Notwithstanding Homer’s recommendation of mules as ‘better far than kine to drag the jointed plough,’ oxen are still used in the greater part of the world. In light soils one yoke of oxen is sufficient, but in heavy fallows, with deep-working ploughs, two, three or more yoke were used; and in feudal times it would appear that the four tenants of a hide or ploughland co-operated with their oxen. A furrow of 40 rods could thus be made easily in one breath, and as this length of a rood coincided approximately with the eighth of a mile, that division of the mile was also called a furrow-long or furlong. When ploughing up fallow-land the oxen, on getting to the end of the ‘shot,’ turned and took breath. The ploughman measured a rod-breadth from the first furrow by means of his goad, Scottice by the ‘fall’ of it, and this rod-breadth down which the oxen turned, the _tornatura_ of Italy, was a rood. Sometimes between the roods a narrow unploughed strip, a balk of land, was left, marking the roods or ‘selions,’ four of which, side by side, made an acre, and forty of which made the square furlong, the ten-acre field. Ploughing in roods, selions, square furlongs, is still far from extinct. In Brittany land is still reckoned by _seillons_ of so many furrows wide, or of so many _gaules_ or 12-foot rods. In Southern France fields are estimated in breadths of a _destre_, of the 12-foot rod corresponding roughly to the width cleared by a couple of mowers. In our Isle of Axholme, in North Lincolnshire, land is reckoned in selions of a rod wide and usually of a furlong in length; these selions or roods being grouped into furlongs, that is, actually or originally, into greater units of a square furlong = 40 roods or 10 acres. 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.