At the present moment, for example, the nature of Force is unknown. A weight released from the hand drops to the earth. Exactly what is the nature of the force with which the earth attracts it? We do not know, but it so happens that it is more than likely that an explanation will be reached in our own day. Gravity will be explained in terms of some more general forces. The mystery will be pushed back another step, and yet another and another. But the progress is not indefinite. If all the mechanical actions of the entire universe were to be formulated as the results of a single law or cause, the cause of that cause would be still to seek, as has been said. We have every right to exult in the amazing achievements of Science; but we have not understood them until we realize that the universe of Science has strict limits, within which all its conquests must necessarily be confined. Humility, and not pride, is the final lesson of scientific work and study. * * * * * The choice of the selections printed in this volume has been necessarily limited by many hampering conditions, that of mere space being one of the most harassing. Each of the chapters might readily be expanded into a volume. Volumes might be added on topics almost untouched here. It has been necessary to pass over almost without notice matters of surpassing interest and importance: Electricity and its wonderful and new applications; the new Biology, with its views upon such fundamental questions as the origins of life and death; modern Astronomy, with its far-reaching pronouncements upon the fate of universes. All these can only be touched lightly, if at all. It is the chief purpose of this volume to point the way towards the most modern and the greatest conclusions of Science, and to lay foundations upon which the reading of a life-time can be laid. [Illustration: Signature: Edward S. Holden] UNITED STATES MILITARY ACADEMY, WEST POINT, _January 1, 1902_. WONDERS OF EARTH, SEA, AND SKY WHAT THE EARTH'S CRUST IS MADE OF (FROM THE WORLD'S FOUNDATIONS.) BY AGNES GIBERNE. "Stand still and consider the wondrous works of God." [Illustration] What is the earth made of--this round earth upon which we human beings live and move? A question more easily asked than answered, as regards a very large portion of it. For the earth is a huge ball nearly eight thousand miles in diameter, and we who dwell on the outside have no means of getting down more than a very little way below the surface. So it is quite impossible for us to speak positively as to the inside of the earth, and what it is made of. Some people believe the earth's inside to be hard and solid, while others believe it to be one enormous lake or furnace of fiery melted rock. But nobody really knows.