214 MACHINERY IN CONNECTION WITH WATEE. 



VELOCITY OF WATER IN DITCHES. 



It is often of great practical utility to know what 

 amount of water may be carried off in draining, or sup- 

 plied in irrigation, by channels of any given size and 

 descent. The following rule will apply to all cases, from 

 the plow-furrow to the mill-race, or even to the large 

 river, and may be used by any boy who understands 

 common arithmetic, and which is illustrated and made 

 plain by the example that follows the rule. 



To ascertain the mean {or average) velocity of water in 

 a straight channel of equal size throughout : 



Let y^the fall in two miles in inches ; 



Let c?=the hydraulic mean depth ; 



Let t'=the velocity in inches per second ; 

 then the rule is thus expressed : t'=0.91 ^fd^ or, in plain 

 words, the velocity is equal to the hydraulic mean depth, 

 multiplied by the fall, with the square root of this prod- 

 uct extracted, and then multiplied by 0.91. 



The " hydraulic mean depth " is found by dividing the 

 cross-section of the channel by the perimeter^ or border. 

 The perimeter is the aggregate breadths of the sides and 

 bottom of the channel. 



The rule will be rendered quite plain by an example. 

 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 quantity 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 six times 4 = 24, which, divided by 14, 

 the perimeter, gives 1.7 = the hydraulic mean depth. 

 Then, by applying the preceding rule: 



tj=9.91 4/ 640 X 1 .7, or ?;=0.91 x 33=30 inches, is the ve- 



