In devising the form of a ship, ho proceeds, as already explained at figs. 3 and 4; but instead of adopting merely arbitrary changes of area, ho arrives at his precise form of midellip and load-water curves by the use of a simple and elegant formula, producing thereby curves of con trary flexure, which are said to facilitate the passage of a solid through a fluid. his experiments have that although the straight bow of wedge-like form presents actually the smallest surface to the friction of the particles of water, yet that in this form such particles, impinging upon the bow, rebound or arc driven off at right angles to the water line, raising a perceptible wave ; while Scott Russell's curve of easy entrance allows the water to roll along the curve to the side of the vessel without such opposition. To illustrate plainly his theory with all possible brevity, let us suppose that, for instance, the area and outline curve of the midship section have been determined : this would represent the greatest beam, not at the centre of the fore and aft line of the ship, but at a point at about two-thirds of the length from the bow. Mr. Scott Russell divides his ship, upon the load-water line, into two unequal and dissimilar portions. He considers that the length of midship portion (which, according to his theory, may consist of horizontal lines perfectly parallel to the keel) does not affect, or scarcely affects, any other property than the displacement, the additional friction of such added midship portion being comparatively trifling. Thus his ship, in its abstract original, is merely a bow and a stern ; but as the curves which form these are, at their junction at the midship section, tangential, and are parallel to the longitudinal axis, they admit of an easy absorption into the general outline curve of the ship.
Let the lino A n represent two-thirds of the whole length of the ship at the load-water line, and be the bow or forward portion. A o representing the half-breadth at her greatest or midship section join c u. Bisect c, and from its centre, o, and with o c as radius, describe the semi circle on A c, and divide it into any number of equal parts—say six, for example. Divide A n also into six equal parts, and draw the ordinates a l', G2', e 3', &c., parallel to A C. Then draw 1.1', 2.2', 8.3', tte., parallel to A n, meeting the first ordinates at the points 1', 2', 3', &e.; then will these points, l', 2', 3', &c., be points in the curve which Mr. Scott linemen calls the wave principle, from some analogies which he considers to exist between it and the form of waves. It will be found that the area of the portion c 3'111 will be equal to the area of the part 3' a an forming an interchange of area without disturbing the amount of displacement or the tonnage of the ship.
The following figure will also, by the same rule, show the shape of Fig. 8.
the stern portion of the load-water line, the outline curve being drawn in precisely the same manner.
In the figure below we have placed the two ends together (at a reduced scale), to show the whole load-water lines of the ship ; and it is remarkable that, in vessels built on the wave principle, the amount of proportionate beam (within ordinary and reasonable limits) is said to very little affect the question of speed.
Another feature in Mr. Scott Russell's ingenious theory deserves mention. If we divide a ship vertically fore and aft into parallel portions of equal thickness, or, which may be more intelligible to the general reader, if we construct the model of such ship with boards of coloured wood of equal thickness throughout, the lines along the bottom of the ship, formed by the line of junction of such boards, and which are called buttock-lines, are made purposely cycloidal at the stern (the difference In the colour of the woods shows the lines more strongly) : such a form of construction is said to facilitate the delivery of water as it passes along the hull.
Sir William Synaonds, late surveyor of the navy, greatly altered the form of ships in her majesty's service, although he had much opposi tion to contend with. We are certainly indebted to him for more height between decks, faster sailing qualities, greater stability, but there are still those who do not wholly admit his theory as fully applicable to heavily armed ships. He gave greater beam, at the expense of hold stowage, fic.—the details of his construction may be seen in an Interesting work bearing his name, published a few years since. As a comparison with the prevailing mode of constructing the midship form of section, the following figure will suffice.
If o a be the beam of a ship of war as usually constructed, he would increase the beam to o b, and adopt a very rising floor ; while a tn? c would be the shape of the old form, b ti c would be that of Sir W. Symonds ; but this extent of beam has its limits, for although the displacement is taken from the bottom and placed at the load-water line, where It exerts a power of buoyancy, in the ratio of the square of its distance from the fore and aft midship line of the ship, it is said to render the rolling of a ship, under certain circumstances, less easy. But there is no question that shipbuilding received an impetus from Sir W. Symonds, at a time when in England the whole science was in a state of torpor.
Most elaborate investigations as to the mathematical principles involved in the rolling motion of a ship have been recently made by Dr. Woolley. Rolling is manifestly of two kinds, deep rolling and uneasy rolling ; the vessel's motion in the latter case is principally affected by the raising of the centre of gravity and the suddenness of the check to oscillation caused by the rapid immersion of the lee-side of the ship, and is often sufficient to endanger masts, and is attendant upon the adoption of very rising floors, for the nearer the midship section approaches a semicircle, the deeper but easier will be the rolling, but this brings us into another extreme, namely, the possibility of danger from capsizing. Dr. Woolley has shown that the mathema tical expression for periods of rolling, or as it is called, the rotation, affords no correct indication of the actual condition of a vessel at such times of oscillation.