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The later "Government" maps

The Committee on Scientific Inquiries of the General Board of Health, the national body charged with investigating the cholera epidemic, also supervised the drawing of a spot map of the Broad Street area (see figure 2). This was printed both in the St James's parish report and in the more voluminous central government report [4, 14]. The precise relation between this map and Cooper's earlier map is unknown; the Parish report refers to their map as "constructed on the authority of the one published in Mr Cooper's Report" [4], but it is unclear whether this refers to the map as a whole, or only to the diagram of the sewers.

The later government maps were (like Cooper's map) more detailed than Snow's. For example, clusters of bars depicting many deaths in the same house were better drawn, compared with Snow's map where some of the bars appeared to be at quite a distance from the actual location of the house. The houses were numbered, making comparisons between the map and other data easier. The government investigators also had the advantage of Whitehead's discovery of the "index case" of the infant at number 40, Broad Street -- that house shows five bars on the government map (see figure 2), compared with four bars on Snow's two maps (see figure 1).

But having a better map to work from assured neither adoption of Snow's theory nor agreement among those who studied the map. The parish committee, as we have seen, supported Snow's claim that the Broad Street pump was the source of contamination, even though they held back from endorsing Snow's entire theory of the pathophysiology of cholera. By publishing the government map, they thought they were providing additional evidence in Snow's favor. By contrast, the Committee on Scientific Inquiries of the General Board of Health flatly rejected the pump theory and insisted that some concentrated noxious atmospheric influence, no doubt emanating from putrefying organic matter, was the cause of the Golden Square outbreak [2]. The committee's thinking paralleled that of another highly respected cholera authority, Edmund A Parkes:

"On examining map given by Dr Snow, it would clearly appear that the center of the outburst was a spot in Broad-street, close to which is the accused pump; and that cases were scattered all round this nearly in a circle, becoming less numerous as the exterior of the circle is approached. This certainly looks more like the effect of an atmospheric cause than any other; if it were owing to the water, why should not the cholera have prevailed equally everywhere where the water was drunk? Dr Snow anticipates this by supposing that those nearest the pump made most use of it; but persons who lived at a greater distance, though they came farther for the water, would still take as much of it ... There are, indeed, so many pumps in this district, that wherever the outbreak had taken place, it would most probably have had one pump or another in its vicinity." [15]

It was not unusual at that time to see spot maps invoked in defense of a miasmatist theory of disease transmission. As early as 1798, Seaman used two spot maps to illustrate a report on deaths from yellow fever in New York [16], and these maps were later used by both contagionists and anti-contagionists to advance their respective causes [17].

Cooper marked with a thick bar each house in which death from cholera had occurred, and indicated only by thin lines how many deaths had occurred in each house. Thus households that had suffered one and ten deaths, respectively, appeared similar on quick inspection. For Cooper, the cause of cholera was obviously a generalized environmental influence, and so what mattered were the locations at which deaths had occurred. By contrast, Snow's theory required the unit of analysis to be the individual, since in any house, some might have drunk the pump water and some not. Nonetheless, the Board of Health adopted Snow's method of marking the frequency of individual deaths (which was the standard method of the day), but without accepting Snow's theory.

The different conclusions drawn by the two official investigations in Broad Street are reflected in the one change that the parish committee made in the government map. The Board of Health, in constructing their map, saw no reason to focus on the Broad Street pump. By contrast, on the otherwise identical map included in the parish report, a circle was drawn, of a radius of 210 yards, with the Broad Street pump at the center. This circle defined the "cholera area" [4], within which the mortality was greatest. The parish committee apparently did not realize that by depicting the high-risk zone as circular, they were inviting a rebuttal such as Parkes', that the diffusion of an impure atmospheric influence would best explain the distribution of deaths. By contrast, Snow in drawing his irregularly-shaped area based on walking distance to the nearest pump, neatly turned aside such a criticism.

The government map-makers disagreed among themselves in one other way. The local inspectors appointed by the General Board of Health to investigate the Broad Street outbreak, Drs Fraser, Ludlow, and Hughes, found themselves walking the pavements at the same time that Cooper was making his survey of the sewers, and the two groups exchanged information on at least one occasion. But, while Cooper thought that his own map conclusively proved that the sewers were not to blame for the epidemic, the Board inspectors stated in their report that the gully-holes and their bad smells had been a major source of the cholera [14].  Neither Cooper nor the Board inspectors offered any sort of numerical analysis in defense of their respective interpretations.