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.