THE FLOW OF WATER IN IRRIGATION CHANNELS. 11 



each other. Through these holes a welhoiled fishline formed a four- 

 tie suspension for a perforated disk. The hole ia the disk was a 

 trifle larger than the punch which passed through it, so that when 

 held truly vertical the disk acted as a plumb bob, not touching the 

 punch at any point. A very shght leaning of the punch caused the 

 disk to touch, which of course warned the operator that the punch 

 must be plumbed. 



In concrete-lined sections and in wooden flumes it was not prac- 

 ticable to use the above device, so a thin headed wire nail was driven 

 horizontally into the side at the water surface. In many cases it 

 was not possible to use a stilUng box with such a mark, but it is 

 beheved that very little error results where the same operator sets 

 all the marks with the same relation to top and bottom of whatever 

 surface fluctuations exist. Wherever the velocity is sufficient to 

 cause much doubt in the mind of the operator as to the accuracy 

 of the mark, then the total fall between the two ends of the reach 

 tested is so great that any error of a few thousandths is but a very 

 small percentage of the total fall. 



Stilling box. — A tin salve box about 3 inches in diameter made a 

 very good stiUing box. A hole about three-eighths inch in diameter 

 was made in the center of the bottom. After the nail had been started 

 in the top of the stake in the canal, the box was set over the stake, 

 with the nail projecting through the hole. When the box rested on 

 top of the stake water entered and withdrew slowly and quietly, so 

 that a quiet, dcfuiite surface was maintained in the box, and the top 

 of the nail was driven to this surface. 



Trimming canal section for measurement. — Where it was necessary 

 to make a current- meter measurement for discharge in a canal and 

 grass or moss was liable to clog the meter and make inaccurate the 

 measurement, or the bottom was slightly uneven, the sides and 

 bottom were neatly trimmed, and all interfering growths removed 

 by means of a sharp short-handled hoe. 



FIELD METHODS. 



The general method uscmI by the writer in conducting the measure- 

 ments for tlx; field data was as follows: Minor changes wore some- 

 times necessary for various reasons. 



A length or reach of the canal to bo tested was chosen. The dis- 

 charge m(!asurement was started innncidiately, preferably at the 

 center of the reach chosen. While the measurement was progressing, 

 the length of the rcuicli wsis can^fully cliained, the final length being 

 deterndned })y actual position of \\\i^ two nnils drivciti to the water 

 surface at the ends of the n;acJi. 'I'lm upsticiaiu nail was always sot 

 first, HO that the party wulkcd (h)Wiistn',tun with any slight change 

 in volume of the dischargo rather than have this change pass them, 



