24 AGRICULTURE IN THE TROPICS [PT. I 



runs for 55 miles along the side valleys, feeding all their tanks, 

 and is about 60 feet wide. 



In many districts irrigation has to be from wells, and every 

 form of appliance is in use, from simple hand carriage of the 

 water the well being made with a sloping side to enable one 

 to walk down to the water through more and more perfect 

 raising implements worked by bullocks or by well sweeps, to 

 modern continuous chain pumps, etc. This form of irrigation 

 is particularly suited to gardening work, tobacco or vegetable 

 cultivation, and so on, and the arrangements for distributing 

 the water are often very perfect, the " flower beds " if such a 

 term may be used, being made with little banks round them, 

 and the water led to them in little canals. 



Irrigation by windmill-pumps, such as is so common in the 

 United States, is almost unknown in the tropics, for the wind 

 in the driest weather is often absent, and blows hardest in the 

 rains. Partly in consequence of this, pumps worked by oil or 

 gas engines are coming in in some places. 



The necessary concomitant of irrigation is drainage, and 

 care must be taken to give the soil no more water than will 

 drain away before the next application of water. Heavy 

 clayey soils are thus in general unsuited to irrigation, which 

 succeeds better on friable soils. A good deal of trouble some- 

 times arises from the top layers of the soil becoming alkaline, 

 often for want of proper drainage. Irrigation results in good 

 and reliable cropping, but is of course exhaustive to the soil, 

 which will probably want manure much sooner 1 . 



1 As many who have read the manuscript have complained of the absence of 

 description of the irrigation works of Egypt, the north of India, South Africa, 

 etc., the opportunity may be taken to point out that none of these are tropical. 



