386 The Land and Its Treatment 



the fall. The water supply must be ample, for when irri- 

 gation is most needed, the air is dry and hot and evapora- 

 tion is rapid. It should be the aim to convey the water 

 in narrow streams or furrows close to the plants, rather 

 than to cover the entire space between the rows. The 

 farther end of the rows should be supplied quickly (by 

 providing sufficient fall, head and quantity), otherwise 

 most of the water will be taken up at the near end of 

 the row. 



Imitation of rainfall is now employed in many high- 

 class gardens. This means an overhead installation, with 

 the water forcibly thrown from small openings in lines 

 of pipe. There should be a good head, for pressure is 

 essential. The mains (consisting probably of ll^-inch 

 iron pipe) are laid either on the surface or beneath it. 

 From these mains, smaller pipes are carried overhead ; they 

 should be about 7 feet above ground to allow of easy work- 

 ing beneath them and to give sufficient " throw " to the 

 jets of water. The pipes are punctured or bored at inter- 

 nals of about 3 feet, special plug-nozzles being employed. 

 The stream is small and solid as it leaves the nozzle, but 

 soon breaks into a rain-like spray. With a head of 30 to 

 40 pounds, the spray should reach 20 to 30 feet, and this, 

 therefore, determines the distance apart of the runs. 

 Gates are provided in the pipes, which, when turned, throw 

 the water in one course or another. 



The overhead irrigation has given good commercial 

 results in certain types of intensive gardening, greatly re- 

 ducing the risks. The system should be installed by an 

 experienced man, for the weak points in the operation 

 have now been well worked out. 



