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To know how the various sounds of Nature and of music are made ; to understand the action of the mechanical contrivances in our throat and ears, with which we speak and hear ; to be able to explain the cause of the different tones of musical instruments ; to know why certain notes sounded together give harmony, while others make discord : such knowledge is certainly valuable, curious, and interesting. You may read about these things, but a better way is to study the things themselves, by making experiments, and these experiments will tell you better than books about the causes and the nature of sounds.

To make an experiment means to put certain things in relation with certain other things, for the purpose of finding out how they act on each other. An experiment is, therefore, a finding out.

It is the aim of this book to show you how to construct your own apparatus out of cheap and common things, and to aid you in becoming an experimenter. The student should, with patience and thoughtfulness, make each ex

periment in order, for they have been arranged so that one leads naturally to the making and understanding of the next. If the first, second, or even third trial does not give success, do not be discouraged, for the oldest and most gifted experimenters often fail ; yet they have made noble discoveries in science by their experiments, because they had patience and perseverance, as well as skill and knowledge. Do not be disheartened, and you will become a skillful experimenter.

In making an experiment, we may work alone, or we may perform the work in the company of our friends, so that a large number may see what we do, and assist in making the experiment. To exhibit an experiment on a large scale, so that all the people in a room may see it, we need a magic-lantern. A lantern with a good artificial light will cost a great deal of money, but by using the water-lantern and heliostat, described in the first book of this series, and employing the sun for a light, we can exhibit many of our experiments in sound, in the most beautiful manner, before a large company, and at a trifling expense.

At the same time, the lantern is not essential, and if you do not wish to use it you can perform all of the experiments without its aid.


The word "heliostat” is formed of two Greek wordshelios, the sun, and statos, standing. There is an instrument so named, because it keeps a reflected beam of sunlight constantly pointing in the same direction. In “Light,” the first book of this series, we have given a description of a simple heliostat ; but, as some of our readers may not have that volume, we here give a short descrip

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