to reckon the population of London, of England, and of Ireland. So far as London is concerned, he had as a basis certain facts—the number of burials and the number of houses—which bear some relation to the number of people. He then multiplied the number of burials by thirty,[1] satisfying himself by quoting Graunt's authority for that number. The result thus obtained he sought to confirm by multiplying the number of houses by a factor assumed to represent the average number of inhabitants to a house. This factor is sometimes six[2] and sometimes eight,[3] as chanced to suit his purpose. He next assumes that the population of England is eleven times that of London, or 7,369,000, because London pays one-eleventh of the assessment, and asserts that the results thus obtained "do pretty well agreee" with the returns of the hearth and poll money and with "the bishops late numbering of the communicants." He does not himself give any of these figures; but it has been discovered[4] that, according to the accepted rules of political arithmetic, the bishops' enumeration accounted for only 82 per cent. of the number that Petty calculated. In all these cases, however, there is some real basis for his calculations; and Petty was himself under no delusions as to the accuracy of his result. Thus he says, "Although the said number of 7 millions, 369 thousand, be not (as it cannot be) a demonstrated Truth, yet it will serve for a good supposition, which is as much as we want at present."[5] Both the strength and the weakness of his method are abundantly exemplified in his writings. Such of his terms of number, weight, and measure as result from actual enumeration are often the basis for conclusions of value; for he had large capacity for distinguishing the essential from the incidental in any economic problem. But the obstacles in the way of enumeration were, in almost all quarters, in-
