1. Data for 1800 and 1950 are from Bureau of the Census, Historical Statistics of the United States, Colonial Times to 1957 (Washington: Government Printing Office, 1960) series K 83–97, p. 281 (hereafter cited as Historical Statistics, Colonial-1957). Data for 1973–1977 are from USDA, Agricultural Statistics 1978 (Washington: Government Printing Office, 1979), Table 642 (hereafter cited as Agricultural Statistics 1978).
2. William L. Cavert, The technological revolution in agriculture, 1910–1955, Agric. Hist. 30 (1956): 18–27.
3. John C. Ellickson and John M. Brewster, Technological advances and the structure of American agriculture, J. Farm Econ. 29 (1947): 827–847.
4. A. N. Johnson, The impact of farm machinery on the farm economy, Agric. Hist. 24 (1950): 58–62.
5. Wayne D. Rasmussen, The impact of technological change on American agriculture, J. Econ. Hist. 22 (1962): 578–591.