TONNAGE, REGISTER TONNAGE, or INTERNATIONAL REGISTER TONNAGE, is the unit on which the assessment of dues and charges on shipping is based. The system at present in force is known as the Moorsom system. A register ton is 100 cubic feet of internal volume. Thus a vessel of 100,000 cubic feet of internal space within the points of measurements prescribed by the law is 1000 tons register. Vessels are sometimes bought and sold under this unit. The tonnage rules, which are very full and elaborate, are contained in part ii. of the Merchant Shipping Act, 1854, sections 20 to 29 inclusive, and in section 9 of the Merchant Shipping Act, 1867, the latter being a special section in reference to a deduction from the gross tonnage in respect of crew space, which space must be fit for the proper accommodation of the men who are to occupy it to entitle to such deduction. This enactment has led to great improvement in seamen's quarters.
Section 60 of the Merchant Shipping Act, 1862, pro-vides on the point of international tonnage as follows :— " Ships belonging to foreign countries which have adopted the British system of tonnage need not be remeasured in this country." The British system has been adopted by the following countries at the dates named :—United States, 1865 ; Denmark, 1867 ; Austria-Hungary, 1871; Germany, 1873 ; France, 1873; Italy, 1873 ; Spain, 1874; Sweden, 1875; Netherlands, 1876; Norway, 1876; Greece, 1878; Bussia, 1879; Finland, 1877; Hayti, 1882; Belgium, 1884; Japan, 1884. It is also under consideration by China.
There are slight differences in the rules for deduction for engine room in some of the countries, but owners or masters of foreign steamships, where this difference exists, may have the engine-rooms remeasured in the United Kingdom if they desire; in other words, their net tonnage may be reduced to exact English measure.
The British system was also mainly adopted by the International Tonnage Commission assembled at Constantinople in 1873, the rules of such commission forming the basis of dues levied on the ships of all countries passing through the Suez Canal. A special certificate is issued in the respective countries for this purpose. The main point of difference from the British system is with respect to the deduction for engine room.
There are three terms used in respect of the tonnage of ships,—namely, tonnage under decks, gross tonnage, and register tonnage.
In obtaining the gross measurement the space under the tonnage deck is first measured—sections 20 and 21 (1), (2), and (3); then the space or spaces, if any, between the tonnage deck (the tonnage deck is the second deck from below in all vessels of more than two decks and the upper deck in all other vessels) and the upper deck— section 21 (5) of Act; and finally the permanent closed-in spaces above the upper deck available for cargo, stores, passengers, or crew—section 21 (4) of Act.
The allowance for engine room is governed by the percentage the net engine room—that is, the space exclusive of the coal bunkers—bears to the gross tonnage, and varies in paddle- and screw-steamers as laid down in section 23 of the Act.
In obtaining the tonnage under tonnage deck, ships are divided in respect of their length into five classes as follows:—
Class 1. Length 50 feet and under into 4 equal parts.
„ 2. ,, 50 ,, and not exceeding 120 feet, 6 ,,
„ 3. ,, 120 ,, „ „ 180 ,, 8 „
„ 4. „ 180 „ „ „ 225 ,, 10 „
„ 5. ,, 225 ,, an 1 upwards 12 ,,
The following is an epitome of the rule for tonnage under the tonnage deck:—
Length is taken inside on tonnage deck, from inside of plank at stem to inside of midship stern timber or plank; the length so taken, allowing for rake of bow and of stern in the thickness of the deck, and one-third of the round of beam, is to be divided into the prescribed number of equal parts (which determines the stations of the areas), according to the length of vessel, as above.
Area 1 is at the extreme limit of the bow. Area 2 is at the first point of division of the length. The rest are numbered in succes-sion, the last being at the extreme limit of the stern.
Depths are taken at each point of division of the length, or station


of each area, from the underside of the tonnage deck to ceiling at inner edge of timber strake, deducting therefrom one-third of the round of the beam. The depths so taken are to be divided into four equal parts, if midship depth should not exceed 16 feet; otherwise into six equal parts.
Breadths are taken at each point of division of the depths and also at the upper and lower points of the depths. The upper breadth of each area is to be set down in its respective column in a line with No. 1 (left-hand numerals), and the rest in succession.
The number of columns for areas will vary according to the length, as in the several classes, and will be equal to the number of parts into which the length is divided plus one.
The space or spaces between decks above the tonnage deck are dealt with by a similar formula. A mean horizontal area of the space, or each space if more than one, is found and multiplied by the mean height.
The permanent closed-in spaces above the upper deck available for cargo, stores, passengers, or crew are measured in the same manner by finding a" mean area and multiplying by a mean height.
The measurement of net engine room is governed by the arrange-ment of the space, and is measured as a whole or in parts as may be required by its particular form.
The following is an example under class 2, depth under 16 feet, of tonnage under tonnage deck:—

== TABLE ==

This formula is also applicable for finding displacement tonnage of ships, that is, the external displacement measured by taking transverse areas to the height of the load water-line to find the cubic content, which content divided by 35 gives the displacement in tons weight, the difference between the light and load displace-ment representing the carrying powers of a vessel in tons.
'' The rule," says Mr Moorsom, '' is founded on the purest mathe-matical principles. It was first published in the Philosophical Transactions of the Royal Society of 1798 by Attwood, in his 'Disquisition on the Stability of Ships,' who there describes it as one of those formulae invented by Sterling for measuring spaces bounded by irregular curves, founded on Sir Isaac Newton's dis-covery of a theorem—a discovery which the immortal author himself considered amongst his happiest inventions—by which the areas of all curvilinear spaces not geometrically quadrible nor discoverable by any known rules of direct investigation are so closely approxi-mated as to amount to geometrical exactness."
Mr Allan Gilmour at the middle of the present century expressed his opinion, after a careful consideration of the tonnage question, which was receiving much attention at that time owing to the law 8 and 9 Vict. cap. 89, which had been adopted in place of the old tonnage law 13 Geo. III. cap. 74, not giving satisfaction, that the " system framed by Mr Moorsom will as it were compel every one to build strong, fast-sailing, and good seagoing ships, and that, in fact, it will stand as long as the world remains." It will he admitted that great progress has been made in every way in British shipping of late years, and for this due praise must be given to the influence of the present tonnage laws. (W. M*.)