The term tonnage, as applied to shipping, Signification was originally intended to express the actual burthen that ton of any ship could carry, in order that the various dues and term customs which are, and always have been, levied upon ships might be proportioned to their carrying powers. To avoid cavilling and uncertainty as to the real tonnage of a vessel (which, it is evident, depends upon the draught of water at which she can safely swim), it must have been soon found necessary to establish one universal mode of calculating the tonnage of merchant-shipping, depending on certain fixed and definite measurements of the hull. A fixed rule, strictly enforced by law, has consequently been adopted by all maritime nations for this purpose. Upon Importance the principles recognised in framing these rules depends, in of good a great measure, the preponderance of good or bad qualities rules for in the ships themselves, the body of shipowners in general tonnage, being found unequal to the temptation of sacrificing the prospective safety, and the weatherly qualities, of their ships to the present sure gain arising from a low rate of register tonnage, when this can be compassed by any peculiarity of build, however extravagant. Legislation on this subject requires, therefore, to be conducted with the utmost caution and circumspection, an irreparable injury having been already done to the merchant-shipping of this country from the erroneous principles on which the measurement for tonnage used to be made.

Not only are all dues and customs levied according to Tonnage tonnage, but ships are also built, bought, and sold for a used for certain price per ton of their admeasurement; and by the fiscal and conditions of Lloyd's classification-list of shipping, they commerce must be timbered and fastened, and must have their official pur- anchors, cables, and boats, all in proportion to the same datum. The tonnage of a ship, therefore, in so far as these considerations are involved, is virtually assumed to be a correct representation of her size.

The true principles upon which the register tonnage of The true shipping should be computed appear to be the following:—principles 1st, It should afford a practically correct measurement of on which all space eligible for stowage or passenger accommodation; tonnage should be 2d, The measurements and dimensions involved should be such, both as regards their number and position, as to effectually prevent even our most ingenious builders from escaping the due influence of the rules, whatever form or dimensions of vessel may be resorted to; and 3d, It should be such as to ensure the dues levied being justly proportional for all classes of vessels. The question has been raised whether the legal tonnage of a ship should not represent numerically the commercial tons actually carried, either of measurement or dead-weight cargoes, or both of them, rather than merely express, as at present, the relative capacity of ships. On this subject it was remarked by Mr G. Moorsom, surveyor-general for tonnage to the Board of Trade, at a meeting of the Institution of Naval Architects (as reported in the Mechanic's Magazine of 6th April 1860), "that the preservation of the expression of the present aggregate tonnage of the kingdom has been the sine qua non with all the public commissions on the question, and is upheld also by the shipping community, as constituting a fairer standard of capacity, under general circumstances, than the estimated cargoes carried, either of measurement or weight, which must necessarily vary with the ever-varying circumstances of longer or shorter voyages—to say nothing of the acknowledged almost impossibility of satisfactorily arriving, by any general rule, at the proper positions of the load and light draughts of water, on which the calculation of the weights carried solely depends." In regard to the question of weight-cargoes, Mr Moorsom observed that "parties were agitating as to the desirability of placing a scale of tonnage (or displacement) on a ship's certificate of registry, to show the weight of cargo carried at different lines of flotation, for the convenience of ship owners, brokers, and masters. He questioned, however, if the utility of that object was at all commensurate with the labour and difficulty of its production, and he had yet to learn that the parties themselves, for whose interest it was proposed, desired such a document. But, if needed, it could be furnished to the ship owner or broker by any respectable builder or surveyor of shipping, and ought not to be prepared at the public expense. It would require ten or twelve practised draughtsmen for a period of nine or ten years to prepare such scales for the existing commercial navy, and two or three others in addition for the ships annually building. Tables had been prepared at the Board of Trade by which it appeared that the weights due to one inch of immersion at the two different draughts of the load and the light lines varied, on an average, to the extent of about 10 per cent only, so that, for all commercial purposes, it would be sufficient to know what weight of cargo corresponded to an inch of depression."

