"Farther Considerations on the Dublin Bills of Mortality," in 1686;
and "Five Essays on Political Arithmetic" (in French and English),
"Observations upon the Cities of London and Rome," in 1687, the last
year of Sir William Petty's life. Other writings of his were
published in his lifetime, or have been published since his death.
He was in the study of political economy one of the most ingenious
and practical thinkers before the days of Adam Smith.

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