Surface.—The square furlong is 10 acres; the acre is 10 sq. chains, or 4840 sq. yards, and may be divided into 160 sq. rods.
Volume.—The cubic foot is approximately 1000 ounces of water, = 62-1/2 lb. The Ton-register is 100 cubic feet.
Weight.—The pound, of 7000 grains, is divided into 16 ounces of 437-1/2 grains.
The Gallon of water weighs 10 lb.
The Cental is 100 lb.
The Ton is 20 long Cwt. of 112 lb.
Capacity.—The Imperial gallon = 277-1/4 c.i. contains exactly 10 lb. of water, or approximately 8 lb. of wheat. It is divided into 8 pints containing 20 oz. of water or 16 oz. of wheat. The Bushel, of 8 gallons, contains 64 lb. of wheat.[[58]] The Quarter is 8 bushels, which is approximately the quarter, either of a short ton, 20 centals, of wheat, or of a freight-ton of 40 cubic feet.
The principal units, foot, pound, gallon, are connected by their common origin in the talent of 1000 ounces of water. Corrections are needed for accuracy since the pound was increased in Elizabethan times by a little more than 1 per 1000 from its original Roman standard, and since the bushel and gallon were increased by 3 per cent. from the original corn-measure to the Imperial standard.
The co-related units, foot, furlong, acre, pound, gallon, are multiplied and divided by the factors found by long use to be the most convenient to the people. When no other influence determines the secondary units, the usual factor is 8, or its double, its half, its quarter.
Any unit may be decimalised for purposes of calculation, and several series have alternative decimal series. Thus—