OUR POPULATION AND ITS DISTRIBUTION. 375 



but their condition as to general healthfulness. The average 

 annual rainfall in this country is 29'6 inches, but the variations 

 range from zero to perhaps one hundred and twenty-five inches. 

 Gauging the distribution of the population in accordance with the 

 average annual rainfall in different localities, some interesting 

 points are observable, not only as to the number of inhabitants in 

 the areas calculated, but as to the density of population. The 

 greater proportion of the people of the United States are living in 

 the regions in which the annual rainfall is between thirty and 

 fifty inches. Mr. Gannett calculates that about three fourths of 

 the inhabitants of the country are found under these conditions ; 

 and, further, that as the rainfall increases or diminishes, the 

 population diminishes rapidly. The density of population in 

 regions where the average rainfall is between thirty and forty 

 inches is 43*1 per square mile ; in regions where it is from forty 

 to fifty inches annually, the density is 59 per square mile; in 

 regions where the rainfall is from fifty to sixty inches annually, 

 the density is 25*1, and in the arid regions of the West, where the 

 rainfall is less than twenty inches, being two fifths of the entire 

 area of the country, less than three per cent, of the population 

 finds its home. The population has increased rapidly in the re- 

 gions having from thirty to forty inches average annual rainfall. 



The importance of the knowledge of this distribution is sup- 

 plemented by that with reference to the mean annual temper- 

 ature, which is in the United States 52°, and the greatest density 

 of population, as might be expected, centers on this pivot, ranging 

 as it does from 50° to 55°. Either side of this range the density of 

 population rapidly diminishes, as it was shown that it decreases 

 rapidly outside the average rainfall between thirty and fifty inches. 

 More than one half of the entire population of the country exists 

 under a temperature between 45° and 55°, while seventy to seventy- 

 five per cent of the inhabitants come within 45° and 50°. Where 

 the temperature reaches 70° on the average, but a little over one 

 per cent of the population finds its home, and the number living 

 under a mean annual temperature above 75° is too trifling for 

 consideration. 



This line of facts leads to the consideration of the distribution 

 of population in accordance with the relative humidity of the 

 atmosphere, by which is understood the amount of moisture con- 

 tained in it in proportion to the amount required to saturate it. 

 This amount varies with the temperature ; the higher the temper- 

 ature, the greater the amount of moisture which it is capable of 

 holding. The term is not a very exact one, but is relative and 

 fairly indicative of conditions. The climate having very great 

 influence upon certain classes of diseases, particularly pulmonary 

 and throat complaints, a knowledge as to the distribution of popu- 



