RELATIONS TO OTHER SCIENCES 61 



supply, water-supply, gas-supply, and drainage, the scientific serv- 

 ices not only of architects, but of engineers, and such public 

 buildings form one small section of the aid which modern engin- 

 eering science is now everywhere rendering to public health science. 

 The present has rightly been called an " age of engineering," and 

 to no other science, excepting only medicine itself, is public health 

 science to-day more indebted than to engineering science. I have 

 referred above to the construction of the first municipal filter at- 

 tached to a public water-supply as that of the Chelsea Company 

 of London, constructed in 1829. How different is it to-day! Not 

 only nearly the whole of London, but also Berlin and Hamburg, 

 and a thousand lesser cities all over the civilized world, are now 

 protected more or less perfectly from epidemics of typhoid fever, 

 Asiatic cholera, and other water-born diseases, by vast municipal 

 filters, ingenious and scientific in design and costly in construction, 

 the w r ork of skillful and faithful engineers, and monuments more 

 precious if less enduring than brass to the contributions of engin- 

 eering science to public health science. Innumerable storage re- 

 servoirs and vast distribution systems for supplies of pure water 

 also bear witness to the enormous debt which public health science 

 owes to engineering science, as do proper street construction and, 

 still more, those splendid systems of sewerage with which so many 

 modern cities are equipped, and which not only serve to remove 

 quickly dangerous liquid waste of human and animal life, but 

 also keep low and wholesome the level of the ground-water, re- 

 ducing dampness and promoting dryness of the environment, and 

 thereby strengthening that physiological resistance by means of 

 which the human mechanism fights against the attacks of infec- 

 tious disease. Nor do the services of engineering science end here, 

 for the fluid content of the sewers must always be safely disposed 

 of, and sewage purification is to-day a problem of engineering 

 science no less important or difficult than that of water purifica- 

 tion. These same processes of the purification of water and sewage 

 are matters of so much moment in public health science that in 

 almost every country experiment stations are now maintained 

 at public and private expense for the purpose of working-out the 

 most practical and most scientific methods of purification. 



In no respect have the services of engineering science to public 

 health science been more conspicuous than in the application and 

 the further study of the principles involved in the processes of water 

 purification. It has lately been shown, for example, that the in- 

 troduction of pure water-supplies has in many cases so conspicu- 

 ously lowered the general death-rate as to make it impossible to 

 escape the conclusions (1) that the germs of a greater number of 

 infectious diseases than was formerly supposed are capable of pro- 



