14 BOTANICAL FEATURES OF THE ALGERIAN SAHARA. 



the desert itself, one station receiving much more heat at certain times of 

 the year than another station. The total heat for each station is arrived 

 at by adding up the daily maxima for the midseasonal months, January, 

 April, July, and October, and dividing by the number of years whose records 

 were consulted. The amounts given in table 1 1 are the averages for three 

 to four years, for three desert stations and for one station in the Tell. 

 Table i i . — Sums of the daily maximum temperatures. 



The variation in total heat received at the three desert stations may 

 probably be explained partly at least by their differences in latitude and 

 in elevation above the sea, as well as the different relations they hold to 

 the highlands of the northern portion of the colony. Laghouat lies im- 

 mediately south of the Saharan Atlas, latitude 33°48'> and at an elevation 

 of 780 meters. Ouargla is approximately 200 miles south of the mountains, 

 latitude 3i°55', and altitude 150 meters. In Salah is in the midst of the 

 western Sahara, latitude 27°i7 / , and about 300 meters above the sea. 

 Fort National lies in the Tell about 30 miles from the sea, and at an alti- 

 tude of 916.3 meters. The latitude of Fort National is 36°38'. 



There are no published records of the soil temperatures of southern 

 Algeria. In the following table is given soil-temperature data taken at 

 Ghardaia by M. Buret. Maximum and minimum standard chemical ther- 

 mometers were used. They were placed in a horizontal position in fixed 

 tubes which were about 30 cm. in length. Precautions were taken to prop- 

 erly insolate the instruments. The depth was 15 cm. 



Table 12. — Soil and air temperatures, Ghardaia, July 2-11, 1911. 



An examination of the meteorological reports for Algeria shows that the 

 direction of the prevailing winds varies considerably, although the varia- 

 tion is possibly greater near the coast than in the interior. The winds un- 