Until January 1836, the rule for computing the tonnage of ships was as follows:—The length was taken on a straight line along the rabbet of the keel of the ship, from the back of the main stern-post to a perpendicular line from the fore part of the main stem under the bowsprit. The breadth was taken from the outside of the outside plank in the broadest part of the ship, either above or below the main wales, exclusively of all manner of doubling planks that might be wrought upon the sides of the ship. If the ship to be measured was afloat, a plumb-line was dropped over the stern, and the distance between such line and the after part of the stern-post, at the load water-mark, was measured; then was taken the length from the top of this plumb-line, in a direction parallel with the water, to a perpendicular immediately over the load water-mark, at the fore part of the main stem. Subtracting from this length the before-mentioned distance between the plumb-line and the after part of the stern-post, the remainder was reckoned to be the ship's extreme length, from which three inches were deducted for every foot of the load draught of water. With the dimensions thus obtained, the rule then was—"From the length taken in either of the ways above mentioned, subtract three-fifths of the breadth taken as above; the remainder is esteemed the just length of the keel to find the tonnage; then multiply this length by the breadth, and that product by half the breadth, and dividing by 94, the quotient is deemed the true contents of the tonnage."

This rule (called "builders' old measurement," or when contracted, B.O.M.) is still much in vogue as a guide for the purchase and sale of ships. It is evident that the tonnage as determined by it was intended to express the size or bulk of the ship, the half-breadth being an assumed equivalent for a mean depth. The evils which arose out of this assumption were very great. As the depth was not at all involved, it might be increased to any extent without increasing the tonnage; while, on the contrary, as the square of the breadth was involved, an undue preponderance was given to this dimension, and it became necessary, on the part of shipowners, to restrict it within the least possible limits. The effect of such a law was obvious. The British merchant-ships, in order to profit by its inconsistencies, were built exceedingly narrow and deep in proportion to the length, so that, according to parliamentary returns, we find, on an average, the mercantile navy would carry a third more weight than its legally registered tonnage. In fact, the ships became little more than oblong boxes, most dangerous as sea-boats, and, from their want of stability, not capable of carrying sufficient sail to insure their safety on lee shores. Hence, after every gale of wind, the leeward coasts were covered with their wrecks; and hence Lloyd's books registered annually the average loss of six ships in four days. This tonnage law, as we have said, was happily altered in January 1836, when the following rule for calculating the tonnage of vessels was substituted for it.

The tonnage of every ship or vessel required by law to be registered shall, previously to her being registered, be measured and ascertained while her hold is clear, and according to the following rule: (that is to say,) divide the length of the upper deck between the after part of the stem and the fore part of the stern-post into six equal parts. Depths—at the foremost, the middle, and the aftermost of those points of division, measure in feet and decimal parts of a foot the depths from the under side of the upper deck to the ceiling at the limber-strake. In the case of a break in the upper deck, the depths are to be measured from a line stretched in a continuation of the deck. Breadths—divide each of those three depths into five equal parts, and measure the inside breadths at the following points: videlicet, at one fifth and four-fifths from the upper deck to the foremost and aftermost depths, and at two-fifths and four-fifths from the upper deck of the midship depth. Length—at half the midship depth measure the length of the vessel from the after part of the stem to the fore part of the stern-post; then to twice the midship depth add the foremost and the aftermost depths for the sum of the depths; add together the upper and lower breadths at the foremost division, three times the upper breadth, and the lower breadth at the midship division, and the upper and twice the lower breadth at the after division, for the sum of the breadths; then multiply the sum of the depths by the sum of the breadths, and this product by the length, and divide the final product by 3500, which will give the number of tons for register. If the vessel have a poop or half-deck, or a break in the upper deck, measure the inside mean length, breadth, and height, of such part thereof as may be included within the bulkhead; multiply these three measurements together, and dividing the product by 92·4, the quotient will be the number of tons to be added to the result as above found. In order to ascertain the tonnage of open vessels, the depths are to be measured from the upper edge of the upper strake.

Mode of ascertaining the Tonnage of Steam-Vessels.

In each of the several rules herein before prescribed, Tonnage, when applied for the purpose of ascertaining the tonnage of any ship or vessel propelled by steam, the tonnage due to the cubical contents of the engine-room shall be deducted from the total tonnage of the vessel, as determined by either of the rules aforesaid, and the remainder shall be deemed the true register tonnage of the said ship or vessel. The tonnage due to the cubical contents of the engine-room shall be determined in the following manner: that is to say, measure the inside length of the engine-room in feet and decimal parts of a foot, from the foremost to the aftermost bulkhead; then multiply the said length by the depth of ship or vessel at the midship division, as aforesaid, and the product by the inside breadth at the same division, at two fifths of the depth from the deck, taken as aforesaid, and divide the last product by 92.4, and the quotient shall be deemed the tonnage due to the cubical contents of the engine-room.

For ascertaining the Tonnage of Vessels when laden.

