CHAP. X IRRIGATION 71 



The water is conducted to the upper trench or canal, and it irrigation 

 overflows and runs along the surface, to collect in time in *''^"^hes. 

 the second trench. After the second trench is full, the water 

 in it overflows and runs down to the third trench, and this 

 goes on in a similar manner until the" water from the last or 

 lowest trench is carried away to the river or to some lower 

 level, where it will do no harm. This is called catch-work 

 irrigation. In some countries, where large rivers run through Overflow of 

 low lying land, as in the case of the river Nile in Egypt, and "^^''^' 

 the river Ganges in India, the river itself is allowed to run 

 over the land when its waters are swollen and full of rich 

 mud. The waters are allowed to remain on the land whilst 

 the mud settles, and in this way the fertility of the soil is 

 enormously increased. The number of feet that the waters The rising of 

 of the Nile rise above the lowest level, is a sure index of the *^^ ^''^' 

 crops to be expected in Egypt. A low Nile means short 

 crops and distress amongst the people, whereas a high Nile 

 indicates large crops and prosperity. In India too, a low 

 Ganges means, sometimes, terrible famine in the lands it is 

 unable to irrigate ; and the great works of irrigation carried The irriga- 

 on by the government of India, have happily prevented, in [Jfi^^J^^'^'"^ 

 many instances, the occurrence of famines, and turned poor 

 districts with their miserable and sickly inhabitants, into Their benefi- 

 highly cultivated lands peopled with a healthy and pros- ^'^' ^^^'^i^- 

 perous population. 



