240 IRRIGATION. 



and to a great extent, to the furrows ; excepting those of 

 the most temporary character. It may be that some 

 repetition of previous statements may be made, but as 

 they will be found pertinent to the matter in hand, no 

 apology may be needed for that. 



In the construction of canals of whatever description, 

 so long as their bed and banks are of earth, the inclina- 

 tion of the bed, the size of the channel, its form, and 

 the nature of the soil through which it is carried, are of 

 the utmost importance ; because upon these depend its 

 capacity for delivering water ; its cost of construction ; 

 its permanence in use; and the prevention of loss of 

 water by filtration through its banks or bed. Upon the 

 inclination of the bed depends the velocity of the current 

 and the stability of the banks. It is necessary to limit 

 the velocity of the stream in the canal, lest the banks 

 should be degraded, and washed down into the bed. 

 "Water flowing at the rate of 120 feet per minute, which 

 is the rate of flow with a fall of one foot in a thousand, 

 is considered the limit of safety in the most consistent 

 soils. "Water flowing at half this speed will wash the 

 banks of a canal made in sand and fine gravel, but it 

 must not be forgotten that the velocity of a stream is 

 greatest in the middle of the surface, and least at the 

 bottom and sides where it comes in contact with the 

 earth. Thus the flow in the center of a wide stream 

 being at the rate of ten feet in a given time, will be only 

 eight feet in a deep canal, and six feet in a shallow one. 



The following table gives the different and the mean 

 velocity of streams : 



Velocity in inches per Velocity in inches per %f ean velocity, 



second at the surface. second at the bottom. 



4 1 ** 



8 3.3 5.6 



12 6. 9.0 



16 9. 13-6 



20 12. I 6 - 



24 15. 19.5 



28 18.4 23-2 



32 16. 26.8 



