many regions destitute of adequate rainfall, and there fore naturally -unfitted for agriculture, the deficiency of moisture is made good by artificial irrig,ation. In this way in various quarters of the world vast tracts of land which otherwise would be useless and uninhabitable have been made extremely fertile and rendered capable of supporting; a teeming agricultural population. This result is attained either by the method of flooding or by that of regulated distribution, as the circum stances or the situation determines.
Irrigation by Flooa'ing is resorted to very freely in hot countries, and especially iu regions adapted for grain-growing, as in Egypt, Southern France, and Italy, the surface being submerged at the proper season by releasing the water confined behind suitably-located dams and embank ments. The area is kept flooded long enough thoroughly to saturate the ground to a suitable depth. Meadow-lands are flooded during the winter season. This method of irrigation has been brought to great perfection in Lombardy, where the waters of the streams descending from the .Alps are most thoroughly utilized for this purpose. As a typical example of this system of irrigation, the Cavour Canal should be named (pi. 52, jig. I). This proceeds from the river Po at Chivasso, near Turin, and is carried over a large number of aqueducts and conducted through numerous tun nels. It has a maximum breadth of 13 feet (at its embouchure) and a maximum depth of II feet. It draws from the Po about 4000 cubic feet of water per second.
Irrigation by another system of irrigation the water is distributed over the surface under cultivation in regulable quantities as it may be needed from time to time. The supply is usually obtained from artificial reservoirs formed by damming the waters of mountain streams in narrow valleys or gorges, and the impounded water is conducted through open or covered channels, sometimes over great distances, and by suitably-disposed channels ramifying from the main conduits is distributed over an extensive surface. In this manner large tracts of land may be kept at all times supplied with the moisture necessary to insure their fer tility, and with comparatively little waste of water, provided only that the natural fall is favorable for the purpose. Where this is not the case, it will be necessary, wherever it is practicable, properly to grade the surface to be irrigated. In the disposition of the works of this order, the distrib uting-canals are conducted along the summits of the ridges or slopes, the drainage-ditches being, situated at the bases of the slopes, and between these are placed a number of smaller parallel ditches, which gradually fill up with water and overflow. The water is permitted to flow a certain
number of days in the year, according to the character of the climate, the soil, the kind of crops that are cultivated, etc. The overflow water may be utilized for irrigating lands at a lower level, and this process may be several times repeated.
ll'orks in the United the United States irrigation is practised to some extent in Colorado, New Mexico, and California. In the arid sections of the south-western portion of this country, the remains of great artificial channels testify to the former existence of extensive irri gation works constructed by the race which prior to historic times inhab ited the region. It is of interest to note in this place that recently a ,9,-reat scheme for the irrigation of the arid lands lying between the Rocky Moun tains and the Missouri River has been proposed by Major Powell, director of the United States Geolog,ical Survey. This region embraces several hundred thousand square miles of territory, all of which iu their present condition are unfit for cultivation. The plan contemplates the construc tion of great dams in the canons of the rivers, at suitable points near their head-waters. These would be built sufficiently strong to resist the flood waters of spring, and behind them would be impounded immense volumes of water, which by means of aqueducts, canals, etc., could be distributed over the entire region. All that is needed, apparently, to render these vast tracts fruitful is the moisture at present lacking, and the scheme presents splendid possibilities. The director expresses the belief that bv the proper application of his plans not less than one hundred and fifty thousand square miles of desert-lands could be made productive, thus increasing- by one third the area of the agricultural lands of the country. It appears possible at this writing that Congress will appropriate a generous sum of money to provide for the cost of making the preliminary surveys required to deter mine the feasibility of this great project, the actual realization of which would be a triumph of science as magnificent as it would be beneficent.