THE INFLUENCE OF URBAN AND RURAL ENVIRONMENT 36 1 



calories per square centimeter per hour, on a surface facing 

 the sun. On surfaces inchned to the sun's rays the heat 

 received is less according to the cosine of the angle of inchna- 

 tion. The earth's surface is more or less inchned first accord- 

 ing to latitude and season, and again according to the 

 hour of the day. 



At latitude 42° the sum of the hourly values of the cosine 

 of the angle of inchnation is 7.61 for the 15 hours of sunhght 

 at the summer solstice, and 5.67 during the 12 hours at the 

 equinoxes. These values then represent the equivalent 

 hours of normal exposure during each day, and their mean 

 value, 6.6, may be taken as a fair representation of the 

 average daily number of hours of normal exposure during 

 the summer. The average daily amount of heat received, 

 therefore, on a square centimeter of surface does not exceed 

 the value for normal exposure, 90 calories per hour times 6.6 

 hours or 594 calories per day. It is always less than this 

 by the proportion of cloudiness, and by the amount of heat 

 intercepted by fog, smoke and dust. In the vicinity of Nev^^ 

 York City, the cloudiness alone diminishes the sunshine to 

 60 per cent of its possible value during the summer months. 



The computed absorption by trees, 6^ calories, is 1 1 per 

 cent of the total heat received over the forest area through 

 clear and dry air, and a much greater proportion of the 

 actual radiation through the average atmosphere. The 

 significance of this value will be appreciated if it be noted 

 that, taking in the hours of normal exposure as computed, 

 the diff'erence between the summer solstice and the equinoxes 

 amounts to only 25 per cent of the former. 



It will be noted in Table i that the death rate from 

 diarrhea and enteritis under two years of age was 48 per 

 cent higher in the urban than in the rural populations (52.2 

 urban, 35.1 rural) in 1920, and in 1910, 53 per cent higher. 

 While diarrheal disease of infants has generally in the past 

 been thought to be due chiefly to the spoilage of food, 

 to bacterial contamination of milk and water and to lack 

 of care in washing and clothing infants, strong evidence has 

 recently been presented by Arnold suggesting that such 

 high effective temperatures as commonly prevail in our 

 cities in summer are a definite predisposing cause to this 