And be it further enacted, that for the purpose of ascertaining the tonnage of all such ships, whether belonging to the United Kingdom or otherwise, as there shall be occasion to measure while their cargoes are on board, the following rule shall be observed and is hereby established: that is to say, measure, first, the length on the upper deck, between the after part of the stem and the fore part of the stern-post; secondly, the inside breadth on the under side of the upper deck, at the middle point of the length; and, thirdly, the depth from the under side of the upper deck, down the pump-well, to the skin; multiply these three dimensions together, and divide the product by 130, and the quotient will be the amount of register tonnage of those ships.

It was soon found that this rule was somewhat partial in its operation in different classes of vessels, and that it could be, within certain limits, evaded by an ingenious builder; but still the evasions were not so destructive to the good qualities of ships as those which were commonly practised during the continuance of the old law.

It is exceedingly difficult, probably even impossible, to frame a rule for computing the tonnage which shall be at once of practical application, and yet not have in some degree the effect of restricting improvement in the qualities of merchant-ships. It is difficult to induce a man to forego a constant and positive gain for one that is only prospective and uncertain. We have seen that the obstacles which oppose themselves to correctly and satisfactorily determining either the light or the load draughts of water, are sufficient to prevent the difference between the light and load displacements from being taken to represent the tonnage. This is, however, the only correct measure of a ship's power to carry cargo; the difference being, of course, exactly equal in weight to the cargo which either has caused or may cause it. All other quantities which can be taken as measures of that power are little more than assumptions, and whether they represent the external dimensions of a ship, or her internal capacity, they scarcely give an approximation even to the power which she may possess of carrying burthen; while, in either of the above cases, the fact that these quantities must be determined by measurements at fixed measuring places, affords opportunity for evasion, and indeed invites it. For if, by any arrangement of the dimensions, or by any peculiarity of the shape, a ship can be enabled to carry a greater burthen than her registered tonnage, the freight of that greater burthen is a premium which is offered to that one proportion between her dimensions, or that one peculiar form for her body, and a restriction is, to a certain extent, placed upon improvement; because the shipowner will content himself with the best ships that he can obtain possessing the advantages of those dimensions or of that form.

The rule last quoted for computing the tonnage assumes it to be the space for stowage, and the internal capacity of the vessel is calculated in order to determine it. As there are necessarily fixed measuring places, the rule may, as we have said, be evaded by a certain build. Its phraseology might also be easily evaded by building accommodations on deck, which would not come within the meaning of the terms that are used in it—"poop," "half-deck," or "break in the deck." Under its operation, vessels might also be advantageously built of very small register tonnage to carry cargoes of heavy goods; for which purpose they should be of the lightest materials, but with very large scantlings, that the internal capacity may bear but a small proportion to the load displacement of the vessel. This rule, however, was far less injurious to the mercantile navy of Great Britain than that which had preceded it.

The present rule for tonnage was introduced in the year New ton-1854, constituting one of the most important sections of the Merchant Shipping Act of that year. It is universally admitted to be a vast improvement upon all that had gone before, and indeed to be one of the greatest benefits ever conferred by the Legislature upon naval architecture, tending, as it does, to advance the character of the merchant marine of this country. The builder is at length free to construct his ship in the way he thinks best for the requirements of her particular trade; the very impossibility of evading the law by any alterations in the dimensions or form making shipowners content to have good trustworthy ships, in place of the dangerous abortions of twenty years since. It is found that the vessels built since the new law came into operation have an average length of 5 times their breadth, in place of 3½ times as formerly, and that their average depth has decreased from above ¼ths of the breadth to ⅓ds ditto, their speed and weatherly qualities being improved in like proportion.

The following is the present law for measurement of tonnage (introduced 1854):

Throughout the following rules, the tonnage-deck shall be taken to be the upper-deck in ships which have less than three decks, and to be the second deck from below in all other ships; and, in carrying such rules into effect, all measurements shall be taken in feet, and fractions of feet, and all fractions of feet shall be expressed in decimals.

The tonnage of every ship to be registered, with the Rule I., for exceptions mentioned in the next section, shall, previously to her being registered, be ascertained by the following rule, hereinafter called Rule I.; and the tonnage of every ship to which such rule can be applied, whether she is about to be registered or not, shall be ascertained by the same rule.

