1 62 EVOLUTION OF THE EARTH 



less than the normal. The less heavily shaded area between 

 the 10 per cent line and the zero line means that on an average 

 the deaths in months having any of the combinations of tem- 

 perature and humidity falling between these lines range from 

 the normal to not more than 10 per cent below it. The next 

 lighter strip indicates conditions under which deaths range 

 from normal to 10 per cent more than the normal, and so on. 

 The lighter the shading the greater the number of deaths, and 

 the less the vitality and energy. The practical use of the 

 diagrams may be illustrated by an example. Suppose, for 

 instance, that we want to find what degree of health we may 

 expect in a month having an average temperature of 50 for 

 night and day together and a mean humidity of 70 per cent. 

 In Figure 31 find 50 in the scale on the left. Follow this 

 level to the right to a point in line with 70 per cent as marked 

 at the top. This point lies a trifle above the zero line and 

 about one-fifth of the way toward the minus 5 per cent line. 

 Therefore we infer that on an average the months with a 

 mean temperature of 50 and a relative humidity of 70 per 

 cent have i per cent less than the normal number of deaths. 

 Such a month in spring after a hard winter may have 10 per 

 cent more than the normal deaths, while in the fall after a 

 cool summer it may have 12 per cent less than the normal, but 

 the average is i per cent below normal. 



The method of preparing these diagrams is as follows. 

 Certain cities were first selected to cover all parts of each 

 region, namely fourteen in France, fourteen in Italy, twenty- 

 two in the northeastern quarter of the United States, and four 

 in California. For each place tables were prepared showing 

 the mean temperature, mean relative humidity, and number 

 of deaths for each month. Next the yearly deaths in the first 

 and last halves of the period under discussion were averaged 

 to see how rapidly the number was increasing or diminishing. 

 On the basis of these figures, and assuming that the change in 



