19 

 The Mariotte Tank 



It is often desirable to make investigations in which the 

 water table in the soil tank does not fluctuate with the demands 

 of the plants. For this purpose the Mariotte supply tank is of 

 practical use. Inverted bottles having connections to the water 

 surface have been used on occasion, but more elaborate arrange- 

 ments embodying the same principles, have been designed by the 

 Division of Irrigation (2, 20, 21). This equipment has given 

 general satisfaction by maintaining a fixed water level in the 

 annular space and in the soil, as well as providing a means of 

 measuring the daily rate of extraction of moisture by the plants. 



In the Santa Ana, Calif,, (2) investigations a battery of 

 Mariotte tanks was used. The following description indicates the 

 relation of supply tank to soil tank and outlines the theory 

 under which operation proceeds: The Mariotte tank, a 12- by 36- 

 inch galvanized-iron range boiler, was chosen because of its 

 solid construction, the rigidity of its connections, and the 

 practicability of keeping it airtight (fig, 1). Mounted on the 

 side of the 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 with the supply 

 pipe between the Mariotte tank and the soil tank, while the upper 

 pipe connects with the top of the supply tank. A graduated scale 

 moiinted beside the glass tube shows the depth of water in the 

 supply tank. A valve in the connecting pipe makes it possible to 

 shut off the flow of water when the supply tank is refilled, A 

 waste pipe in the connecting pipe discharges 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, 



A small vent tube passes through the rubber stopper at the 

 top of the glass gage. This tube is open at both ends (the lower 

 end in water and the upper in air) and the level of the soil water 

 is determined by the elevation of the bottom end of the vent. In 



