306 HUMAN BIOLOGY 



death rate. In other words chmatic conditions , regardless 

 of their geographic location, take the place of distances 

 east and west or north and south, and excess or deficiency 

 of deaths is substituted for height of the land above or 

 below sea level. Figure 2 illustrates how the matter works 

 out for 3,700,000 deaths in the cities of France and Italy. 

 In the upper left hand corner the number 31.6 is located 

 at the high monthly temperature of 82° to 83° and the low 

 relative humidity of 45 to 50 per cent. It is based on all 

 months with that kind of weather no matter what city or 

 year they occurred in. For each month the death rate for 

 the city in question is first expressed as a percentage of 

 the normal death rate, that is, of the rate to be expected 

 in that city in that particular year when due allowance 

 is made for the steady improvement in medical methods. 

 In the case before us the percentages for several months in 

 several cities averaged 31.6 higher than the normals for the 

 places and years of their occurrence. 



Run down the figures below 31.6. The relative humidity 

 remains constant, but the temperature becomes lower. 

 The percentages hkewise diminish steadily, then they 

 begin to have minus signs; and finally at a temperature of 

 67° to 68° the deaths average 7.8 less than the normal. 

 Skip now to the next column and begin with —9.4 at a 

 temperature of about 64° and a relative humidity of 50 

 to S5 pel' cent. Passing from this number to the right, we 

 maintain the same temperature but reach higher degrees 

 of atmospheric humidity. The death rate steadily diminishes 

 until at a relative humidity of 85 to 90 per cent it averages 

 14 per cent less than the normal. 



Now that we understand what the figures on our isograph 

 mean, we can draw isopleths which will be like contour 

 lines. Each will pass through all points having a given 

 departure of the death rate from the normal. The central 

 solid line in Figure 2 is the isopleth indicating 10 per cent 

 less than the normal number of deaths; the lines above and 

 below represent 5 per cent less than normal, then come two 

 solid lines, the normal. Beyond that the dotted lines indicate 

 a greater and greater excess of deaths above the normal. 

 In spite of some irregularities on the edges where the number 



