CLIMATE AND HEALTH 67 



nations of temperature and humidity indicated by those parts 

 of the diagrams. 



To take a concrete example, suppose we want to know the 

 healthfulness of a month having a mean temperature of 60°F and 

 a mean relative humidity of 80 per cent in northern France. On 

 the left margin of Figure 8 B we find the number 60° beside the 

 word "temperature." Then we ascertain the point where the level 

 thus indicated lies directly below the figure 80 per cent among the 

 humidity figures at the top. The point thus determined lies close 

 to the curved line marked —10, and on the edge of the most 

 heavily shaded part of the diagram. The curved lines are called 

 *'isopracts," or lines of equal efficiency. The numbers, such a^ 

 -zl Oj 0, -j -10, etc ^ mean that oi^ an averag e all the montlisnaving 

 the combinations of temperatnrft an d h^irpidJtyJm^lnrlpH within the 

 — 10 lin c i fpy,, fix^TYipIp, havp an. average deathrate at least lO 

 per cent less than that of the year as a whole, A month having 

 an average temperature of 70° and a humidity of 80 per cent 

 would also fall close to the —10 isopract, but on the upper side 

 of the heavily shaded area. Similarly a month having a tempera- 

 ture of 65° and a humidity of 85 per cent would fall well within 

 this isopract, which means that in such months the deaths usually 

 fall below the yearly average by even more than 10 per cent. 



On the other hand, suppose the temperature averages 60°, as 

 in the first case, but the humidity is low, only 60 per cent. This 

 combination of weather conditions occupies a position close to 

 the isopract marked 0. That is, in northern France a very dry 

 month with a temperature of 60° is not nearly so healthful as a 

 wetter month having the same temperature. The health in such 

 months averages no better than the normal for the year as a 

 whole. Again, if the temperature averages only 50° and the 

 humidity is very low — less than 60 per cent — the deathrate rises 

 notably, for Figure 8 shows that in such cases it averages 15 

 per cent above the yearly average. In wet months, however, an 

 equally high deathrate, as may be judged from the curve of the 