(1.) Measure the length of the ship in a straight line above the upper side of the tonnage-deck from the inside of the inner plank (average thickness) at the side of the stern, to the inside of the midship stern-timber or plank there, as the case may be (average thickness), deducting from this length what is due to the rake of the bow in the thickness of the deck, and what is due to the rake of the stern-timber in the thickness of the deck, and also what is due to the rake of the stern-timber in one-third of the round of the beam; divide the length so taken into the number of equal parts required by the following table, according to the class in such table to which the ship belongs.

Table.—Class 1. Ships of which the tonnage-deck is Table of (according to the above measurement) 50 feet long or under, into 4 equal parts. 2. Ships of which the tonnage-deck is (according to the above measurement) above 50 feet long, and not exceeding 120, into 6 equal parts. 3. Ships of which the tonnage-deck is (according to the above measurement) above 120 feet long, and not exceeding 180, into 8 equal parts. 4. Ships of which the tonnage-deck is (according to the above measurement) above 180 feet long, and not exceeding 225, into 10 equal parts. 5. Ships of... which the tonnage-deck is (according to the above measurement) above 225 feet, into 12 equal parts.

(2.) Then, the hold being first sufficiently cleared to admit of the required depths and breadths being properly taken, find the transverse area of such ship, at each point of division of the length, as follows:—Measure the depth at each point of division from a point at a distance of one-third of the round of the beam below such deck; or, in case of a break, below a line stretched in continuation thereof, to the upper side of the floor-timber at the inside of the limber-strake, after deducting the average thickness of the ceiling which is between the bilge-planks and limber-strake; then, if the depth at the midship division of the length do not exceed 16 feet, divide each depth into four equal parts; then measure the inside horizontal breadths at each of the three points of division, and also at the upper and lower points of the depth, extending each measurement to the average thickness of that part of the ceiling which is between the points of measurement; number these breadths from above (i.e., number the upper breadth one, and so on down to the lowest breadth); multiply the second and fourth by 4, and the third by 2; add these products together, and to the sum add the first breadth and the fifth; multiply the quantity thus obtained by one-third of the common interval between the breadths, and the product shall be deemed the transverse area; but if the midship depth exceed 16 feet, divide each depth into 6 equal parts instead of 4, and measure, as before directed, the horizontal breadths at the five points of division, and also at the upper and lower points of the depth; number them from above as before; multiply the second, fourth, and sixth by 4, and the third and fifth by 2; add these products together, and to the sum add the first breadth and the seventh; multiply the quantity thus obtained by one-third of the common interval between the breadths, and the product shall be deemed the transverse area.

(3.) Having thus ascertained the transverse area at each point of division of the length of the ship, as required by the above table, proceed to ascertain the register tonnage of the ship in the following manner:—Number the areas successively 1, 2, 3, &c., No. 1 being at the extreme limit of the length at the bow, and the last number at the extreme limit of the length at the stern; then, whether the length be divided according to the table into 4 or 12 parts, as in classes 1 and 5, or any intermediate number, as in classes 2, 3, and 4, multiply the second and every even-numbered area by 4, and the third and every odd-numbered area (except the first and last) by 2; add these products together, and to the sum add the first and last if they yield anything; multiply the quantity thus obtained by one-third of the common interval between the areas, and the product will be the cubical contents of the space under the tonnage-deck; divide this product by 100, and the quotient being the tonnage under the tonnage-deck shall be deemed to be the register tonnage of the ship, subject to the additions and deductions hereinafter mentioned.

(4.) If there be a break, a poop, or any other permanent closed-in space on the upper-deck, available for cargo or stores, or for the berthing or accommodation of passengers or crew, the tonnage of such space shall be ascertained as follows:—Measure the internal mean length of such space in feet, and divide it into two equal parts; measure at the middle of its height three inside breadths—namely, one at each end, and the other at the middle of the length; then to the sum of the end-breadths add four times the middle breadth, and multiply the whole sum by one-third of the common interval between the breadths, the product will give the mean horizontal area of such space; then measure the mean height, and multiply by it the mean horizontal area; divide the product by 100, and the quotient shall be deemed to be the tonnage of such space, and tonnage shall be added to the tonnage under the tonnage-deck, ascertained as aforesaid, subject to the following provisos:

—First, that nothing shall be added for a closed-in space, solely appropriated to the berthing of the crew, unless such space exceeds one-twentieth of the remaining tonnage of the ship,—and in case of such excess, the excess only shall be added; and, secondly, that nothing shall be added in respect of any building erected for the shelter of deck-passengers and approved by the Board of Trade.

