30 DIVISION OF WATER RESOURCES 



was lowered iuto the outer, where it hung suspended from the heavy 

 iron rim around the top. 



As a means of comparing results of evaporation from similar soil 

 placed in tanks by different methods, three tanks of the same size as 

 those described were filled with loose soil taken from an excavation 

 made to a depth of 6 feet. In making the excavation each foot of soil 

 removed was kept separate from the others and placed in the tanks in 

 the original order, foot bj^ foot. Before the tank was completely filled, 

 it was flooded with water to compact and settle the material in a 

 uniform manner. Water was added from time to time until it drained 

 from the bottom through a pipe connection. Very little settlement 

 occurred after the initial settlement and there is no doubt as to this 

 method producing a more uniform soil density' than is obtained by 

 placing and tamping the soil in layers. 



The Mariotte Tank 



This device consists essentially of a supply tank equipped on the 

 principle of the Mariotte flask to supply water to the soil tank through 

 a connecting pipe. A 12- by 36-inch galvanized-iron range boiler, 

 chosen because of its solid construction, rigidity of its connections, 

 and practicability of keeping it air-tight, was found satisfactory for 

 the purpose. Mounted upon the side of the supply tank is a vertical 

 length of glass tubing, each end of which is fitted with a rubber 

 stopper perforated to admit a small connecting pipe. The lower pipe 

 connects -svith the supply pipe between the Mariotte tank and the soil 

 tank, while the upper connects with the top of the supply tank. Upon 

 the glass tube is mounted a meter stick or scale upon which differences 

 in daily readings determine the amount of water withdrawn. A valve 

 in the connecting pipe makes it possible to shut off the flow of water 

 when the supply tank is being refilled. A waste pipe also is set into 

 the connecting pipe to discharge excess water from the soil tank into 

 a receiving vessel. The lip of the waste pipe is set at the level of the 

 water in the soil tank. The tank equipment is shown in Plate II. 



A vent tube of small diameter passes through the rubber stopper at 

 the top of the glass gage. This tube is open at both ends and the level 

 of the soil water is determined bj' the elevation of the bottom end of 

 the vent. In Plate II, the water table in the soil tank is shown as 

 being at such a depth that it is necessary to extend the vent tube 

 downward into an extension well below the supply pipe. 



Previous experience has determined that A'ariations in temperature 

 cause changes in the vapor pressure in the jMariotte tank above the 

 water surface, causing fluctuations in the water level. Every effort 

 was made, therefore, to insulate the IMariotte tanks against temperature 

 changes. The Santa Ana ]\Iariotte tanks were completely buried in 

 the ground except for a small entrance provided with a narrow door- 

 way for making readings of the graduated scale. They were further 

 protected from the cooling effect of rainfall on the surrounding soil 

 by a galvanized iron roof, beneath which was free circulation of air. 



For the benefit of those who may be interested in the theory of 

 operation of the Mariotte tank, a brief description follows. The vent 

 tube provides the Mariotte control feature which maintains a constant 

 water level in the connected soil tank. In operation, the Mariotte tank 

 is filled -with water and the valve in the connecting pipe is opened. 



