When a ship is propelled by paddles, the motion of the vessel creates, in consequence of the friction of the fluid against the sides and bottom, a current of water which flows in the direction in which the ship is moving, and forms a current following the ship for a time, and finally losing all motion by contact with the surrounding mass of water. All the power expended in the production of this great stream is, in the case of the paddle-steamer, entirely lost. In screw-steamers, however, the propelling instrument works in this following current, and the tendency of its action is to bring the agitated fluid to rest, taking up and thus restoring, usefully, a large part of that energy which would otherwise have been lost. The screw is also completely covered by the water, and acts with comparative efficiency in consequence of its submersion. The rotation of the screw is comparatively rapid and smooth, also, and this permits the use of small, light, fast-running engines. The latter condition leads to economy of weight and space, and consequently saves not only the cost of transportation of the excess of weight of the larger kind of engine, but, leaving so much more room for paying cargo, the gain is found to be a double one. Still further, the quick-running engine is, other things being equal, the most economical of steam; and thus some expense is saved not only in the purchase of fuel, but in its transportation, and some still additional gain is derived from the increased amount of paying cargo which the vessel is thus enabled to carry. The change here described was thus found to be productive of enormous direct gain. Indirectly, also, some advantage was derived from the greater convenience of a deck clear from machinery and the great paddle-shaft, in the better storage of the lading, the greater facility with which the masts and sails could be fitted and used; and directly, again, in clear sides unencumbered by great paddle-boxes which impeded the vessel by catching both sea and wind. The screw was, for some years, generally regarded as simply auxiliary in large vessels, assisting the sails. Ultimately the screw became the essential feature, and vessels were lightly sparred and were given smaller areas of sail, the latter becoming the auxiliary power.