WATER LOSSES FROM WET AREAS 29 



The evaporalioii pan was set upon a \v()()(U'ii grillage in accordance 

 with iiistnictions issued by the Ignited States AVeather Bureau for pans 

 of this type. Evaporation losses Avere measured by a hook gage gradu- 

 ated to thousandths of a foot and mounted on the side of the pan. At 

 the northwest corner of the grillage an anemometer was mounted on a 

 stand so that the level of the cups was about V2 inches above the top 

 of the pan and about 24 inches above the ground surface. Originally, 

 a second anemometer was set 12 inches above the ground surface, mid- 

 way in the row of soil tanks, to measure ground winil, but it was used 

 only during the tir.st year of the investigation. 



Method of Filling Soil Tanks 



Heretofore a method frequently used for placing soil in experi- 

 ment tanks has been to separate layers of soil as excavated from a 

 trench and place them in the tank in the same order as originally found 

 in the ground. This process broke up the soil structure, increased the 

 volume, and changed the density. If tamped into the tank, alternate 

 layers of loose and dense material resulted, with structural arrange- 

 ments of soil particles entirely different' from the original. Soil mois- 

 ture experiments with soil so placed have not always proved satisfac- 

 tory. To rectify this condition the first tanks used in this investigation 

 were filled without materially changing the original soil structure. The 

 plan followed was to fill each tank by forcing the bottomless inner shell 

 over a core of soil of the same diameter of the tank until full, at the 

 same time excavating around the tank shell as the filling proceeded. 



At first use was made of a heavy screw jack resting upon a crib 

 of timber blocks, which in turn rested upon the angle-iron rim, with 

 the jack working against an overhead cable anchored in the ground 

 on each side of the excavation. As the tank sank into the soil under the 

 pressure, a gradually increasing pile of cribbing was used to support 

 the jack against the cable. The anchors to which the cable was fas- 

 tened were of a type generally used to anchor gaw wires and were 

 set in auger holes bored in the ground. 



Friction of soil against the outside of the tank was relieved by 

 excavating around the tank as the work proceeded. This excavation 

 generally kept a few inches ahead of the cutting edge of the tank, 

 cutting a core slightly larger than the tank diameter, the core being 

 shaved to the proper size as the cutting edge of the tank moved 

 doAvnward. 



As the tank gradually filled, the skin friction on the inside rapidly 

 increased, tending to cause compression in the soil. After the first 

 two or three attempts, it was found that the tank shell would slide 

 over the trimmed core of soil more readily if a sharp blow was given 

 at the to]) of the tank, using a short piece of timber as a driver. This 

 impact broke the bond of the inside friction, resulting in less tendency 

 toward soil compression and allowed increased speed in the work. 

 A few of the last tanks to be filled were driven over the trimmed core 

 of soil by impact alone. 



When the inner tank shell was filled, the soil column was cut off 

 l)y jacking the bottom plate across the bottom edge of the tank and 

 l)olting it to the angle-iron rim at the top. The whole was then hoisted 

 above the ground by chain block and tripod. The outer shell was ther. 

 set in place in the excavation, and the inner slu'U with its soil content 



