THE INFLUENCE OF IRRIGATION ON CLIMATE AND HEALTH. 69 



feeling of depression. In the Salt River 

 Valley these periods are usually limited to 

 say a week in February and a week in Aug- 

 ust, very much less in duration than in any 

 other locality within my observation. 

 There is very little wind here. The mean 

 average hourly movement at Phoenix for a 

 period of years is stated by the United 

 States Signal Service at 2 and 37-100 

 miles. It would be interesting to compare 

 the humidity of the higher lands of the 

 valley near the foothills with that in the 

 lower valley, but no data exist for such 

 statement. We know that it is 10 to 15 

 degrees warmer in winter and cooler in 

 summer, for instance, on the lands under 

 the Rio Verde Canal on the north side 

 and the Highland Canal on the south side 

 of the Salt River than near the river at 

 Phoenix. The extremes of temperature be- 

 tween day and night are much less on 

 the higher lands than in the lower valley, 

 and the danger of taking cold is propor- 

 tionately reduced. 



It is probable that the effects of irrigation 

 on climate and health under the high tem- 

 perature and low relative humidity of this 

 valley are somewhat different from those 

 in regions of lower temperature, greater 

 humidity and either higher or lower alti- 

 tude. 



It is almost impossible without accurate 

 observations to make comparisons, or to 

 arrive at safe, definite conclusions as to 

 the influence of irrigation on climate, 

 either in a general way or in a given lo- 

 cality. I have been unable to procure any 

 data whatever as related to this valley, or 

 to any similar locality, showing the rela- 

 tive humidity before and after irrigation. 

 Without such facts I can only state con- 

 clusions arrived at from personal observa- 

 tion and study of its effects on this local- 

 ity. 



I am decidedly of the opinion that upon 

 the deserts of Arizona, without irrigation, 

 the moisture in the atmosphere is some- 

 times so little as to interfere with health 

 and comfort, and produce feverish condi- 

 tions. The evaporation of water from the 

 irrigated land supplies this deficiency to 

 the air and obviates the injurious tend- 

 ency. 



I have frequently had this experience. 

 The " wetting down " of my well shaded 

 porch on a hot summer day lowers the 

 temperature, shown by the thermometer 

 hanging upon the wall, 10 to 15 degrees. 



This results from the refrigeration of the 

 air in the process of evaporation of the 

 water. 



It is well known that a well shaded 

 dwelling in the midst of an alfalfa field 

 is much cooler than the same residence 

 surrounded by bare ground. This is due 

 in part, perhaps, to the absence of reflec- 

 tion from the earth, but chiefly I think to 

 a similar slight refrigeration of the air by 

 the evaporation of the moisture in the earth 

 and vegetation of the surrounding field. 

 The effect becomes still more marked 

 when a gentle breeze is blowing. 



At Phoenix, during the summer months, 

 the. air is so dry that the midday registra- 

 tion of relative humidity ranges from 6 to 

 15 per cent. It rarely goes above the 

 latter point, and if it were not for irriga- 

 tion it would drop still lower, which is not 

 desirable. 



My conclusion is that the evaporation 

 of moisture from irrigated surfaces slight- 

 ly increases the moisture in the air and 

 promotes the healthfulness of both animal 

 and plant life. 



That the evaporation from irrigation 

 has but slight influence in increasing the 

 dampness in the surrounding air will be 

 readily understood when we recall the fol- 

 lowing facts: That moist air is lighter in 

 weight than is dry air. That moisture is 

 evaporated as an invisible gas. That 

 being lighter and a gas it is not a disturb- 

 ing atmospheric element. That it in- 

 stantly rises with great velocity to a point 

 in the atmosphere where the temperature 

 is below its own dew point, where it 

 becomes visible in the form of clouds. 

 But a very small portion of the evaporated 

 moisture is retained in the lower and 

 warmer strata of air. The hotter the air 

 the greater is the evaporation from the 

 irrigated ground. This evaporation low- 

 ers the earth's temperature and also that 

 of the surrounding air. 



During the winter months the tempera- 

 ture ranges much lower, evaporation is 

 much less and the air is constantly so dry 

 that the slight influence it exerts is scarcely 

 noticeable. Duringthelastwinter the mean 

 relative humidity was as follows: 1895, 

 Oct. 53%, Nov. 68%, Dec. 58%; 1896, 

 Jan. 54%, Feb. 45%, Mar. 38%, Apr. 

 32%, with a rainfall during these same 

 months of but 2.70 inches. 



It is well known that the best qualities 

 of citrus fruits can only be grown where 



