238 



THE IRRIGATION AGE. 



above one-fourth the depth for depths from five to 

 twenty-four inches ; 



c. That the distance of the sill of the weir from 

 the bottom of the canal be at least three times the depth 

 on the water; 



d. That the distance of the sill of the weir from 

 the sides of the channel be at least twice the depth of 

 the water flowing over the weir; 



more practical method of obtaining the discharge is by 

 use of what is called a current meter which, as its name 

 implies, is a measurer of the current or velocity of flow. 

 The instrument consists essentially of a metal frame 

 in which is pivoted a small paddle wheel which turns on 

 its axis with but very little friction. The theory of its 

 operation is that when immersed in moving water there 

 is a mathematical ratio betwen the velocity of the flow 



T 



H 



C 



C 



Fig. 3. Longitudinal Section Through Weir-box. 



e. That the lateral contraction remaining undis- 

 turbed, the length of the weir shall be three or, better, 

 four times the depth of the water flowing over ; 



f. That the length of the water flowing over the 

 "weir shall not be less than two inches. 



3. The velocity of approach must be very small; 

 for weirs three feet long and depth of twelve inches, it 

 ought not to be greater than six inches per second; for 

 weirs of six feet long and depth of twenty-four inches 

 it ought not to be above eight inches per second. In all 

 these cases the cross-section of the canal of approach 

 ought to be at least seven times that of the weir. Other 

 conditions effecting the velocity are included in c, d, and 

 e, respecting complete contraction. 



4. The layer of falling water should be perfectly 

 free from the walls below the weir, in order that air may 

 freely circulate underneath. For short weirs it is suf- 

 ficient that the lateral walls of the lower canal be free 

 from the sides of the weir. In such case, when air 

 freely passes underneath, the level of the water in the 

 lower canal has no influence on the discharge of the 

 weir, unless it reaches or exceeds the level of the crest. 



5. The- depth of the water should be measured with 

 accuracy where the suction of the flow does not affect 

 the height and where it is free from the influences such 

 as the wind, or the movement of the water which can 

 effect the true level. The height should be read to within 

 one-three hundredth of the depth in order that the error 

 may be within one-half of one per cent. 



6. The weir ought to be constructed with care 

 and carefully located. It should not vary more than 

 lour degrees from being perpendicular to the channel. 

 Its sill should be horizontal. 



THE CURRENT METER. 



To use the weir as a measuring device, it is neces- 

 sary, as has been seen, to obstruct the channel and inter- 

 fere with the free flow of the water. Obviously this can 

 only be done where the stream is of comparatively small 

 size. The large canal and natural stream are without 

 its sphere of usefulness, unless costly works can be em- 

 ployed. For the larger streams, therefore, say where the 

 discharge exceeds 40 cubic feet per second, by far the 



of the stream and the velocity of the revolution of the 

 vane. This relation may be represented by an equation 



Fig. 4. Small Electric Current Meter and Outfit. 



which, as the friction in the instrument becomes less, 

 approaches the equation of a straight line. 



To obtain the velocity of any stream it is necessary, 



