Readers who have followed our description of the physical conditions encountered in Transcaspia will have grasped the fact that its tillage depends wholly on the timely supply of water by artificial means. The Turkoman farmer is not, like his European comrade, at the mercy of the seasons, for he taps the rivers and streams descending from the Persian and Afghan highlands, which enjoy a fairly constant rainfall.[668] Dams erected in channels give a “head” of water which is drawn off into distributories or _arīks_, and these, again, are subdivided into tiny rills which afford to every plot of cultivated land its portion of the precious fluid. The parent stream thus gradually loses its speed and volume, and finally disappears in the arid desert sands. Where visible water is not met with, the springs on the mountain flanks are reached by a system of tunnelling. A well is sunk at a higher level than the area to be irrigated, and, when water is found, a lateral tunnel is excavated which carries the subterranean water several hundred feet nearer its object. At its extremity another well is dug, and the piercing process is repeated till the thirsty tract is reached. A well-known natural law compels the water in the last of the chain of wells to rise to the level of that first sunk; and thus a head is formed which supplies a system of distributors. The method is known as the Persian, and is of extreme antiquity. So great is the skill of the older labourers practising it that the mole-like excavations in which they work are barely two feet in diameter by four in height.