254 WORLD-POWER AND EVOLUTION 



One such line begins on the left at a temperature of about 74° and 

 extends nearly straight to the right with a bend upward at the end. 

 Just above it comes a dotted line. This means that although at 74° 

 the mortality rate was normal, at 76° it had increased to 5 per cent 

 more than the normal. The next dotted line shows a mortality rate 

 of 10 per cent and so on. The dotted lines in this and in other dia- 

 grams indicate more than the normal number of deaths, while the 

 solid lines indicate less. Such a diagram is called a "climograph." 

 The lines may be called "iso-practs" or "lines of equal effect," from 

 the Greek word "isos" meaning "equal" and "practicos" meaning 

 "effective." They are analogous to the "iso-bars" or lines of "equal 

 pressure" on a weather map. The isopracts divide the climograph 

 into zones. Each zone indicates the degree of health to be expected 

 among people living under its particular climatic conditions. 



In the central portion of Figure 30 the isopractic lines are quite 

 regular, for that part represents the temperatures and humidities that 

 generally prevail. On the borders of the diagram especially at the 

 top and bottom the isopracts become irregular because the number 

 of months having such extreme conditions is small. In order to do 

 away with the irregularity and the resulting confusion, the diagrams 

 in the body of the text have been simplified. That is, the isopracts 

 have been smoothed, the figures for departures have been omitted, 

 and the zones between the isopracts have been shaded. The darkest 

 shading indicates that the mortality is more than 10 per cent below 

 the normal ; the next shading indicates a mortality ranging from normal 

 to — 10 per cent, and so on. The degree to which the diagrams in 

 the text have been smoothed may be judged by comparing Figures 8 A, 

 12 A, and 14 A with the corresponding diagrams numbered 8 B, 12 B, 

 and 14 B.* 



*The term "climograph" was introduced by Dr. GriflSth Taylor in his 

 interesting paper on "The Control of Settlement by Humidity and Tempera- 

 ture." (Commonwealth Bureau of Meteorology, Bulletin No. 14, Melbourne, 

 1916.) His climograph differs somewhat from the form here shown. (1) He 

 uses wet bulb temperature instead of the ordinary dry bulb temperature. 

 This gives double weight to humidity, and is of doubtful expediency. (2) He 

 does not use isopracts. Instead of this he plots the points indicating the 

 mean wet bulb temperature and the mean relative humidity for each month. 



