198 EXPLORATIONS ACROSS THE GREAT BASIN OF UTAH. 



tained in Table F. The most characteristic one is again No. VIII for Woodruff Val- 

 ley, in the last days of May. It illustrates the extremely arid climate of the interior 

 of Utah. The maximum of saturation at the time of sunrise, at 4.30 a. m., was 74 . 

 saturation being- 100. If the soil was covered with .mass instead of being nearly bare| 

 dew, or rather frost, might then have been formed, under else favorable circumstances, 

 by a very slightly farther decrease of temperature. This, however, was only due to 

 the great depression of the temperature during the night to below the freezing point, 

 not to a large quantity of vapor in the atmosphere, which actually amounted to only 

 1 .5 grains in a cubic foot of air. With the rapid increase of temperature, the degree 

 of saturation decreased so much that at 9 a. m. it was only 22, because the ground 

 was extremely dry, and the little vapor which was formed was carried on high by the 

 ascending currents of air. 



While the maximum of the temperature took place at 3 p. m. the humidity, there- 

 fore, continued to decrease ; at 6 p. m. it was 5.3, and the minimum seemed to take 

 place at about 20 minutes past 6, an hour before sunset, with 4.5. From that time 

 to sunrise of next morning the relative humidity increased nearly uniformly. The 

 amplitude was 69.5 and the mean degree of saturation 29.1. At the same season at 

 Philadelphia, with nearly the same mean temperature, the maximum of saturation at 

 sunrise is about 91, the minimum, between 3 and 4 p. m., about 64, the amplitude, 

 therefore, only 27, and the mean 78.9. A glance at these numbers is sufficient to 

 convince anybody that agriculture can never be carried on there except by irrigation, 

 and they prove, at the same time, that water for that purpose must be exceedingly 

 scarce, so that only a few acres might be cultivated out of stretches of many miles in 

 extent. The plants, however, withstand such extreme changes much better than it 

 might be expected, because the very dryness of the atmosphere .protects them from 

 being injured by the night frosts. 



At Camp Floyd, in the middle of September, as represented in diagram No. VI, 

 the mean humidity was still less, viz, 21.9 ; but the amplitude was not as large, only 

 39; the maximum, at 5.30 a. m., being 42.5, and the minimum, at 3.30 p. m./bein'a- 

 3.5. The corresponding numbers I'm- Philadelphia are -82.5 as mean, 94 as maximum 



At Woodruff Valley it increased proportionally 

 «, because the vapor carried off by rising cur- 



prevailed dunn- the i 



In August, at Ca 



being 38, and in Apri 



