The London Bills of Mortality were printed every Thursday for sixty years before anyone added them up. Parish clerks walked their wards, counted the burials, recorded what had killed each person after 1629, and sent the tallies to the Worshipful Company of Parish Clerks, which had been charged with producing them by charter in 1611. The sheets were sold. Londoners bought them the way people now check a weather forecast, to judge whether the plague was close enough to justify leaving the city.
So the data was public, weekly, continuous from 1603, and read by thousands of people with a direct personal interest in what it said. For six decades nobody asked the Bills anything except whether the plague was getting closer.
What a haberdasher did with them
John Graunt sold small wares in Birchin Lane. He had no university training, no patron, and no standing in the natural philosophy of his day. In 1662 he published eighty-five pages called Natural and Political Observations Made upon the Bills of Mortality, working from the annual summaries running back to 1604.
He wanted to know how many people lived in London. Nobody knew. The Bills did not say, because the Bills were not built to say, and no census existed to consult. What the Bills contained was burials, christenings, and after 1629 a cause of death, none of which is a population figure.
Graunt got one anyway, three separate ways. From christenings, assuming a rate of births per woman of childbearing age and a ratio of such women to the whole, he reached about 384,000. From burials, working the same logic in reverse, he reached 381,338. Then he abandoned the vital records entirely, took a map, counted the households he estimated would fit inside the walls at a plausible density, and reached 380,160.
Three routes, three numbers, agreeing inside one percent. That is the moment worth stopping on. Any one of the three estimates is a guess resting on assumptions a reader can dispute. All three landing together is evidence, because it is very hard to construct three wrong methods that fail in the same direction by the same amount.
The other things he found
More boys are born than girls, at a ratio he put at roughly fourteen to thirteen. London grew despite burying more people than it christened, which meant something like six thousand people were arriving from the countryside every year. Roughly a quarter of plague deaths were being recorded as other causes, because a household with a plague death was shut up and searchers could be paid to write down something else.
None of those facts was in the Bills. All of them were derivable from the Bills by someone willing to treat a plague warning as evidence about a city.
What was actually new
Very little of this was unprecedented on its own, and the histories that make Graunt the beginning of something are usually skipping the parts that make him interesting.
People had counted polities for centuries. Domesday in 1086, the Hundred Rolls in 1279, the Florentine catasto of 1427, English parish registers from 1538. Giovanni Villani estimated the population of Florence around 1338 from the city's daily bread consumption, which is the same inferential move Graunt made from burials, three hundred years earlier. Causal reasoning about economies was older still. Azpilcueta explained Spanish inflation by the abundance of silver in 1556, and by the 1620s Gerard de Malynes and Thomas Mun were arguing in print about whether a nation's trade balance could be measured at all.
What Graunt did was join two things that had stayed apart. He took an administrative series kept continuously for another purpose, and he asked it questions its compilers were not asking, showing the arithmetic so that anyone could contest a step. A snapshot survey tells you what someone decided to record. A serial record kept by clerks who had no theory to defend will answer questions nobody thought to put to it, provided somebody thinks to put them.
He called his method the Mathematiques of my Shop-Arithmetique, which is both accurate and a little pointed, coming from a tradesman publishing in a field owned by gentlemen. The Royal Society elected him that same year on the king's recommendation.
Where it breaks
His most famous table is not a life table, whatever it is usually called. A life table needs age at death, and the Bills did not record age until 1727. Graunt has a column showing how many of a hundred people survive to each decade, and the column is built from an assumed infant mortality rate, an assumed figure for the very old, and an interpolation across the middle that he never explains.
He knew the gap was there. He could not close it with the records he had, and it stayed open for thirty-one years until a set of registers turned up in Breslau that recorded the one thing the London clerks had never been asked to write down.