struction, remained unchanged. But any such rule, resting upon no scientific basis, might possibly be evaded by various devices, and in course of time might become an evil which could no longer be borne with.

This was undoubtedly true of the oldest law for tonnage to which reference will be made, the so-called "builders' old measurement," or "B.O.M.," of which the use can be traced back for centuries, and which remained in force until 1836 as the basis for all dues on British shipping. Although then abolished, and replaced by another tonnage law, this older rule continues in use, with some modifications, up to the present time; its imperfections being condoned probably on account of its great age. Until 1872, the B.O.M. tonnage was the only one given in the Navy List for her Majesty's ships; but since then the displacement, as well as the B.O.M. tonnage, has been given. In the United States Navy List the old tonnage measurement, modified in some cases, is still retained. Yachts are also usually measured by a rule which is a slight modification of the B.O.M. rule. And yet it is a generally acknowledged fact that such great injury to the mercantile marine was done by this rule as to justify the repeal of the law that enforced it no less than forty years ago.

From 1836 to 1854 another law was in force for British merchant ships; but no account of this is needed, as it gave place at the latter date to the present law, which introduced "register tonnage"; a mode of measuring ships which has since been adopted by the greatest maritime powers, has been made the basis of charge for passing through the Suez Canal, and will probably be universally adopted before many years have passed. This fact alone is sufficient evidence of the fairness and superiority of the latest tonnage law; it is not absolutely free from objections, as applied to steam-ships, but these are not of great importance when compared with the general scope and working of the measure.

Still another unit of measurement is in common use, viz.

the "freight-ton" or unit of "measurement cargo for a given ship. This is a purely arbitrary measure, but it has a definite meaning, and is of considerable value in the stowage of ships.

It appears, therefore, that there are in common use the following tonnage measurements :

(1) Displacement.

(2) Builders' old measurement.

(3) Yacht measurement.

(4) Register tonnage.

(5) Freight tonnage.

It is not to be wondered at that some confusion exists as to the exact meaning of the term "tonnage," seeing it is used in so many senses, sometimes expressing weight, at others capacity, and at others a purely arbitrary function of the principal dimensions. In the following pages a brief description of these different tonnages will be given.

Respecting "displacement tonnage," it will suffice to say that it expresses the total weight of a ship (in tons) when immersed to her maximum draught, or "load-line." In the previous chapter allusion has been made to the process by which the naval architect from the drawings of a ship can estimate her displacement. This is a measurement especially suited for war-ships, which are designed to carry certain maximum weights, and to float at certain load-lines, that are fixed with reference to desired conditions, such as height of the guns above water, or limitations of draught imposed by the character of the service. Hence the displacement tonnage furnishes the fairest means of comparison between different types of war-ships; and it has long been in use in the navies of France, Italy, and other European countries, while it alone now appears in the Navy List for the ships of recent design, and in course of time will doubtless entirely replace builders' old measurement.

Merchant ships, as was shown in the previous chapter,

have not to carry certain fixed weights, nor have they usually a fixed maximum load-line. The very various conditions of cargo stowage to which these ships are liable entirely separate them from war-ships; the decision as to the draught to which they can be safely laden must vary with the character of the cargo, its mode of stowage, the nature of the voyage, and the season of the year. With some cargoes the ships might be safe at a deeper draught than would make them unsafe with other cargoes on board. Hence displacement tonnage would not be so fair a measure for merchant ships as it is for ships of war.

A modification of displacement tonnage has been proposed for merchant ships, and styled "dead-weight measurement." The idea is to express the tonnage of a ship by the number of tons weight of cargo which is or can be carried. Two methods of applying this scheme have been suggested. First, to fix the maximum load-line, calculate the corresponding displacement, and then to estimate the number of tons of cargo which she could carry, when floating at that line, in addition to her equipment—coals, stores, machinery &c. Second, it has been proposed to allow the tonnage to vary according to the number of tons of cargo on board at any time; this being ascertained by means of an officially guaranteed "curve of displacement." Against the first

proposal may be urged the difficulty just referred to of fixing by law the maximum load-line for any merchant ship; the second would make the tonnage of any ship a varying quantity, which would be most objectionable. In addition, dead-weight measurement has been objected to by the highest authorities on other than professional grounds. All changes so far made in the tonnage laws have intentionally been framed so as to leave unchanged the nominal aggregate tonnage of the British mercantile marine; but the use of dead-weight measurement would entirely change this aggregate tonnage; and there is no reason for incurring this

* See page 7 for a description of such curves.

serious practical disadvantage when there are other good grounds for objecting to the methods of measurement which would cause the change. Dead-weight measurement has been strongly supported, and on the first glance appears simple and practical, but it is not likely to supplant register tonnage for merchant ships.

Coming next to builders' old measurement, the rule may be briefly stated as follows:

(a) The length was taken on a straight line along the rabbet of the keel of the ship from the back of the main sternpost to a perpendicular line from the fore part of the main stem, under the bowsprit. Fig. 27 shows this; CA

FIG 27.

Upper Deck

D

Rabbet of Keel

is the perpendicular line, and AB is the length required. If the ship was afloat when the measurements for tonnage were made, the length AB could not be taken; and to allow for the rake of the sternpost (BE), and the consequent shortening of the keel, as compared with the length along the deck or water-line, a deduction was permitted of 3 inches for every foot of draught of water from the length measured along the water-line from the perpendicular line AC to the back of the sternpost. Long after raking sternposts ceased to be used in war-ships, a deduction continued to be made for the "rake" of a post which was upright, in order to secure a small diminution of the tonnage.

(b) The breadth was taken from the outside of the outside plank in the broadest part of the ship, exclusive of any additional thickness of planking or doubling strakes that might be wrought at that part. This reduction from the extreme breadth to obtain the "breadth for tonnage " amounted to 10

or 11 inches in large vessels, decreasing to 3 or 4 inches in small vessels; it expressed the excess in thickness of the "wales," worked in the neighbourhood of the water-line, over the ordinary bottom planking. In iron ships the breadth extreme and breadth for tonnage are usually identical, except in cases where the armour shelf "overhangs" the hull proper. The Devastation is a The Devastation is a case in point. Her breadth extreme (to outside of armour) is 62 feet; the armour and backing (on both sides) project some 4 feet beyond the hull beneath, and the breadth for tonnage is consequently only 58 feet. In the American monitors, with overhanging armour, similar deductions are made from the extreme breadth in estimating the breadth for tonnage. For example, the Dictator had a breadth extreme of 50 feet, and a breadth for tonnage of 41 feet 8 inches.

(c) From the length (obtained as described in (a)) was deducted three-fifths of the breadth for tonnage, the remainder being termed the "length for tonnage." This was multiplied by the breadth, and their product by half the breadth, and dividing by 94, the quotient expressed the tonnage.

In algebraical language, if L = the measured length along the rabbet of keel; B = breadth for tonnage,

Length for tonnage = (L-3 B);

As an example, take a ship for which L = 200 feet, B =

It will be noted that the continued product in the numerator expresses capacity; the divisor 94 was chosen originally with reference to the carrying power of the ships