But the interest of those “Essays in Political Arithmetic” lies chiefly in the facts presented by so trustworthy an authority. London had become in the time of the Stuarts the most populous city in Europe, if not in the world. This Sir William Petty sought to prove against the doubts of foreign and other critics, and his “Political Arithmetic” was an endeavour to determine the relative strength in population of the chief cities of England, France, and Holland. His application of arithmetic in the first of these essays to a census of the population at the Day of Judgment he himself spoke of slightingly. It is a curious example of a bygone form of theological discussion. But his tables and his reasonings upon them grow in interest as he attempts his numbering of the people in the reign of James II. by collecting facts upon which his deductions might be founded. The references to the deaths by Plague in London before the cleansing of the town by the great fire of 1666 are very suggestive; and in one passage there is incidental note of delay in the coming of the Plague then due, without reckoning the change made in conditions of health by the rebuilding. Nobody knew, and no one even now can calculate, how many lives the Fire of London saved. There was in Petty’s time no direct numbering of the people. The first census in this country was not until more than a hundred years after Sir William Petty’s death, although he points out in these essays how easily it could be established, and what useful information it would give. There was a census taken at Rome 566 years before Christ. But the first census in Great Britain was taken in 1801, under provision of an Act passed on the last day of the year 1800, to secure a numbering of the population every ten years. Ireland was not included in the return; the first census in Ireland was not until the year 1813. Sir William Petty had to base his calculations partly upon the Bills of Mortality, which had been imperfectly begun under Elizabeth, but fell into disuse, and were revived, as a weekly record of the number of deaths, beginning on the 29th of October, 1603; notices of diseases first appeared in them in 1629. The weekly bills were published every Thursday, and any householder could have them supplied to him for four shillings a year. These essays will show how inferences as to the number of the living were drawn from the number of the dead. And even now our Political Arithmetic depends too much upon rough calculations made from the death register. It is seven years since the last census; we have lost count of the changes in our population to a very great extent, and have to wait three years before our reckoning can be made sure. The interval should be reduced to five years.