Essays on Mankind and Political Arithmetic — A Closer Reading

Counting Houses and Hearth Taxes

Petty employs multiple methods to estimate the number of houses in London. He cites the 13,200 houses burned in the Great Fire of 1666, then uses burial ratios to project housing totals for 1686. He also references a map from 1682 that counted above 84,000 houses, and hearth tax data from Dublin and Bristol to cross-check London's figures. The hearth tax returns show 388,000 hearths in London, which Petty converts into house counts using ratios from other cities: Dublin yields 87,000 houses, Bristol 123,000. He settles on a medium estimate of 105,000 houses, later confirmed by a certificate from the hearth office. This triangulation of sources reveals Petty's method: no single dataset is trusted alone.

Plague Mortality as a Census Tool

Petty uses plague deaths to estimate total population. He notes that in Nero's time, 300,000 died of plague in Rome, and if three of ten died then (versus two of ten in London), Rome's population was about one million. London's plague deaths are then compared to infer its population of roughly 700,000. He also compares the built areas: Rome's walled area of about seven square miles plus suburbs totals thirteen square miles, while London's built ground is about nine square miles. The ratios of area and population align, leading Petty to conclude that old Rome was "half as big again" as contemporary London. This reasoning shows how Petty uses historical parallels to validate his estimates.

Households and Heads per Family

Petty estimates the number of families per house and heads per family. He suggests that in Paris there might be three or four families per house, while in London about one-tenth of houses contain two families. For heads per family, he relies on John Graunt's observation that tradesmen's families average eight persons, higher ranks above ten, and the poorest near five. Petty had previously calculated a national average of six and one-third heads per family, but for London he adopts a different figure, agreeing with "Monsieur Auzout" (likely the French astronomer Adrien Auzout). This section illustrates Petty's willingness to adjust assumptions based on local data and expert opinion.

Petty's essays are best read as a demonstration of method rather than a definitive census. His willingness to combine disparate sources—hearth taxes, burial records, maps, and foreign comparisons—shows a mind intent on extracting order from imperfect data. Readers should note that Petty often states his assumptions explicitly, making his reasoning transparent even when his numbers are rough. The essays reward attention to his arithmetic steps and his candid acknowledgment of uncertainty.