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[Extract from The Discourse Concerning the
Use of Duplicate Proportion[1], 1674.]

The Eleventh Instance.

In the Life of Man, and its Duration.

It is found by Experience, that there are more persons living of between 16 and 26 years old[2], than of any other Age or Decade of years in the whole life of Man (which David and Experience say to be between 70 and 80 years:) The reasons whereof are not abstruse, viz. because those of 16 have passed the danger of Teeth, Convulsions, Worms, Ricketts, Measles, and Smallpox for the most part: And for that those of 26. are scarce come to the Gout, Stone, Dropsie, Palsies, Lethargies, Apoplexies, and other Infirmities of Old

  1. The fundamental idea of Petty's "Discourse of Duplicate Proportion" is that certain phenomena, capable of expression in terms of number, weight and measure, stand related to one another as the squares or cubes, or as the square or cube roots of their respective quantities. Petty illustrates his theory by a number of "instances," drawn for the larger part from the physical sciences. Some of his instances are correct, some are fantastic. Only two of them, the eleventh and the sixteenth, are at all closely connected with the subject of his economic writings, and these instances are reprinted as apposite illustrations of an idea which was not without influence upon his work in political arithmetick. The eleventh instance is found at pages 82–88, the sixteenth at pages 106–109 of the "Discourse," as printed in 1674. See Bibliography. Cf. also Birch, iii. 156, Fitzmaurice, 268. Bishop Barlow's Remains contain a sharp criticism of the "Discourse."
  2. Cf. Graunt's "Observations," p. 387.