462 THE POPULAR SCIENCE MONTHLY. 



when the focus is so distant that a strong echo is returned from the 

 opposite wall. Their object is rather to control such rays of sound as 

 would otherwise go up to the roof and be retained as a disagreeable 

 echo. A perfectly flat surface placed over the pulpit and not too far 

 from the speaker's head, will often do this ; but it is better to have the 

 wall back of the pulpit gradually curve forward until it completely 

 overhangs the speaker's head. The speaker must be near to this wall, 

 so that the direct and reflected sounds may be as nearly as possible 

 identical. 



There is a belief, prevalent among some architects, that a hall, in 

 order to be acoustically good, must have its length, breadth, and height 

 in harmonic proportion. There seems to be no good foundation for 

 this belief, and the author, after careful experimental inquiry, has 

 failed to find that this is the case. 



It frequently happens that, in a building in which there is consid- 

 erable resonance, the speaker, by timing his syllables so that the reso- 

 nance of one shall have disappeared before the next is uttered, may 

 make himself understood to a large audience with comparative ease. 

 This is recognized by most public speakers, and it is not uncommon 

 to hear them speak of the "key-note" of any particular hall. 



The act of striking the " key-note " consists not so much in pitch- 

 ing the voice at any particular key as in carefully timing the rate at 

 which the syllables succeed each other. 



+++- 



PEOGEESS OF THE GEEM THEOEY OE DISEASE* 



By JOHN TYNDALL, F.E.S., LL.D. (M. D., TUBINGEN). 



THE virtual triumph of the antiseptic system of surgery, based as 

 that system is on the recognition of living contagia as the agents 

 of putrefaction, is of good augury as regards the receptivity of the 

 public mind to new views respecting the nature of contagia generally. 



To the credit of English surgeons it stands recorded that, guided 

 by their practical sagacity, they had adopted in their hospitals meas- 

 ures of amelioration which reduced, almost to a minimum, the rate of 

 mortality arising from the "mortification" of wounds. They had dis- 

 covered the evils incident to " dirt " ; and, by keeping dirt far away 

 from them, they had saved innumerable lives, which would undoubt- 

 edly have succumbed under conditions prevalent in many of the hos- 

 pitals of Continental Europe. 



In thus acting, English surgeons were, for the most part, "wiser 

 than they knew." Their knowledge, however momentous in its prac- 



* Introductory Note reprinted from Professor Tyndall's " Essays on the Floating Mat- 

 ter of the Air, in Relation to Putrefaction and Infection." 



