21 



free circulation of air above the tanks. 



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 and the valve in the con- 

 necting pipe is opened, admitting water to the soil tank. As 

 the water level drops in the supply tank a partial vacuum is 

 formed above the water surface and the water drops in the vent 

 tube from the original level to a point the position of which 

 depends upon the degree of vacuum established. This point is 

 determined by the difference in the pressure heads due to atmos- _ 

 pheric pressure and the partial vacuum in the supply tank. Water 

 will continue to fall in the vent tube, but at a greater rate 

 than in the Mariotte tank, until the pressure head corresponding 

 to the atmospheric pressure, minus the pressure head caused by 

 the partial vacuum, is balanced by a column of water equal to 

 the difference in elevati9n between the water surface in the 

 Mariotte tank and the bottom of the vent. Water will then stand 

 in the vent at the bottom of the tube with the pressure at this 

 point atmospheric. 



If the water continues to flow, air will enter the glass 

 gage through the vent tube, bubbling upward through the water and 

 into the top of the supply tank. Water will continue to rise in 

 the soil tank to the level of the lower end of the vent, at which 

 point the atmospheric pressure in the soil tank and in the bottom 

 of the vent tube is the same. As there is no difference in pres- 

 sure and both points are at the same level, there is no head to 

 cause further flow and bubbling will cease. When the water table 

 in the soil falls below the bottom of the vent, the balance of 

 pressures is again disturbed and flow will once more start from 

 the Mariotte tank, replacing the quantity of water used. 



As a partial vacuum must be maintained at all times, pipe 

 connections must be airtight. Air leaks through the many joints 

 of the system disturb the balance of pressure necessary for full 

 automatic control. Thorough insulation against temperature 



