is no good reason to suppose that further successes than have already been attained will be achieved in any other way than by improving the conditions that now obtain, both as regards form of ship and method of propulsion, inasmuch as the physical causes which combine to retard the motion of a vessel, and the physical forces which are employed in overcoming that resistance, remain to-day as they ever were, and are—in fact, Nature’s immutable laws. The commercial question is also one that presses very hardly at all times and must continue to do so more and more, as will be seen later on. The Atlantic greyhound of to-day is, in immersed form, substantially that of the viking’s craft of more than a thousand years ago. And if we look to Nature for our study we shall find that the swiftest fish are not unlike in general form to the submerged part of a ship; and the comparison is the more easily accepted when it is remembered that the fish is wholly submerged while the ship is only partially so. The one has to contend with waves and other surface disturbances, and must perforce keep above the water, while the other is free from such disturbing elements and conditions, and pursues its course in practically smooth water. H. B. M. S. Polyphemus is the nearest approach to the fish conditions in a sea-going ship that has proved successful. [Illustration: H. B. M. S. Polyphemus at Full Speed—18-5/8 knots.] In order to produce motion at all, the _inertia_ of the ship, or that quality which every concrete body possesses of remaining at rest until disturbed, has to be overcome, and when the ship is in motion through the water there is resistance of a two-fold kind—that due to the disturbance of the water, and that due to the frictional resistance of the immersed surface. If a thin sheet of metal is moved edgewise through water it offers a decided resistance, even if its surface be smooth and bright; it will also be noted that this resistance increases very rapidly as the speed is increased, and that the larger the area the greater is the resistance. If this sheet of metal is moved in a direction at right angles to its surface the resistance is of course great: in fact, it is very great compared with that of the previous experiment, and the disturbance of the water is considerable. If a log of timber is to be towed from one place to another, it is a common observation that an experienced boatman causes it to move with its big end first, because he finds it easier work that way than with the smaller end first; in the latter case he has the same section of timber offering resistance to the log’s passage, but owing to its wedge-like form the pressure on its long sides is greater than when towed the other way, and the friction of the water past these sides—which are generally more or less rough—causes very great resistance; no doubt, for the same reason, those forms of ships adopted for centuries by some European nations, and known to mariners as “cod’s-head and