The desert is a desert for want of water only. There is no lack of nutrition in the soil, nor is there anything in surface or temperature that makes a desert unproductive. Temperature and winds reach great extremes in fierceness, however. The temperature of the air in the noonday sun will often exceed one hundred and forty-five degrees; it may reach one hundred and fifty-five degrees. In the shade it frequently climbs to one hundred and thirty degrees in the vicinity of the tropics. Unless one is at a considerable altitude there is not much relief at night, though the thermometer may drop to ninety degrees. Farther north, however, and at an altitude of five thousand feet or more, the temperature of the night is even more cruel than that of the day. Immediately after sunset a sharp chill becomes perceptible. At first it is a welcome relief from the intolerable heat. By nine o'clock it begins to cut like a stiletto, and at midnight the water suspended in shallow dishes clinks into ice. The drivers burrow deep into the sand and wrap woollen baracans about them; the camels shiver and even blubber like whipped bullies. The air is so dry, however, that the extreme heat of day is by no means insupportable. Sunstroke is almost unknown, and even the tragedy of perishing for want of water is very rare; for the caravan drivers know just where to find water, and there are many hidden watering places that are known to the crafty Tuaregs and Bedouins. Many of the watering places are wells that have been sunk in various localities along the caravan trails. The intense heat, great depth of rock waste, and dry air are not favorable to the above-ground flow of rivers. But nearly every river has an underground flow that is pretty likely to exist all the year round. One may follow a stream of considerable volume down the southern slope of the Atlas Mountains. The volume of water grows less and less until at last it apparently disappears. Not all is lost by evaporation, however; possibly the greater part sinks into the porous rock waste. And the rock waste?--perhaps it may be twenty, fifty, or one hundred and fifty feet deep. At all events, the water sinks until it reaches bed rock or clay through which it cannot pass. Then it flows along what may once have been an above-ground channel until fierce winds and cloud-bursts buried it deep. But the half-savage dwellers of the desert know just where to tap these underground reservoirs and streams; even the dumb animals know instinctively where to look for water. It is merely a question of instinct coupled with experience, and the animal's judgment is about as good as the man's. When one finds the spot, it is necessary only to dig. The water may be two feet below the surface or it may be ten feet. When the moist sand is reached the task is half over. A foot or two more and the hole begins to fill. The water is hot, brackish, and repulsive to the taste, but it is water--and in the desert, water is water!