This historic 1941 diagram, which had been digitally enhanced, illustrated a method of altering tank-type flush toilets, to prevent back-siphoning, or the sudden loss of pressure within a plumbing system, allowing dirty water to be sucked into the clean water supply pipes. Such a phenomenon could involve just the plumbing in a single house, if it was on its own well, or it could occur inside a municipal water system. The focus of these images was on protecting water supplies from bacterial contamination due to incorrect disposal of waste, in order to prevent water-borne diseases. The descriptive information accompanying this drawing referred to a governmental publication, which contained instructions describing how to correctly place, and build such a device. The specific citation was, “Minnesota Department of Health, Division of Sanitation, Manual of Water Supply Sanitation, 1938, Section 8, p. 14”.