which his power of giving motion is increased, and as he learns more of the properties of matter he constructs engines, by which these properties work for him in the directions in which he guides them. Meantime he has learned that clay, when heated, becomes hard as stone, and the arts of pottery take their rise; while glass-making follows on the discovery that ashes and sand fuse into a transparent mass. Yet, whether in their rude beginning or finished elegance, man in these arts does no more than bring together the rough materials and apply to them heat, then their own inherent properties effect the result. Science--that is, knowledge of natural laws of matter--guides his hand, but his hand only moves matter; it gives no property and takes away none; it does not even enable one property to work; it does absolutely nothing except to place matter where its own laws work, to bring or to remove matter which is needed, or to remove matter which is superfluous. Let us analyze every complicated triumph of human knowledge and skill, and we shall find it all reduced to the knowledge of what the properties of matter are, and the skill which imparts to it motion just sufficient to permit these properties to operate. Man's power over Nature is therefore limited to the power of giving motion to matter, or of stopping or resisting motion in matter. Now, to give motion or to resist motion is itself either a breach or a use of a law of Nature, according as we express that law. The law is (as usually expressed), that matter at rest remains at rest till moved by a force, and that matter in motion continues in motion till stayed by a force. This is the law of inertia. If we consider that rest or motion when once established is the normal state of matter, then the force which causes a change causes a breach of the law of inertia. But if we consider that the liability to be moved, or to have motion stopped by force, is itself a property of matter, then the application of force with such result is merely calling into operation the law of inertia. It really does not signify which view we take, so long as we recognize that such are the facts. But since it is more familiar to associate rest with inertia, it will perhaps be most convenient and simple to consider rest and motion as the laws of matter, till the law is interfered with. Therefore in what follows we shall say, that when matter at rest is moved, or when matter in motion is stayed, or its movement by a natural force is prevented, a breach of the law of inertia is committed.