CONDITIONS INFLUENCING PREVALENCE 187 



During the dry weather some 26 inches are spread over a 

 very large area, the security of the spring crops in the 

 Punjab being almost entirely dependent on irrigation (either 

 from canals or wells). For the autumn crops, however, the 

 cultivator relies mainly on the natural rainfall, and it is 

 mainly in the case of certain valuable crops, such as rice 

 and sugar, that he requires to supplement it by irrigation. 



Hence, for the cultivation of the autumn crops, which 

 commences in the dry weather and is continued through the* 

 rains, some 56 inches of water are given, although the area 

 so treated is much smaller than that irrigated for the spring 

 crops ; and this, it must be remembered, is in addition to a 

 natural rainfall of some 20 inches, which is mainly concen- 

 trated in the same season of the year. Except in the case 

 of rice, however, the water never lies long enough on the 

 ground to admit of the development of a generation of 

 Mosquitoes, as for most crops only a few waterings and 

 those at long intervals are given, as may be seen from the 

 table given below, taken from Mr. Kennedy's above-quoted 

 paper. 



Minimum Amount of Water Required to Irrigate Different Crops. 



Mr. H. Frost, the present executive engineer of the same 

 canal, tells me that except in the case of rice it is rare for 

 the water given to the land to remain visible on the surface 

 for more than a day, and my own casual observation of 

 irrigated land has given me the same impression. It will 

 be observed in the above table that the number of waterings 

 given to rice is not stated. In other words, during the 

 earlier and more critical stages of the crop, it is kept con- 

 tinuously under water. The water is fairly clean, though 



