Agricultural historians have long been aware that a major increase in productivity and output characterised the so-called ‘agricultural revolution’. Usually, however, this has been measured by indirect means: the fact, for example, that English farmers were able to feed some 3 million more people in 1700 than in 1540, and almost 20 million more in 1880 than in 1750. Since mouths were fed without recourse to massive imports -which would have had significant economic implications for the industrial revolution -and since these increases in output were achieved while the agricultural labour force was in steep relative decline, the obvious implication is that productivity was increasing. Measuring such changes has proved complex, partly because data were not collected in a systematic fashion prior to the 1870s, and partly because such evidence as we have relating to prices and rents hardly represents an adequate proxy for productivity. In general terms, the best material has been for the grain acreage, particularly for wheat and barley.