MAN'S IMPROVEMENT OF HIS ENVIRONMENT 375 



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This map shows how cases of tuberculosis are found recurring in the same locality 

 and in the same houses year after year. Each black dot is one case of tubercu- 

 losis. 



Heating. Houses in the country are often heated by open 

 fires, stoves or hot-air furnaces, all of which make use of heated 

 currents of air to warm the rooms. But in the city apartments, 

 usually pipes conduct steam or hot water from a central plant to our 

 rooms. The difficulty with this system is that it does not give us 



