I 



HYGROSCOPIC MOISTURE. 37 



From the foregoing facts the principle is clear that the annual vegeta- 

 tion of the desert is supported by capillary moisture derived from local 

 rainfall; and furthermore, that of the two phenomena, growth of annual 

 vegetation and presence of capillary moisture in the soil, each is an 

 index to the other as to beginning, quantity, and duration. 



The subject of hygroscopic moisture is unfortunately one about 

 which very little is known. It has been found that a mass of earth, 

 kiln dried and exposed to atmospheric air, absorbs from it a certain 

 amount of moisture. According to the experiments of Schubler, 1 

 1 kilogram of kiln-dried calcareous sand, exposed at a temperature of 

 12° to 15° C.to an atmosphere saturated with moisture, took up, in 72; 

 hours, 3 grams of water; the same amount of pure blue clay, under 1 

 identical conditions, took up 49 grams; and humus, 120 grams. 



The amount that would be absorbed by various soils in the dry air 

 of Death Valley would be much less, yet it is believed to be of 

 very great importance in the economy of desert perennial vegetation. 

 Whether or not a plant can abstract a sufficient amount of water for 

 its continued existence from hygroscopic moisture alone, in any soil, 

 can be determined only by experiment. From several months' obser- 

 vation of desert conditions and phenomena, it appears to the writer 

 extremely probable that a very important proportion, if not indeed in 

 some cases all, of the summer water supply of certain desert shrubs is 

 conserved in such a maimer. 



TEMPERATURE. 



The observations on the temperature of Death Valley in 1891 for the 

 hottest month of the year, July, show an average daily maximum of 116°, 

 an average daily minimum of 87° and a mean of 102° F. A maximum 

 temperature of 1 22° occurred five times during the summer. Although 

 this maximum has been exceeded in a few instances in India and in the 

 Colorado Desert of California, the average temperatures given above 

 are considerably in excess of any averages recorded in India, the Sahara, 

 or at any other point in our own deserts. Moreover, these tempera- 

 tures were taken under cover of a shelter constructed especially to 

 eliminate the effect of radiation from the ground. The temperature 

 in the shade under normal conditions is therefore a few degrees higher; 

 and in direct sunlight bodies that readily absorb heat must reach a 

 temperature many degrees in excess of the recorded maximum. 



The only accessible records of the winter temperature in Death Valley 

 are those kept by the expedition at Bennett Wells in January and Feb- 

 ruary, 1891. The lowest observed temperature was 30° F., and it was 

 not unusual for thin ice to form on water left over night in camp 

 utensils. A yearly range of 92° obtained from these records is doubt 

 less several degrees too low. The average daily range during August 

 was 32°, which is unusually large. The observed range for our second 



1 Cited in Stich^e's Lelirljucb tier Agriculturchemie, p. 208 (1888). 



