VELOCITY OF WATER IN DITCHES. 193 



border. The perimeter is the aggregate breadths of 

 the sides and bottom of the channel. 



The rule will be rendered quite plain by an exam- 

 ple. Suppose a smooth furrow is cut six inches wide 

 and four inches deep, with perpendicular sides, and 

 that it descends one inch in a rod ; to find the quanti- 

 ty of water that will flow through it. One inch fall in a 

 rod is 320 inches in a mile, or 640 in two miles. The 

 perimeter in contact with the water will be six inches 

 on the bottom, and four inches in each side = 14 inches. 

 The area of the cross-section will be 6 times4 = 24, which 

 divided by 14, the perimeter, gives 1.7 = the hydraulic 

 mean depth. Then, by applying the preceding rule : 



V = 0.91^640x1.7, or t^ = 0.91 X 33 = 30 inches, 

 the velocity per second, which would be about three 

 gallons per second, or three hogsheads per minute. 



An open ditch, therefore, with smooth sides, convey- 

 ing a stream of this size, would carry oft' in one hour, 

 from an acre of land, all the water which might fall 

 by half an inch of rain during the wet season ; for half 

 an inch of ram would be 180 hogsheads per acre, which 

 would pass oft" in one hour ; or it would supply in one 

 hour, by the process of irrigation, as much water as a 

 heavy shower of half an inch. Where the descent is 

 greater, the increased quantity may be readily calcu- 

 lated by the rule given. The capacity of smooth-sided 

 underground channels may be determined in the same 

 way; but if built of rough stones, great allowance 

 must be made, as they will retard the flow of water. 



In common practice, too, even with straight, open 

 ditches, the velocity will be much dmiinished by the 

 rough sides, 



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