November 10, 1905.] 



SCIENCE. 



583 



may mature the crop. These, which are 

 known in India as inundation canals, are 

 of high value. 



SOUTHERN INDIA. 



In southern India there are three great 

 rivers, drawing their supply from the line 

 of hills called the Ghats, running parallel 

 to and near the western coast, and after a 

 long course discharging into the Bay of 

 Bengal on the east coast. Against the 

 Ghats beats the whole fury of the tropical 

 southwest monsoon, and these rivers for a 

 few months are in high flood. As they 

 approach the sea they spread out in the 

 usual deltaic form. Dams have been built 

 across the apex of these deltas, from which 

 canals have been drawn, and the flood 

 waters are easily diverted over the fields, 

 raising a rice crop of untold value in a 

 land where drought and famine are too 

 common. But for the other months of the 

 year these rivers contain very little water, 

 and there is noAV a proposition for supple- 

 menting them with very large reservoirs. 



A very bold and successful piece of irri- 

 gation engineering was carried out a few 

 years ago in south India, which deserves 

 notice. A river named the Periyar took its 

 rise in the Ghats, and descended to the sea 

 on the west coast, where there was no 

 means of utilizing the water, and a good 

 deal of money had periodically to be spent 

 in controlling its furious floods. A dam 

 has now been built across its course, and a 

 tunnel has been made through the moun- 

 tains, enabling the reservoir to be dis- 

 charged into a system of canals to the east, 

 where there is a vast plain much in need 

 of water. 



In the native state of Mysore, in southern 

 India, there are on the register about 

 40,000 irrigation reservoirs (or tanks, as 

 they are called), or about three to every 

 four square miles, and the nature of the 

 country is such that hundreds may be 



found in the basin of one river— small 

 tanks in the upper branches and larger ones 

 in the lower, as the valley widens out, and 

 these require constant watchful attention. 

 From time to time tropical rainstorms 

 sweep over the country. If then even a 

 small tank has been neglected, and rats and 

 porcupines have been allowed to burrow in 

 the dam, the flood may burst through it, 

 and sweep on and over the dam of the next 

 village, lower down. One dam may then 

 burst after another, like a pack of cards, 

 and terrible loss occurs. 



In this state of Mysore a very remark- 

 able irrigation reservoir is now under con- 

 struction at a place called Mari Kanave. 

 Nature seems here to have formed an ideal 

 site for a reservoir, so that it is almost 

 irresistible for the engineer to do his part, 

 even although irrigation is not so badly 

 wanted here as elsewhere. The compara- 

 tively narrow neck of a valley containing 

 2,075 square miles is being closed by a 

 masonry dam 142 feet high. The reservoir 

 thus formed will contain 30,000 million 

 cubic feet of water, but it is not considered 

 that it will fill more than once in thirty 

 years. Nor is there irrigable land requiring 

 so great a volume of water. Much less 

 would be sufficient, so such a high dam is 

 not needed ; but the construction of a waste 

 weir to prevent the submergence of a lower 

 dam would require such heavy excavation 

 through one of the limiting hills that it is 

 cheaper to raise the dam and utilize a nat- 

 ural hollow in the hillside for a waste weir. 



IRRIGATION IN EGYPT. 



No lecture on irrigation would be com- 

 plete without describing what has been 

 done in Egypt. You are generally familiar 

 \^^th the shape of that famous little coun- 

 try. Egypt proper extends northwards 

 from a point in the Nile about 780 miles 

 above Cairo — a long valley, never eight 

 miles wide, sometimes not a half a mile. 



