510 ELEMENTARY BIOLOGY 



of various crops, the best methods for eradicating weeds or 

 exterminating certain insects, how to get the best results from 

 feeding cattle or hens, the best kinds of plants or animals to 

 raise for various purposes, and so on. 



The United States Department of Agriculture is not only 

 cooperating with the various state agencies, but is directing 

 special investigations of a kind that may be too costly for a 

 single station to undertake, or on problems that concern the 

 people of more than a single state. 



In every large city the department of health has a number 

 of men and women whose business it is to make special inves- 

 tigations on special phases of the local health problems. They 

 study the water supply, the milk supply, the markets, the 

 sewerage system, the disposal of garbage ; they examine speci- 

 mens for the more accurate diagnosis of disease, and they con- 

 duct experiments with a view to increasing the accuracy of 

 diagnosis or to shortening the time of diagnosis, for in some 

 diseases a few hours may be of great significance. They ex- 

 periment for the purpose of improving materials and methods 

 in the preparation of vaccines and serums, and they investigate 

 the relative efficiency of different methods of fighting flies or 

 of ventilating factories or schoolhouses. 



The departments of health in the various states are also 

 doing more and more systematic work in extending our knowl- 

 edge of the conditions that make for health. The United 

 States government contributes to the solution of these problems 

 through the work of the Public Health Service, which not only 

 has the supervision of the marine hospitals, but conducts im- 

 portant investigations on special diseases 1 and on methods for 

 preventing epidemics. 



1 Important investigations were conducted by the United States govern- 

 ment scientists, leading to the discovery of the hookworm disease and 

 to the development of methods for curing it among the victims, as well 

 as for preventing it in the future. They made important studies on the 

 relation of rats and fleas to the plague, on the relation of pellagra to the 

 diet, and on other health problems. 



