The system of the United States only differs from the Imperial system in its retention of the wine-gallon = 231 c.i. and of the corn-gallon = 268·8 c.i.; and in its rejection of the long cwt. for the cental.
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—
Itinerary and Land measures were decimalised three centuries ago by the chain-series.
The Ton-register of 100 cubic feet, used throughout the world, has a complete decimal series of divisions.
The pound-gallon-cental series are fully decimalised, from the 100-lb. cental down to the septem, 1/1000 of a pound.
A decimal series of weights from the pound upwards is perfectly lawful. It may be confidently expected that it will before long replace for most purposes the stone and long-hundredweight series imposed in the fourteenth century, and fought against ever since.
Apothecaries’ weight, abolished by the Medical Council half a century ago, still lingers in the Board of Trade list of standards. Mint-weight is still on the Troy system. The half-crown is one grain less than an Imperial half-ounce. It may be hoped that it, and other silver coins, will before long be brought exactly to that standard. Already the bronze penny is one-third of the Imperial ounce.
Further improvements will be made. Some adjustment of the inconvenient 112-lb. hundredweight with the cental series, that of our ancient hundredweight, returned to us from America, will probably be effected. In the meantime we know that our system is progressive.