70 THE DATE PALM. 



duce offshoots, which are imported f rorn the Oued Rirh country when 

 needed to plant new gardens. A single palm may be worth from $80 to 

 $100 arid may produce as much as 330 pounds of dates, which bring the 

 highest price of any in the Sahara. There can be little doubt that the 

 superior quality of these dates is due to the accumulation of heat in 

 the still air of the sunken gardens by reflection from the bare sand of 

 the sloping sides. 



In the Salton Basin the Deglet Noor date can doubtless attain the 

 same perfection with infinitely less expense and trouble, since the 

 higher summer temperature will give the same heat in level orchards 

 that is reached in the sunken gardens of the Souf . 



EFFECTS OF WIND ON THE DATE PALM. 



In the large deserts there are frequently high winds which are 

 usually very hot and dry and sometimes so violent as to carry great 

 quantities of dust and sand. Delicate foliage is injured by such winds 

 in two ways; first, by being lacerated by the violence of the wind and 

 also bruised and abraded when sand is carried; second, by the drying 

 action of the intensely hot, dry air, especially on leaves which have 

 been torn or injured. Such winds often cause great discomfort and 

 even grave danger to caravans in the desert. ' ' The spectacle is fright- 

 ful, the impression most painful, the danger real; sand obscures the 

 air and singes the face, it fills the eyes, the mouth, the ears; it hurts 

 the throat and dries up the water skins of the native caravans, which 

 are thereby in danger of perishing. " a 



Such winds, called "simoons" or "siroccos" in the Sahara, often 

 blow several days in succession, sometimes keeping up all night. 

 During such winds the relative humidity sometimes falls as low as 2 

 per cent at a temperature of 33 C., corresponding to 0.75 mm. 

 pressure of water vapor, J whereas the mean humidity of the driest 

 month at Paris, for example, is 57 per cent, and at Biskra 25 per cent 

 (see p. 53). 



Observations made by the writer at Biskra during a sirocco at 3 

 o'clock p. m., May 13, 1900, showed even less humidity. The dry 

 thermometer read 38.5 C. and the wet bulb sling thermometer 

 15.3 C., corresponding to a relative humidity of 2 per cent and 

 an absolute pressure of water vapor of 1.02 mm/ Sometimes the air 

 is so dry in the interior of the Sahara that the instruments such as 

 have been used do not indicate the presence of any water vapor 

 whatever. 



a Holland, Georges. Geologic du Sahara algerien, p. 225. 



&Massart, Jean. Un voyage botanique au Sahara. In Bui. Soc. Roy., Bot. de 

 Belgique, vol. 37 (1898), I, p. 273, observations made near Ouargla at, noon, May 

 23, 1898; the wet-bulb sling thermometer registered 14.2 C., which gives nearly 7 

 per cent relative humidity by Prof. C. F. Marvin's tables (Weather Bureau Publica- 

 tion No. 235, 1900). 



c Calculated by Prof. C. F. Marvin, Weather Bureau, U. S. Dept. of Agriculture. 



