November 10, 1905.] 



SCIENCE. 



581 



dinate length, and compensation for, the 

 area of land to be submerged may become 

 a very large item in the estimate. I have 

 known of districts so flat that in order to 

 irrigate an acre more than an acre must be 

 drowned. This looks ridiculous, but is not 

 really so, for the yield of an irrigated acre 

 may be eight or ten times that of an unirri- 

 gated one; and after the storage reservoir 

 has been emptied it is often possible to 

 raise a good crop on the saturated bed. 



The advantage of a deep reservoir is, 

 however, very great, for the evaporation 

 'is in proportion to the area of the surface, 

 and if two reservoirs contain the same vol- 

 ume of water, and the depth of one is 

 double that of the other, the loss by evap- 

 oration from the shallow one will be double 

 that of the deep one. In India, from time 

 immemorial, it has been the practise to 

 store water for irrigation, and there are 

 many thousands of reservoirs, from the 

 great artificial lakes holding as much as 

 5,000 or 6,000 millions of cubic feet, down 

 to the humble village tank holding not a 

 million. There are few of which the dam 

 exceeds 80 feet in height, and such are 

 nearly always built of masonry or concrete. 

 For these it is absolutely necessary to have 

 sound rock foundations. If the dam is to 

 be of earth, the quality of the soil must be 

 carefully seen to, and there should be a 

 central core of puddle resting on rock and 

 rising to the maximum height of water 

 surface. If the dam is of masonry, there 

 may perhaps be no harm done should the 

 water spill over the top. If it is of earth, 

 this must never happen, and a waste weir 

 must be provided, if possible cut out of 

 rock or built of the best masonry, and large 

 enough to discharge the greatest possible 

 flood. More accidents occur to reservoirs 

 through the want of sufficient waste weirs 

 or their faulty construction than from any 

 other cause. 



As important as the waste weir are the 

 outlet sluices through which the water is 

 conveyed for the irrigation of the fields. 

 If possible they should be arranged to serve 

 at the same time as scouring sluices to 

 carry off the deposit that accumulates at 

 the bottom of the reservoir. For, unless 

 provided with very powerful scouring 

 sluices, sooner or later the bed of the reser- 

 voir will become silted up, and the space 

 available for water storage will keep dim- 

 inishing. As this happens in India, it is 

 usual to go on raising the embankment (for 

 it does not pay to dig out the deposit), and 

 so the life of a reservoir may be prolonged 

 for many years. Ultimately it is aban- 

 doned, as it is cheaper to make a new reser- 

 voir altogether than to dig out the old one. 



ITALIAN IRRIGATION. ■ 



For the study of high-class irrigation 

 there is probably no school so good as is to 

 be found in the plains of Piedmont and 

 Lombardy. Every variety of condition is 

 to be found here. The engineering works 

 are of a very high class, and from long gen- 

 erations of experience the farmer knows 

 how best to use his water. 



The great river Po has its rise in the foot- 

 hills to the west of Piedmont. It is not fed 

 from glaciers, but by rain and snow. It 

 carries with it a considerable fertilizing 

 matter. Its temperature is higher than 

 that of glacial water — a point to which 

 much importance is attached for the very 

 valuable meadow irrigation of winter. 

 From the left bank of the Po, a few miles 

 below Turin, the great Cavour Canal takes 

 its rise, cutting right across the whole 

 drainage of the country. It has a full- 

 supply discharge of 3,800 cubic feet per 

 second ; but it is only from October to May 

 that it carries anything like this volume. 

 In summer the discharge does not exceed 

 2,200 cubic feet per second, which would 

 greatly cripple the value of the work were 



