A certain number of my readers have been for some time kindly expressing a wish for a new edition of this early book. But strictly speaking I could not do this without considerably enlarging my original plan and composing an entirely new work. The daily routine of my astronomical labors has constantly hindered me from devoting myself to that task. The starry heaven is a vast and absorbing field of work, and it is difficult to turn aside (even for a relaxation in itself scientific) from the exacting claims of a science which goes on developing unceasingly at a most prodigious rate. Still, the present work may be considered as, in a sense, an enlarged edition of the earlier one. The foregoing citation of a little book written for the purpose of proving the existence of unknown forces in nature has seemed to me necessary here; useful in this new volume, brought out for the same purpose after more than forty years of study, since it may serve to show the continuity and consistent development of my thought on the subject. MYSTERIOUS PSYCHIC FORCES CHAPTER I ON CERTAIN UNKNOWN NATURAL FORCES I purpose to show in this book what truth there is in the phenomena of table-turnings, table-movings, and table-rappings, in the communications received therefrom, in levitations that contradict the laws of gravity, in the moving of objects without contact, in unexplained noises, in the stories told of haunted houses,--all to be considered from the physical and mechanical point of view. Under all the just mentioned heads we can group material facts produced by causes still unknown to science, and it is with these physical phenomena that we shall specially occupy ourselves here; for the first point is to definitely prove, by sufficient observations, their real existence. Hypotheses, theories, doctrines, will come later. In the country of Rabelais, of Montaigne, of Voltaire, we are inclined to smile at everything that relates to the marvellous, to tales of enchantment, the extravagances of occultism, the mysteries of magic. This arises from a reasonable prudence. But it does not go far enough. To deny and prejudge a phenomenon has never proved anything. The truth of almost every fact which constitutes the sum of the positive sciences of our day has been denied. What we ought to do is to admit no unverified statement, to apply to every subject of study, no matter what, the experimental method, without any preconceived idea whatever, either for or against. We are dealing here with a great problem, which touches on that of the survival of human consciousness. We may study it, in spite of smiles.