case the brewers mostly tended to drink beer, not water (Snow 1855: 42). At other
addresses, closer to another water pump than to Broad Street, Snow learned that the
deceased residents had preferred, for one reason or another, to take water at the Broad
Street pump.
3
One victim, a widow who lived in the Hampstead district of London and
had not been in the neighborhood of Broad Street for months, used to send for water from
the Broad Street pump, and she drank this water in the two days before dying of cholera
during the epidemic (Snow 1855: 44).
Snow’s experience as a clinician, his studies of the pathology of cholera deaths, his
investigation of previous epidemics, and his spot map showing the proximity of victims
to the Broad Street pump all provided bits of evidence, which suggested that cholera
might indeed be an infectious disease carried by waste or water. However, although
these “causal process observations” (Brady and Collier 2004) were crucial both for
allowing Snow to formulate a hypothesis about the causes of cholera transmission and for
providing evidence of the plausibility of this hypothesis, his strongest piece of evidence
came from a “natural experiment” which he exploited during the epidemic of 1853-54.
Large areas of London were served by two water companies, the Lambeth company and
the Southwark and Vauxhall company. In 1852, the Lambeth company had moved its
intake pipe further upstream on the Thames, thereby “obtaining a supply of water quite
free from the sewage of London,” while the Southwark and Vauxhall company left its
intake pipe in place (Snow 1855: 68).
3
Snow writes, “There were only ten deaths in houses situated decidedly nearer to another street pump. In
five of these cases the families of the deceased persons informed me that they always sent to the pump in
Broad Street, as they preferred the water to that of the pump which was nearer. In three other cases, the
deceased were children who went to school near the pump in Broad Street. Two of them were known to
drink the water; and the parents of the third think it probable that it did so. The other two deaths, beyond
the district which this pump supplies, represent only the amount of mortality from cholera that was
occurring before the irruption took place” (1855: 39-40).
Convention