150 



HORTICULTURE FOR SCHOOLS 



water maintained at a constant level 4 inches above the 

 center of the opening. This is equal to ^ cubic foot a 

 second, or 1^ cubic feet a minute. Since 1 cubic foot con- 

 tains 7.48 gallons, it fol- 

 lows that 1 miner's inch 

 delivers practically 

 9 gallons a minute. 



246. Weir meas- 

 urement. A n o t h e r 

 very common device 

 for calculating stream 

 flow is known as the 

 weir; but this is 

 limited in its usefulness 

 to comparatively small 

 streams. It is com- 

 monly constructed of 

 wood, faced with met- 

 al, especially along the 

 crest of the weir. In 

 calculating stream 

 flow, the observer 

 takes the depth of 

 water flowing over the 

 crest of the weir, being 

 careful to secure the 

 reading at a point far 

 enough back to avoid 

 the curved surface 

 which is formed as 

 the water flows over 

 the crest. This reading 

 can then be changed to cubic feet a second by using the table 

 on page 156. 

 The Gippoletti weir was invented by an Italian of that 



