IRRIGATION 273 



lar kind of soil and a particular kind of work. Disc plows (Fig. 230) 

 have a revolving cutting instrument, called a disc, instead of a plow- 

 share. The friction is less than with an ordinary plow, because the 

 cutting tool rotates through the soil instead of sliding through it (cf. 

 206). 



Gang plows consist of many plowshares attached to one frame. 

 They can be used on large, level areas, and are drawn by a large num- 

 ber of horses or by engines (tractors; Fig. 231). 



Fig. 231. 



A Gang Plow. This one has ten plowshares; behind it is a cultivator. 

 Courtesy of Deere and Co. 



297. Irrigation. By irrigation we mean the adding of 

 water to a soil to make up for a lack of rain. Irrigation 

 may be carried on anywhere, but it is especially im- 

 portant in desert areas. In this country it is used to 

 supply water for farming and fruit-raising to the sections 

 having a small rainfall (cf. 42 and 272). Even in 

 almost rainless regions there is usually a lake or stream 

 that has a large amount of water during the rainy season, 

 or in the spring, when the snows melt in the mountains. 

 But in the summer, the growing season of crops, there is no 

 water. Men therefore impound the water in large reser- 



