TONNAGE, in regard to ships, is the measure of capacity, the ton being one not of weight, but of cubic content—i.e., 40 cubic feet. Very early in the history of naviga tion, some scale must have been established by which the relative capacity of different vessels could be determined. In England there are early laws upon the subject, set tling the data upon which the should be made. The present system, called "new measurement," dates from 1835; but the prior system, established in 1719, and now known as O. M. (old measurement), still subsists among yachts and some other ves sels. The old measurement was greatly erroneous, for the actual depth of the ship was not taken into account, but was assumed to be equal to her breadth. The tonnage was then obtained by multiplying together length, breadth, and assumed depth in feet, and by dividing the product by 94. As harbor-dues and such like taxes were levied accord ing to each vessel's tonnage, it naturally followed under such a system that traders built their ships with as little beam and as great depth as they possibly could. The ships thus became highly dangerous in rough weather, and, moreover, every principle of cor rect naval architecture was set at nought, to produce deep wooden boxes capable of car rying a maximum of cargo with a minimum of beam. The absurdity of a law by which, in consequence of an inch more beam, a two-decked vessel might appear of greater capa city than a three-decked ship of like length, was so palpable, that many efforts were made at improvement, though without success until 1835.
By the act of that year, the new system established the depth of hold as a necessary ingredient of the calculation. As, however, the section crosswise of a ship varies so considerably at different points in her keel both in superficies and shape, more than an approximation to her cubic content cannot be attained. To arrive at this approximation, the total length of the upper deck, or, if the ship be not wholly devoted to Cargo, of the upper portion of the space for cargo, is taken, and divided into 6 equal parts at the points, a, b, c, d, e. From the foremost, center, and aftmost of these points, the depths
to the bottom of the hold are measured as af, cg, eh. Each depth has to be divided into 5 equal parts; at the fore and after depth, the width inboard of the ship is measured at k and the depth from the top; and the center-depth at a and t from the top. These lateral measurements at i, k„ m, m„ and m., are developed on the plan, and curves drawn representing cross-sections of the ship at the given points. All dimensions are supposed to have been taken in feet and decimal parts, and they are thus used in computing the tonnage. The "length," no, is measured from stem (internal side) to stern post, at half the height of the center-depth from the keel. To twice the depth amidships (cg), add the depths forward and aft (af, eh). The result is the "sum of the depths." Add together the two breadths taken at the foremost depth: of the breadths. taken at the center-depth (cg), add together three times the breadth at I, and twice the b•eadth at 4: of the after-breadths, add together the breadth at and twice the breadth at t. The sum of these three totals is the "sum of the breadths." Hav ing obtained these quantities, the tonnage is approximated to in a somewhat arbitrary manner by the following formula: Tonnage of depths X sum of breadths X length • 3500 or, if expressed in terms of the figure: Tonnage — {2cg af-1- H (pa rv) (3t,g,d- 2r,v,)+ 2r.v,)1X • In computing the measurement of a steamer, the same system is folloN4d, but the tonnage of the engine-room (which is supposed to be capable of floating engines and boilers) is deducted from the total to express the tonnage of the ship.