THE FLOW OF WATER IN IRRIGATION CHANNELS. 15 
OFFICE EQUIPMENT AND METHODS. 
Multiplication, division, and addition were performed on mechan- 
ical devices. Areas included within vertical velocity curves were 
determined with planimetcrs and checked by arithmetical ordinates. 
Vertical velocity curves were drawn through platted points by using 
xylonite parabolas. The wetted perimeters were determined by 
stepping around cross sections, plotted to such a scale that the length 
of the perimeter could be read as closely as the measurement in the 
field would warrant. Checking was done by means of slide rules and 
plotted curves showing the values of n for various other hydraulic 
elements. 
CORRECTION FOR VELOCITY HEADS. 
In order to bring out the influence of the correction for the differ- 
ence in velocity heads when there is a difference in areas at the ends 
of the reach, a concrete example is given: 
Test No. 175, made on the Salt River Valley Canal, Arizona. The 
various items of the data were as follows: 
Example showing correction for change in velocity heads. 
Length of reach feet. . 900. 
Assumed elevation of water surface at station do 90. 000 
Elevation of water surface at station 9 do 89. 319 
Fall in water surface do 00. 681 
Area at station square feet. . 38. 610 
Area at station 3 do 40. 41 
Area at station 6 do 45. 09 
Area at station 9 do 43. 35 
Discharge, found by current meter second-feet. . 131. 33 
Mean area throughout reach, A square feet. . 42. 16 
Mean velocity throughout reach, V feet per second . . 3. 115 
Mean velocity at station do 3. 40 
Mean velocity at station 9 do 3. 03 
Velocity head for velocity at station foot. . . 1797 
Velocity head for velocity at station 9 do 1427 
Difference in velocity heads do 0370 
This difference added to fall, =00.681+ 0.0370= . . .do 718 
Slope, s, without correction for velocity heads 00075667 
Slope, s, corrected for velocity heads 00079778 
Mean wetted perimeter throughout reach feet.. 19. 52 
Mean hydraulic radius throughout reach do 2. 158 
Using the above data and solving equation 4, page 4, the value of 
0.0217 is found for n if the slope as found by the level, uncorrected for 
velocity heads, is used. If the corrected slope is used, a value of 
0.0223 for n is found. The change due to this correction is academic 
rather than practical for the lower velocities, but in such as are found 
in some flumes and concrete sections the change is very material and 
should not be disregarded. 
