draught, due to alterations in the weights carried, from causing changes in the nominal tonnage, as they would do if the actual displacement tonnages were employed. There seems no weighty objection, however, to taking the maximum displacement for a yacht as her tonnage, and neglecting the effects of comparatively small changes of draught.

The New York Yacht Club settle time allowances quite apart from "tonnage." Their tables are based upon the area obtained by multiplying the extreme length of the yacht on or under the water-line from the fore side of the stem to the aft side of the sternpost by the extreme breadth wherever found. It would appear to be assumed in this rule that the power to carry sail varied with the area obtained by multiplying these two dimensions; but this is only an approximate rule, the true law for sail-carrying power depending upon more complex conditions, not now to be discussed. This rule is said to have given satisfaction in New York, but it has not been adopted here.

Summing up these different rules for yacht measurement, it must be admitted that no plan yet proposed is free from objection; the two most trustworthy methods appear to be "displacement tonnage" and "register tonnage." All yachts are measured for their register tonnage, under the law of 1854, just as other ships are measured; but no use is made of the measurement in racing. With certain limitations as to the thickness of the sides in yachts, the register tonnage might be made to give a very fair approximation to a constant fraction of the total bulk, measured to the outside of the skin and the top of the planking of the upper deck. Here, however, as in the Corinthian Club rule, there would be an inducement to decrease freeboard in order to diminish the tonnage; whereas with displacement tonnage there would be no such inducement.

Turning to register tonnage, the legal measurement for British merchant ships since 1854, one finds a method

resting upon a scientific basis, not easily evaded by persons desirous of making the nominal tonnage of their ships small, and generally commending itself after more than twenty years' experience. Many other nations have adopted the same or very similar laws; Germany, Austria, France, Italy, Denmark, and the United States among the number; and the Suez Canal tariff is based upon the same method of measurement. Register tonnage was thus defined by Mr. Moorsom, who had most to do with its introduction:"It is simply the internal capacity of the hold of the ship "in cubic feet, with any additional spaces built on deck, divided by one hundred. . . . . The nominal ton of the "present law consists simply of one hundred cubic feet." The tonnage of a merchant ship is usually stated both as "gross" and "nett." "Gross" tonnage expresses (in tons of 100 cubic feet) the total internal capacity of the ship, together with that of any closed-in spaces, such as deckhouses, &c., erected upon the deck, for purposes of accommodation or stowage. "Nett" tonnage is intended to express in the same units the cubical content of the space actually available for remunerative service, the conveyance of passengers, or stowage of cargo; it is sometimes styled the "register" tonnage. In calculating the tonnage, the surveyors of the Board of Trade take certain measurements in the interior of the ship, if that is accessible, the number and position of these measurements being settled by the tonnage law, in proportion to the size of the ship, and being sufficiently close to one another, to prevent unfair decrease of the tonnage by local thickening of the inside lining, or any such devices. These measurements are made the basis of a calculation very similar in character to that by which the displacement of a ship is estimated by the naval architect; and so an exceedingly close approximation is made to the internal capacity below the tonnage deck. To this is added the space between decks, and the closed-in spaces (if any), the final result being the gross tonnage.

In some cases the holds of ships cannot be cleared for purposes of measurement, and then the gross tonnage under the deck is estimated as follows. The length is taken at the upper deck from the fore point of the rabbet of the stem to the after point of the rabbet of the post. The extreme breadth of the ship is also taken, and a chain is passed under her at this place, in order to determine the girth of the ship, as high up as the upper deck. The formulæ now employed by the surveyors in calculating the approximate tonnage by rule 2 are:

But these rules are only used when internal measurement is either very inconvenient or impracticable.

In the Merchant Shipping Act of 1854, rule 2 was stated as follows. The length was taken on the upper deck between the outside of the outer plank at the stem and the after side of the sternpost, deducting from this the distance from the after side of the sternpost and the rabbet of the sternpost at the point where the counter plank crossed it. The girth and breadth extreme were taken as described above; and the formulæ for the gross tonnage were:

These coefficients were, however, exchanged many years ago for the smaller coefficients given above.

Mr. Moorsom also gave the following approximate rules for the gross register tonnage of merchant ships in terms

* See an able article on Register Tonnage in Practice," in Naval Science for 1873.

of their principal dimensions in a paper contributed

to vol. i. of the Transactions of the Institution of Naval Architects.

Let L = inside length on upper deck from plank at bow to plank at stern;

B = inside main breadth from ceiling to ceiling;

inside midship depth from upper deck to ceiling at limber strake.

Then the register tonnage for a ship in the under-mentioned

As an example take the case (from the Mercantile Navy List) of a steamer for which L = 137 feet; B = 20 feet; D = 11 feet.

All her Majesty's ships are now measured by surveyors of the Board of Trade, and their register tonnage is recorded in their papers; the above rules may, however, prove of some service, enabling a fair approximation to the tonnage to be rapidly made in terms of dimensions that are readily ascertainable.

The "nett" register tonnage of sailing ships differs very

little from the "gross" tonnage; the only deductions being spaces occupied solely by the crew, provided they do not fall below 72 cubic feet per man; cargo must not be carried in any such spaces, or the deductions cease to be made. In steam-ships further deductions are permitted on account of the spaces occupied by the machinery and coals, such spaces being regarded as lost to the cargo-carrying capacity of the vessel, and therefore not remunerative. The fundamental principle, that nett register tonnage (upon which the dues are estimated for any ship) shall only include spaces used for cargo-carrying or passenger accommodation, is thus maintained; but the fairness of making any such allowances to steamers, or, if any, how great allowances, has been the subject of much discussion. The Act of 1854 is still in force, however, although confessedly imperfect, and under it the deductions are made in one of two ways. The space "solely occupied by and necessary for the "proper working of the boilers and machinery" is measured (shaft passages, funnel casings, ventilation trunks, &c., being included herein). If this space has a tonnage, in screwsteamers, above 13 per cent. of the gross tonnage and under 20 per cent., the total deduction permitted, for machinery and coal-space, is 32 per cent. of the gross tonnage. In paddle-steamers, if the measured space has a tonnage above 20 per cent. and under 30 per cent. of the gross tonnage, the total deduction permitted is 37 per cent. This is the first, or "percentage," method supposed to be applicable to all ordinary steamers. The second method is applied where the space occupied for the machinery is either unusually large or small; the space may then be measured (as before), and the total deduction from the gross tonnage is to be 50 per cent. more than the measured space in paddle-steamers, and 75 per cent. in screw-steamers. These additions to the measured space are considered to allow fairly for the coal-stowage required for the propulsion of the ship. Slight variations, such as may be made in the erections on deck, might