The cubic foot and the cubic inch are the usual measures of solidity. The cubic yard is used as a measure of masonry, earthwork, or reservoirs of water.

The cubic foot has many points of concordance with weights and with measures of capacity, and is the basis of ship and cargo measurement.

The definition of the Imperial gallon as 277·274 cubic inches, the volume of 10 lb. of water at 62°, a pound of water measuring 27·7274 cubic inches, led to attempts to determine accurately the weight of a cubic inch and of a cubic foot of water. These experiments are interesting in consequence of the recognition, in 1685,^[29] that the cubic foot of water weighed approximately 1000 ounces, and of the probability that this weight of water in Roman ounces, = 437 grains, was the source of our Imperial system. It has already been shown how difficult it is either to construct accurately a measure containing a certain weight of water or conversely to determine the weight of water in a standard measure.^[30]

The statute definition of the cubic inch of water as = 252·458 grains at 62° corresponds to 62·326 lb., or 997·21 ounces, for the cubic foot. Reduction of these weights to the standard of maximum density of water at 39·2° increases the weight of the cubic inch by 0·29 grain, and of the cubic foot by 1·1 ounce, making it = 62·4 lb. or 998·3 ounces. An Order in Council of 1889 gives 252·286 grains as the weight of the cubic inch of water. But the exact weight is uncertain, and the 1824 statute definition seems to be as accurate as the more recent determinations, all different.

It may be taken that the cubic foot of water weighs very approximately—

And 1000 ounces of water at the original weight of the averdepois ounce, of Roman standard = 437 grains, would weigh 999·5 of such ounces, at 62° in air.

Practically measures of capacity need only approximate coincidence with standards; they are used for convenience in order to avoid weighing, especially in retail trade. Corn and many other kinds of produce are more conveniently measured than weighed, the average weight being ascertained, if desired, by a sample bushel.

Fluids may also require corrections for temperature when bought or sold by measure. Water increases in volume 1 per 1000 between 39° and 61°; and another 1 per 1000 between 61° and 70°; other fluids have their peculiar coefficients of expansion.

Allowing then for small temperature-corrections, the cubic foot may be taken as equal to 62-1/2 lb. or 1000 ounces of water, and at this sufficiently approximate standard it becomes the basis of a series of measures for ship and other purposes.

The Ton Register

The capacity of ships has for centuries been reckoned in tons. The term arose from the custom, in French and other wine ports, to take as the unit of cargo-bulk the tun of wine usually contained in four hogsheads, each of 63 wine-gallons. The number of hogsheads divided by 4 gave the tonnage to be charged.

This cargo-ton, the tonneau d’encombrement, was equal to 42 French cubic feet = 51 English cubic feet.

The Ton Register appears to have arisen in the ports of Northern Europe. There the unit was usually the skippund (ship-pound) of about 360 lb. for wool and light goods. But the Last was also a wide-spread, though variable, measure; in the Baltic trade it was usually reckoned at 11-1/4 quarters of wheat = 90 bushels or 5400 lb. In England it was usually 10 quarters = 80 bushels = 5000 lb. Now this bulk of wheat measures about 100 cubic feet, so 100 English cubic feet has become the unit adopted in all maritime countries, as the Ton Register. In France it is called the tonneau de jaugage and is taken as = 2·83 cubic metres.

A ship of 2000 tons register is of a capacity = 200,000 cubic feet below decks. The register tonnage is thus obtained:

Mean length × 0·94 of maximum beam × depth from upper deck to keel, the measure being taken inside, and in feet. The product is cubic feet, which divided by 100 gives register tonnage.

In France these measurements have to be made in metres; the product in cubic metres is divided by 0·38 to get tonnage.

Net tonnage, as distinguished from gross tonnage, is the latter less the space occupied by cabins below deck, by engines and bunkers, in short all that is not ‘hold.’

This deduction gives the space available for cargo, a very large proportion in a sailing-ship, a very small proportion in a steam-yacht or tug.

The Cargo Ton is usually reckoned at 40 cubic feet; the space occupied by 20 centals = 4 quarters of wheat, or 25 centals of water.

A steamship of 4500 tons register may be 3000 tons net; as each of these net tons will contain 2-1/2 tons of cargo of about the same weight as wheat, after allowing for cases, dunnage, &c., the ship may be described as carrying 7500 tons dead-weight. Of course, this would only apply to goods of medium weight; not to iron rails or to ore, which could only be taken as a limited part of the cargo, the rest of the space being either filled with light goods or remaining empty.

The ship-owner has the choice of charging freight by measurement, usually at 40 c. ft. to the ton, or by the ton weight for metal and other heavy goods.