580 



SCIENCE. 



[N. S. Vol. XXII. No. 567. 



WELL IRRIGATION". 



It is evident that where water has to be 

 raised on to the field there is an outlay of 

 human or mechanical power which may be 

 saved if it can be brought to flow over the 

 fields by gravitation. But there is one 

 practical advantage in irrigating with the 

 water raised from one's own well or from 

 a river. It is in the farmer's own hands. 

 He can work his pump and flood his lands 

 when he thinks best. He is independent 

 of his neighbors, and can have no disputes 

 with them as to when he may be able to 

 get water and when it may be denied to 

 him. In Eastern countries, where corrup- 

 tion is rife among the lower subordinates 

 of government, the farmer who sticks to his 

 well knows that he will not require to bribe 

 any one ; and so it is that in India about 

 thirteen millions of acres, or 30 per cent, 

 of the whole annual irrigation, is effected 

 by wells. Government may see fit to make 

 advances to enable the farmer to find his 

 water and to purchase the machinery for 

 raising it ; or joint-stock companies may be 

 formed with the same object. Beyond this 

 all is in the hands of the landowner himself. 



CANAL IRRIGATION. 



Irrigation on a large scale is best effected 

 by diverting water from a river or lake 

 into an artificial channel, and thence on to 

 the fields. If the water surface of a river 

 has a slope of two feet per mile, and a canal 

 be drawn from it with a surface slope of 

 one foot per mile, it is evident that at the 

 end of a mile the water in the canal will be 

 one foot higher than that in the river ; and 

 if the water in the river is ten feet below 

 the plain, at the end of ten miles the water 

 in the canal will be flush with the plain, 

 and henceforth irrigation can be effected 

 by simple gravitation. 



When theje is no question of fertilizing 

 deposit, and only pure water is to be had, 

 the most favorable condition of irrigation 



is where the canal or the river has its source 

 of supply in a great lake. For, be the rain- 

 fall ever so heavy, the water surface in the 

 lake will not rise very*much, nor will it 

 greatly sink at the end of a long drought. 

 Where there is no moderating lake, a river 

 fed from a glacier has a precious source of 

 supply. The hotter the weather, the more 

 rapidly will the ice melt, and this is just 

 when irrigation is most wanted. 



Elsewhere, if crops are to be raised and 

 the rain can not be counted on, nor well 

 irrigation be practised, water storage be- 

 comes necessary, and it is with the help of 

 water storage that in most countries irri- 

 gation is carried on. 



WATER STORAGE. 



To one who has not given the subject 

 attention surprise is often expressed at the 

 large volume of water that has to be stored 

 to water an acre of land. In the case of 

 rice irrigation in India, it is found that the 

 storage of a million cubic feet does not 

 suffice for more than from six to eight acres. 

 For the irrigation of wheat about one third 

 this quantity is enough. It would never 

 pay to excavate on a level plain a hollow 

 large enough to hold a million cubic feet of 

 water. It is invariably done by throwing 

 a dam across the bed of a river or a valley 

 and ponding up the water behind it. Many 

 points have here to be considered: The 

 length of dam necessary, its height, the 

 material of which it is to be constructed, 

 the area and the value of the land that 

 must be submerged, the area of the land 

 that may be watered. The limits of the 

 height of a dam are from about 150 to 15 

 feet. If the slope of the valley is great it 

 may be that the volume which can be pond- 

 ed up with a dam of even 150 feet is incon- 

 siderable, and the cost may be prohibitory. 

 On the other hand, if the country is very 

 flat, it may be that a dam of only 20 feet 

 high may require to be of quite an inor- 



