John Snow, Asiatic Cholera and the inductive-deductive method

Lecture 13: Farr’s Law of epidemics

We have seen the spatial distribution of deaths in the area of the Broad Street pump. Snow adds to this with a time distribution showing that the peak of the outbreak occurred in the first two days of September 1854. There had been sporadic cases up to then, but all of a sudden, 143 new cases arose on 1 September, followed almost immediately by a peak in deaths.

The common explanation for the downward curve is that Snow removed the pump handle, and the outbreak soon subsided. This is sheer nonsense for a variety of reasons.

The only authority for any such action was the St James’s Parish Council. To this body, Snow delivered an explanation and a request for the removal of the handle on the evening of 7 September when he knew the worst was already over. 

“I had an interview with the Board of Guardians of St. James's parish, on the evening of Thursday, 7th September, and represented the above circumstances to them. In consequence of what I said, the handle of the pump was removed on the fo…