4i6 THE POPULAR SCIENCE MONTHLY. 



sponding withdrawal of the vibrations from/', and a consequent still- 

 ing of tluit flame. 



With thinner calico or cambric it would require a greater number 

 of layers to intercept the entire sound ; hence, with such cambric we 

 should have echoes returned from a greater distance, and therefore of 

 greater duration. Eight layers of the calico employed in these experi- 

 ments, stretched on a wire frame, and placed close together as a kind 

 of pad, may be taken to rejiresent a very dense acoustic cloud. Such 

 a pad, placed at the proper angle beyond it, cuts off the sound which 

 in its absence reaches/', almost as effectually as an impervious solid 

 plate; the flame/' is thereby stilled, while / is far more powerfully 

 agitated than by the reflection from a single layer. With the source 

 of sound close at hand, the eclioes from such a pad would be of insen- 

 sible duration. Thus close at hand do I suppose the acoustic clouds 

 surrounding Villejuif to have been, a similar shortness of echo being 

 the consequence. 



A further step is here taken in the illustration of the analogy be- 

 tween light and sound. Our pad acts chiefly by internal reflection. 

 The sound from the reed is a composite one, made up of partial sounds, 

 differing in pitch. If these sounds be ejected from the pad in their 

 pristine proportions, the pad is acoustically lohite ; if they return with 

 their proportions altered, the pad is acoustically colored. 



In these experiments my assistant, Mr. Cottrell, has rendered me 

 material assistance. 



n. STATE OF THE RESEAECH.> 



In preparing this new edition of " Sound," I have carefully gone 

 over the last one, amended, as far as possible, its defects of style and 

 matter, and paid at the same time respectful attention to the criti- 

 cisms and suggestions which the former editions called forth. 



The cases are few in which I have been content to reproduce what 

 I have read of the works of acousticians. I have sought to make my- 

 self experimentally familiar with the ground occupied ; trying, in all 

 cases, to present the illustrations in the form and connection most 

 suitable for educational purposes. 



Though bearing, it may be, an undue share of the imperfection 

 which cleaves to all human effort, the work has already found its way 

 into the literature of various nations of diverse intellectual standinj?. 

 Last year, for example, a new German edition was published " under 

 the special supervision" of Helmholtz and Wiedemann. That men 

 so eminent, and so overladen with ofiicial duties, should add to these 

 the labor of examining and correcting every proof-sheet of a work like 

 this, shows that they consider it to be what it was meant to be a 

 serious attempt to improve the public knowledge of science. It is 

 especially gratifying to me to be thus assured that not in England 



> Preface to forthcoming third edition of " Sound." By John Tyndall, F. R. S. 