(5.) If the ship has a third deck, commonly called a spar-deck, the tonnage of the space between it and the tonnage-deck shall be ascertained as follows:—Measure in feet the inside length of the space at the middle of its height from the plank at the side of the stem to the lining on the timbers at the stern, and divide the length into the same number of equal parts, into which the length of the tonnage-deck is divided as above directed; measure (also at the middle of its height) the inside breadth of the space at each of the points of division, also the breadth of the stem and the breadth at the stern; number them successively, 1, 2, 3, &c., commencing at the stem; multiply the second, and all the other even-numbered breadths by four, and the third and all the other odd-numbered breadths (except the first and last) by two; to the sum of these products add the first and last breadths; multiply the whole sum by one-third of the common interval between the breadths, and the result will give, in superficial feet, the mean horizontal area of such space; measure the mean height of such space, and multiply by it the mean horizontal area, and the product will be the cubical contents of the space; divide this product by 100, and the quotient shall be deemed to be the tonnage of such space, and shall be added to the other tonnage of the ship, ascertained as aforesaid; and if the ship has more than three decks, the tonnage of each space between decks above the tonnage-deck shall be severally ascertained in manner above described, and shall be added to the tonnage of the ship ascertained as aforesaid.

In every ship propelled by steam, or other power requiring engine-room, an allowance shall be made for the space occupied by the propelling power, and the amount so allowed shall be deducted from the gross tonnage of the ship, ascertained as aforesaid, and the remainder shall be deemed to be the register tonnage of such ship; and such deduction shall be estimated as follows (that is to say):

(a) As regards ships propelled by paddle-wheels, in which the tonnage of the space solely occupied by and necessary for the proper working of the boilers and machinery is above 20 per cent. of the gross tonnage of the ship, such deduction shall be 37 hundredths of such gross tonnage; and in ships propelled by screws, in which the tonnage of such space is above 13 per cent. and under 20 per cent. of such gross tonnage, such deduction shall be 32 hundredths of such gross tonnage.

(b) As regards all other ships, the deduction shall, if the commissioners of customs and the owner both agree thereto, be estimated in the same manner, but either they or he may, in their or his discretion, require the space to be measured, and the deduction estimated accordingly; and whenever such measurement is so required, the deduction shall consist of the tonnage of the space actually occupied by, or required to be inclosed for the proper working of the boilers and machinery, with the addition in the case of ships propelled by paddle-wheels of one-half, and in the case of ships propelled by screws of three-fourths, of the tonnage of such space. In the case of screw-steamers, the contents of the shaft-trunk shall be added to and deemed to form part of such space.

In every registered British ship the number denoting the register tonnage, ascertained as hereinbefore directed, and the number of her certificate of registry, shall be Tonnage deeply carved, or otherwise permanently marked on her main beam, and shall be so continued; and if at any time cease to be so continued, such ship shall no longer be recognised as a British ship.

These rules have now been in operation for a period of five years, during which about 16,000 British, and a much greater number of foreign ships, have been measured by them. The experience of their operation thus afforded has proved highly satisfactory, with the single exception of the method of estimating the allowance made to steamers for their propelling power, by the plan of percentages on their gross tonnage. After explaining certain abuses under the old law, which led to the adoption of this mode of allowance, Mr Moorsom, in the paper before alluded to, stated, that "it had been found practically to admit of the intended allowance being anomalously increased by other means, and had given dissatisfaction even to the owners themselves, by its unjust action between steamer and steamer." In illustration of this, Mr Moorsom gave a paper of examples, by which it was seen that, in two paddle-ships of about the same gross tonnage and power, there could be, and frequently was, a difference in their allowances to the extent of 20 per cent.; and that in a similar case of screw-vessels, the difference was to the still greater extent of about 40 per cent. He likewise found that "the system of percentages frequently gave to large-powered coasting steamers undue allowances, to extreme-power tugs to within 1 per cent. of a negative tonnage, and to long-voyage auxiliary-power vessels, on the contrary, a less allowance than the old system." Steam-vessels seem to have gained by the new law, as regards their register tonnage, an average advantage over sailing vessels of a decrease of about 6½ per cent., which is very unequally and unjustly distributed between steamer and steamer. To give an example, the steam-tug United States, of Cardiff, 60 horse-power, is but 4 tons register. This anomaly will probably be soon remedied, such an alteration in the law being already provided for, without further legislative interference, under the provisions of the 29th section of the Merchant Shipping Act of 1854.