 The sound directly from his voice, and that from reflection immedi- 

 ately behind him, is thrown forward upon the audience ; and as the 

 difference of distance travelled by the two rays is much within the 

 limit of perceptibility, no confusion is produced by direct and reflected 

 sound. 



Again, on account of the oblique walls behind the speaker, and 

 the multitude of surfaces, including the gallery, pillars, stair-screens, 

 &c., as well as the audience, directly in front_, all reverberation is 

 stopped. 



No echo is given off from the ceiling, for this is also within the 

 limit of perceptibility, while it assists the hearing in the gallery by 

 the reflection to that place of the oblique rays. 



The architecture of this room is due to Captain Alexander, of the 

 corps of topographical engineers. He fully appreciated all the prin- 

 ciples of sound which I have given, and varied his plans until all the 

 required conditions, as far as possible, were fulfilled. 



