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struction and use of this apparatus. In my descriptions of the experiments I have endeavored to be clear ; but in this I may have failed. If I have, I am sure that the experiments themselves are true, honest, and of good report, and will supply all the shortcomings of language, which, even from the best pens, gives but a weak and incomplete conception of an experiment.

In Chapter II. is given an account of the order of the experiments. These have been carefully selected, and arranged so that one leads to the next. Each experiment has been made by me over and over again, and the series has been performed before me by beginners in the art. I therefore know that they will all succeed if my directions are perseveringly followed. The experiments are numbered in order up to 130, so that they may be referred to from this work, and from the other books of the series.

Several of the instruments described are new, and many of the experiments are so pleasing in their action that they may be of interest to my scientific brethren, and to those engaged with college classes. I would refer to the instruments or experiments described in Experiments 1, 2, 17, 33, 34, 43 to 59, 61, 65, 66, 67, 68, 69, 70, 73, 74, 78, 79, 100, 104, 105, 107, 108, 110, 112, 121, 122, 125, 126, 127.

A lively interest has recently been excited in the subject of Sound by two of the most remarkable inventions of this century : Bell's Telephone, and the Speaking and Singing Phonograph of Mr. Thomas A. Edison. The first

named of these inventions will be described in the fourth book of the series ; the second I describe, with two excellent engravings, at the end of this volume.

The experiments have been completed for the remaining books of the series, which will appear in the following order (I. “Light;" II. “Sound,” already published): III.

Vision, and the Nature of Light ;” IV. “ Electricity and Magnetism ;” V. “Heat ;” VI. “Mechanics ; " VII. “ Chemistry ; " VIII. “ The Art of Experimenting with Cheap and Simple Instruments."

Mr. Barnard, who was associated with me in writing the book on “ Light,” found that his engagements did not permit him to continue his work on the series.

Since the publication of “Light” I have received the request, from various parts of the country, that I should make arrangements with some competent instrumentmaker, who will supply sets of apparatus to go with the books of the series. This I have done, and Samuel Hawkridge, instrument-maker to the Stevens Institute of Technology, Hoboken, New Jersey, will supply the sets of apparatus for “Light” and “Sound” at the rates given in his price-list at the end of this volume. The separate pieces of the apparatus for “Sound” are numbered to correspond to the numbers of the experiments in the book. By this plan the purchaser knows which pieces of apparatus go together, and is also informed of their uses. The student may find it cheaper to hunt up the materials, and then make his own apparatus ; but so

many have desired to have the sets ready for use that I have complied with their request. Of course it will be understood that the instrument-maker must be paid for the time taken in finding the objects in the market, and for the labor and skill spent in making the apparatus, and in packing it in convenient boxes.

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