Table 11.—Sums of the daily maximum temperatures.
| Station. | January. | April. | July. | October. |
|---|---|---|---|---|
| °C. | °C. | °C. | °C. | |
| Laghouat | 439 | 476 | 1,135 | 668 |
| In Salah | 641 | 1.027 | 1.455 | 1.121 |
| Ouargla | 570 | 909 | 1.444 | 923 |
| Ft. National | 230 | 470 | 697 | 564 |
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 immediately 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 31°55′, and altitude 150 meters. In Salah is in the midst of the western Sahara, latitude 27°17′, and about 300 meters above the sea. Fort National lies in the Tell about 30 miles from the sea, and at an altitude 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 thermometers were used. They were placed in a horizontal position in fixed tubes which were about 30 cm. in length. Precautions were taken to properly insolate the instruments. The depth was 15 cm.
Table 12.—Soil and air temperatures, Ghardaia, July 2-11, 1911.
| Date. | Soil. | Air. | |||
|---|---|---|---|---|---|
| Maximum. | Minimum. | Maximum. | Minimum. | ||
| °C. | °C. | °C. | °C. | ||
| July | 2 | 36.0 | 31.0 | 41.0 | 21.0 |
| July | 3 | 36.0 | 31.0 | 41.0 | 22.0 |
| July | 4 | 36.0 | 31.0 | 41.0 | 23.0 |
| July | 5 | 36.5 | 31.5 | 41.0 | 23.5 |
| July | 6 | 33.7 | 32.0 | 41.0 | 25.0 |
| July | 7 | 36.0 | 32.5 | 40.0 | 26.0 |
| July | 8 | 36.0 | 32.5 | 38.0 | 26.5 |
| July | 9 | 37.0 | 25.0 | ||
| July | 10 | 37.0 | 32.5 | 40.0 | 24.0 |
| July | 11 | 33.0 | 23.0 | ||
An examination of the meteorological reports for Algeria shows that the direction of the prevailing winds varies considerably, although the variation is possibly greater near the coast than in the interior. The winds undoubtedly play an important rôle in the environment of the vegetation of the country. At Algiers, in 1907, winds were reported from the north 288 times, from the east 203 times, from the west 224 times, and from the south 51 times. At Barika, on the High Plateau, the number of times and the directions of the winds for the same year are as follows: northeast, 203 times; east 242; south 21, west 89, and southwest 93. At El Golea, on the desert, the winds were as follows: north 192, northeast 614, southeast 107. The winds from the north, or northerly winds, are cool and laden with moisture, but they are most effective in regions near the coast or in the mountains. Winds from the south are dry winds, and are probably of great importance in limiting the distribution of plants through the increased aridity caused by them.
The most important of these desert winds is the sirocco, a wind that is most likely to blow in spring and summer, although occurring in autumn also, and to a very limited degree in the winter season. The sirocco crosses the Mediterranean and is felt in the southern portions of France, on the Italian Riviera, and in other parts of southern Europe. It is especially common on the High Plateau; for example, during five years it was reported on an average of 28.4 days each year. It does not generally last more than three days, but at Batna, in July, 1902, it was recorded on eight consecutive days. The sirocco operates to lower the relative humidity and to raise the temperature. For instance, on the first day of the eight-day sirocco above alluded to, the average relative humidity was 16 per cent, while the average on the preceding day was 25.6 per cent. It has already been remarked that on the days of the greatest temperature variation the sirocco was usually blowing. In the desert the winds often bring with them much dust and sometimes last during several days; for example, one is reported at In Salah, in May, 1904, for six consecutive days. Such winds on the desert may come from the north, or at least may be northerly, and when storms of this character are in progress, although possibly destructive of animal life or at least comfort, the effects as regards decreasing the relative humidity and raising the air temperature are not so marked as is the case when the sirocco is blowing.
SOME CHARACTERISTICS OF THE VEGETATION OF THE TELL.
Because of the similarity in the flora of Algeria and southern Europe, a very good introduction to Algeria is by the way of southern Spain, France, or Italy. As one approaches southern France, for instance, he begins to see evidences of increasing aridity. Upon passing Lyons grassy fields and heavy forests are left behind, the hills become bare or covered with a chaparral-like growth, and the practice of irrigation is observed on the plains. The vegetation, especially of the region between Avignon and Nimes, recalls that of portions of California, and one sees the mulberry, the olive, and the pomegranate in abundance, and occasionally the orange. Very much the same conditions greet one when he arrives in Algiers, except that along the littoral, at and in the vicinity of Algiers, there is a wealth of native and especially of introduced plants, which give little hint of the arid regions close at hand. The hills which make a part of the beautiful city of Algiers contain fine plantations of foreign trees, such as eucalyptus, conifers of various species, acacias, figs, and a variety of fruit trees like the apricot, peach, plum, apple, almond, orange, and others. In the fine public squares one sees large palms also, and in the botanical garden are bamboos, palms, bananas, India-rubber trees, and a large number of forms from the subtropics. The great variety of introduced plants which appear to thrive on the littoral again recalls portions of California, where the kinds and numbers of introduced plants which grow well are likewise large, and where the floral strangers are gathered from the antipodes. But among the species there are also many natives to the colony and which one will see when he begins to travel away from the coast; for example, the cedar from the higher mountains (Cedrus atlantica) and the fine pine (Pinus halepensis) from lower altitudes; there are junipers and oaks, among the latter the cork oak (Quercus suber), and fine specimens of Pistacia atlantica and its protector the jujube (Zizyphus lotus), the relationship of which will be given later in this paper. Both of the last-named species are native to the Sahara and are of rather frequent occurrence in the regions visited.