Simple country-folk, whose only ideas of land-measure were taken from the length of the ox-goad and of the furrow, and from the breadth of the long acre-strip of land, came slowly to understand that the surface of a field of irregular shape might be reckoned in acres and rods. A statute of Edward II gave a table of the different breadths of the acre when it was less than forty rods or perches in length:
‘When an acre of land containeth ten perches in length, then it shall be in breadth sixteen perches; when it containeth eleven perches in length, then it shall be in breadth fourteen and a half and three-quarters of a foot’—and so on through the different lengths an acre might be.
So people came gradually to abstract the idea of superficial measure from shape and to apply it to land of any figure, however different from a square or a rectangle. Thus measures, always _concrete_ at first and taken from some known object of comparison, became _abstract_ in men’s minds for purposes of calculation. Then came the land-surveyor introducing arithmetic and geometry into the art of measurement, and using the cord or chain instead of the measuring rod; and it was also found that decimal calculation would be an improvement in this art.
For purposes of accurate measurement and calculation, Edward Gunter introduced, nearly three centuries ago, measurement by a chain of a hundred links and twenty-two yards or four rods in length. Its adoption decimalised the land-measures without disturbing them. Ten chains go to a furlong and ten square chains to an acre.
Norden (‘Surveior’s Dialogue,’ 1610) mentions the ‘standard chaine, that is by the chaine of 16-1/2 foote.’ It was soon after this that the chain was increased to 66 feet or 4 rods, which length was a current unit, the ‘brede’ or acre-brede, the breadth of an acre.
In the following table each superficial unit is placed opposite the lineal unit of which it is the square:
─────────────────────────┬─────────────────────────────────────────────── LINEAL MEASURES │ SUPERFICIAL MEASURES ─────────────────────────┼─────────────────────────────────────────────── 12 inches 1 foot │ 144 square inches 1 sq. foot. 3 feet 1 yard │ 9 square feet 1 sq. yard. 5-1/2 yards 1 rod │ 30-1/4 square yards 1 sq. rod. │⎧ 40 square rods 1 rood 40 rods 1 furlong │⎨ (4 roods or 160 square rods 1 acre). │⎩ 40 roods (10 acres) 1 sq. furlong. 8 furlongs 1 mile │ 64 square furlongs (640 acres) 1 sq. mile. ─────────────────────────┴───────────────────────────────────────────────