Page:Life of Sir William Petty 1623 – 1687.djvu/209

 the receipts;' to 'guess at our strength and wealth by the general stock employed in trade;' and to compute the population by the returns of the number of houses paying the tax. But even these data were very imperfect; for example, the farmers of the excise were never obliged to render a real account of their receipts prior to 1674, nor was there any correct return of the gross produce of the hearth money prior to 1679.

The study of the bills of mortality of the City of London, which comprised 134 parishes in Middlesex and Surrey, probably furnished him with his most valuable data; but it is obvious that the difficulties with which he had to contend from the insufficiency of his materials could not fail greatly to impede the accuracy of his conclusions; and Davenant goes on to regret that the 'excellent wit' and 'skilful hand' of the author had not survived till a later date, when the fuller information which had accumulated under the variety of taxes that had been lately levied in the kingdom would have been at his command for purposes of comparison.

Of the method of these calculations, admittedly founded on insufficient data, his views in regard to the amount of the population of London may be given as an example. He starts from the number of houses prior to the Plague and Fire, which appear by the register to have been 105,315; he then estimates that one-tenth of the houses held two families, the remaining nine-tenths only one; he takes the average number of persons to a family at ten in the wealthier, five in the poorer, eight in the middle class. He then checks his calculation in two ways: by multiplying the annual death rate by thirty, and by taking the number of deaths in the Great Plague—estimated at one-fifth of the total population; and then calculates the natural rate of increase subsequently. The three methods produce results approximating to the same result: the first, 695,076; the second, 690,360; the third, 653,000. His calculation of the stock of Ireland he bases on