Page:Popular Science Monthly Volume 40.djvu/391

Rh 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 regions having from thirty to forty inches average annual rainfall.

The importance of the knowledge of this distribution is supplemented by that with reference to the mean annual temperature, 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 contained in it in proportion to the amount required to saturate it. This amount varies with the temperature; the higher the temperature, 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