"We have in this mode of measurement all the accuracy we can desire; and we find in practice in the workshop that it is easier to work to the ten-thousandth of an inch from standards of end measurements, than to one-hundredth of an inch from lines on a two-foot rule. In all cases of fitting, end measure of length should be used, instead of lines."(20) Improvements in accuracy continued to be made, and by 1896 a Swede, Carl Johansson, was able to make blocks of steel, now called "Jo" blocks, accurate to four-millionths of an inch, to be used for the cali- bration of other measuring devices. The assembly line found its first application in pig processing in the 1860s. Pigs slaughtered at the top level of a building were conveyed on hooks past various workmen, each of whom removed an individual part until the carcass was completely processed.21 Interchangeable parts, precision measurement, and the assembly line, coupled with the increased 20 20Quote from Joseph Whitworth, cited by Robert S. Woodbury, "Machines and Tools," in Technology in Western Civilization, ed. by Kranzberg and Pursell, op. cit., p. 621. Woodbury's article is an excellent source for information concerning developments in precision measurement and machine tools during the nineteenth century, and their relation to mass production by precision methods. 2Giedion, Mechanization Takes Command, pp. 239-45. Henry Ford later reversed the same process by assembling (rather than disassembling) parts as they moved along a conveyor. In 1913 Ford's experiment using the production line method on the assembly of magnetos realized a substantial savings in labor and time; he later successfully applied the technique to automobiles. Eventually, of course, the system became commonplace.