1 62 EVOLUTION OF THE EARTH 



less than the normal. The less heavily shaded area betweei 

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

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

 perature and humidity falling between these lines range froi 

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

 lighter strip indicates conditions under which deaths range 

 from normal to lo per cent more than the normal, and so on\ 

 The lighter the shading the greater the number of deaths, an( 

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

 diagrams may be illustrated by an example. Suppose, foi 

 instance, that we want to find what degree of health we ma] 

 expect in a month having an average temperature of 50° foi 

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

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

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

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

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

 Therefore we infer that on an average the months with 

 mean temperature of 50° and a relative humidity of 70 pel 

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

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

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

 cool summer it may have 1 2 per cent less than the normal, bul 

 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 eacl 

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

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

 in California. For each place tables were prepared showin| 

 the mean temperature, mean relative humidity, and numbei 

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

 and last halves of the period under discussion were averagec 

 to see how rapidly the number was increasing or diminishing^ 

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



