Encyclopædia Britannica, Ninth Edition/Acre (1.)

Acre, a measure of surface, being the principal denomination of land-measure used in Great Britain. The word (akin to the Saxon acer, the German acker, and the Latin ager, a field) did not originally signify a determinate quantity of land, but any open ground. The English standard or imperial acre contains 4840 square yards, or 10 square chains, and is also divided into roods, of which it contains 4, the rood again being divided in 40 perches. The imperial acre has, by the Act 5 Geo. IV. c. 74, superseded the acres, of very different extent, that were in use in different parts of the country. The old Scottish acre was equal to 1.26118345 imperial acres. The Irish acre contains 7840 square yards. The acre is equivalent to .40467, i.e., about 2/5ths, of the French hectare (now the basis of superficial measurement in Germany, Italy and Spain, as well as in France), .7 of the Austrian joch, .37 of the Russian desätine, and 1.62 Roman jugera. The hectare corresponds to 2 acres 1 rood 35.38 perches.