need, and as part of the best bounty which enriches our days. And to make this indubitably clear, let us hasten to meditate that simple and central law which governs this matter and at the same time many others. And the law is, that every definite action is conditioned upon a definite resistance, and is impossible without it. We walk in virtue of the earth's resistance to the foot, and are unable to tread the elements of air and water only because they are too complaisant, and deny the foot that opposition which it requires. Precisely that, accordingly, which makes the difficulty of an action may at the same time make its possibility. Why is flight difficult? Because the weight of every creature draws it toward the earth. But without this downward proclivity, the wing of the bird would have no power upon the air. Why is it difficult for a solid body to make rapid progress in water? Because the water presses powerfully upon it, and at every inch of progress must be overcome and displaced. Yet the ship is able to float only in virtue of this same hindering pressure, and without it would not sail, but sink. The bird and the steamer, moreover,--the one with its wings and the other with its paddles,--apply themselves to this hindrance to progression as their only means of making progress; so that, were not their motion obstructed, it would be impossible. The law governs not actions only, but all definite effects whatsoever. If the luminiferous ether did not resist the sun's influence, it could not be wrought into those undulations wherein light consists; if the air did not resist the vibrations of a resonant object, and strive to preserve its own form, the sound-waves could not be created and propagated: if the tympanum did not resist these waves, it would not transmit their suggestion to the brain; if any given object does not resist the sun's rays,--in other words, reflect them,--it will not be visible; neither can the eye mediate between any object and the brain save by a like opposing of rays on the part of the retina. These instances might be multiplied _ad libitum_, since there is literally _no_ exception to the law. Observe, however, what the law is, namely, that _some_ resistance is indispensable,--by no means that this alone is so, or that all modes and kinds of resistance are of equal service. Resistance and Affinity concur for all right effects; but it is the former that, in some of its aspects, is much accused as a calamity to man and a contumely to the universe; and of this, therefore, we consider here.