Page:Popular Science Monthly Volume 7.djvu/432

 withdrawal of the vibrations from f', and a consequent stilling of that 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 experiments, stretched on a wire frame, and placed close together as a kind of pad, may be taken to represent a very dense acousticaccoustic [sic] cloud. Such a pad, placed at the proper angle beyond it, cuts off the sound which in its absence reaches f', almost as effectually as an impervious solid plate; the flame f' is thereby stilled, while f is far more powerfully agitated than by the reflection from a single layer. With the source of sound close at hand, the echoes from such a pad would be of insensible 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 between 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 white; if they return with their proportions altered, the pad is acoustically colored.

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

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 criticisms 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 myself 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 standing. 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 official 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