AND ITS INHABITANTS 163 



the number of deaths due to increasing population and to im- 

 proved sanitation progresses uniformly, the normal for each 

 month was obtained. Allowance was made for the varying 

 number of days in different months. Then for each month 

 the departure of the death-rate from the normal was calculated 

 in percentages. These departures were next tabulated accord- 

 ing to temperature and humidity. Contagious diseases were 

 of necessity included and cause some irregularities which have 

 been smoothed out in the diagrams here presented. It must 

 be clearly understood, however, that the irregularities are 

 slight, and their effect is merely to cause the lines separating 

 the different zones to be wavy instead of smooth. As a matter 

 of fact, contagious diseases are in general worst under the same 

 conditions which are worst for other diseases. 



By the method described above, each place is compared 

 only with itself. This avoids the serious difficulty which arises 

 if actual death-rates are employed, and if all places are com- 

 pared with the same standard. The death-rate of one town 

 may be high because of poor sanitation, or because a great 

 number of people work in factories. In another it 'may be 

 low because a large number of young people have migrated 

 thither or because the occupations are unusually healthful. 

 Therefore comparisons of the death-rate of different places 

 are misleading. According to the method here employed, 

 however, it makes no difference what the actual death-rate is. 

 We deal simply with the degree to which the deaths fall above 

 or below the normal for the particular place and period in 

 question. 



It will readily be seen that diagrams 31 to 33 present the 

 same general features. This would be much more evident if 

 the range of temperature in California and of humidity in the 

 eastern United States were not so small. Nevertheless, in 

 each diagram the outstanding feature is a central heavily 

 shaded area of few deaths and high physical energy. When 



