Page:Life of Sir William Petty 1623 – 1687.djvu/210

 the area of the pasture land of the country, 'supposing them to be competently well stocked,' and he 'guesses' that one-third of the small occupiers have one horse, and 'supposes' that 16,000 wealthier families have 40,000, and so on. He is of opinion that, because the export of Irish butter and cattle in 1664 had increased one-third since 1641, the population had increased one-third since the latter date also. He calculates that the population doubles itself in 40 years, and that the present population of London being about 670,000, the population of the 133 parishes would in 1840 be 10,718,880, almost equal to the population of the whole of the rest of the country, a result which he thinks impossible; and he anticipates that the highest point in population will be reached about 1800, and that afterwards there will be a falling-off. Such calculations are manifestly hazardous, and based on very imperfect premises; but they were the best of which the existing materials admitted. Nor was anyone better aware of their defective character than the author himself. 'Curious dissections,' he says, 'cannot be made without variety of proper instruments, whereas I have had only a common knife and a clout, instead of the many more helps which such a work requires;' and his works contain constant and oft-repeated pleas for the collection of more accurate information, and for the intervention of the State, especially in regard to a correct enumeration of the people, statistics of trade, and a register of lands and houses: until then everything will be 'by hit rather than by wit, and all calculations merely conjectural.'

Of Sir William's contributions to the infant science five have achieved a permanent reputation: the 'Treatise on Taxes and Contributions,' published in 1662; the 'Discourse on Political Arithmetick,' written in 1671, but not published till 1691; and a tract entitled 'Quantulumcumque concerning