It is believed by the author that a classification and presentation of selected groups of the devices produced in the past by which it was by the inventor believed, self-motive power had been attained, will save much work in fields already thoroughly exploited.

So far as the author knows no book on the subject has appeared since 1870. The various encyclopedias published contain articles on the subject, but they are necessarily brief, and not satisfying to young men who have become interested in the subject.

In 1861, Henry Dircks, a civil engineer, of London, published a work entitled "Perpetuum Mobile; or, Search for Self-Motive Power, During the Seventeenth, Eighteenth and Nineteenth Centuries." The book contains 599 pages, and was followed in 1870, by a second series by the same author entitled "Perpetuum Mobile, or a History of the Search for Self-Motive Power from the Thirteenth, to the Nineteenth Century." In these two books there is amassed a wonderful amount of material showing on the part of the author diligence, great patience and wide and thorough search.

The author of these works was not enamoured of his subject, and his books clearly show that he was not writing them because of any interest he had in the subject of Perpetual Motion. On the contrary, they appear to have been written because of a deep detestation entertained by the author for the subject of Perpetual Motion, and a contemptuous pity for any one seriously interested in the subject. Mr. Dircks's works may be said to be the works of a scold. His sentiments were deep, and his impulses strong, which accounts for the vast amount of labor he did in the preparation of his books. Those books are now out of print, and it is believed by the author of this book that they may well remain so. They contain much material that no one would be justified in wading through. The most complicated mechanisms devised by enthusiastic dreamers are shown in the same detail with which the inventors described them in presenting them to the public, or to the patent offices. Little is to be gained by this.

So complicated are many of the devices that only technically trained engineers could read them understandingly, and few technically trained engineers are now greatly interested in self-motive power devices. We believe that every useful or interesting purpose is served if enough devices are collected, classified and presented to show the various principles relied upon by the inventors; with an explanation of why they failed—i. e., wherein the principles relied upon are wrong, and while possibly not out of harmony with any mechanical principles then known, are entirely out of harmony with principles since discovered and now well known.

Perpetual Motion as used in this book is to be taken in its conventional, and not in its strict literal sense. The strict literal analysis of the two words implies unceasing motion. Of this we have many illustrations—the tides, the waves of the ocean, the course of the earth around the sun, and in the movements of all heavenly and astronomical bodies. In fact, it is difficult to conceive in a strictly scientific sense of any substance having an entire absence of motion.

Perpetual Motion as used in this book means what it is usually understood to mean—Self-Motive Power—a machine that furnishes the power to keep its parts going as a machine.