8 BULLETIN 854, U. S. DEPARTMENT OF AGRICULTURE. 



either side of the tile in the flume was prevented. After the tile 

 were covered, the remaining space in the flume was filled with earth. 



PIEZOMETERS AND PIEZOMETER TILES. 



In order to measure the depth of flow in the tile drain, piezometer 

 tubes of graduated glass were placed on the side of the flume and 

 connected to the lower part of the tile line. Twelve tiles of uniform 

 shape, for each size and kind, were selected, and a small hole was 

 drilled through the wall of each. In each hole a f-inch iron pipe, 

 2 inches long, was inserted, care being taken that the tube did not 

 project inside the tile bore. This tube was set in cement (PI. II, 

 fig. 2), and any unevenness on the inside wall of the tile at the en- 

 trance of the tube was removed by coating the surface with a little 

 cement. This method of inserting the tube was deemed the best as 

 determined by Hiram F. Mills from a study of the results of some 

 6,000 observations on various piezometer connections (see Trans. 

 Amer. Academy of Science, 1878). Mills found that with an orifice 

 whose edges are in the plane of the side of the conduit and with the 

 bore of the tube normal to the plane of the wall, the piezometer 

 column indicates the true height of the water surface in any open 

 conduit, or the pressure in a closed conduit. 



At first these piezometer tile were so turned as to have the tube 

 on the bottom of the tile in the flume. Much trouble was experienced 

 from the tube openings filling up, so the piezometer tile were then 

 laid with the tube leading toward the side of the flume but turned 

 slightly downward. The connection was made by rubber tubing to 

 a steel nipple inserted through the wall of the flume (PL VII). On 

 the side of the flume at each piezometer tile, a frame holding the glass 

 tube was set. This glass tube (PI. VII) was graduated in tenths and 

 hundredths of a foot. Its zero was set at a definite distance below 

 the top of the flume. A rubber tube connected the piezometer glass 

 to the nipple in the wall of the flume. 



The zero of each piezometer gage was 17| inches below the top of 

 the flume. The invert of the tile in the flume was always laid 16^ 

 inches below the top of the flume. The capillarity of the glass tubes 

 used was found to be 0.01 foot. Thus, with water just entering the 

 tile drain, the piezometer tube read 0.09 foot. In other words, in 

 order to obtain the true depth of flow in the drain, 0.09 foot was 

 subtracted from each piezometer reading. 



With the exception of the two piezometers near the tile entrance, 

 which were only 8 feet apart, these tubes were distributed along the 

 flume approximately 55 feet apart, the last piezometer being within 

 a few feet of the outlet of the tile drain. 



