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son's Rocket. While nearly all of these illustrations are of things that did not exist a few years ago, one of them, the section of the Eddystone lighthouse, is given to show that the method of securing stability in such a structure has been known for many years. Railroad trains show the most recent method of applying steam and electricity as motive power, while the Zeppelin airship and the aeroplane are examples of what has been done to secure a means of traveling through the most unstable of fluids, the air. The Gatun locks of the Panama Canal are triumphs of mechanical engineering. The work of the electrical engineer is shown in the electric power station, the electric train, and the wireless telegraph. The page of sound waves is a striking proof of the accurate way in which photography makes a permanent record of a pulse that is invisible to the eye, while the marvelous results of the three-color method of printing are beautifully shown in the frontispiece.
The hope is expressed that those who study the essential principles of Physics, as treated in this book, will master them so thoroughly that they will take pleasure in tracing the dependence of our way of living upon these underlying principles.
Acknowledgment is made of many helpful suggestions made by teachers of Physics and to publishers who have granted the use of subjects for illustration.
GEORGE A. HOADLEY.