22 



changes has been mentioned. Such changes may cause expansion or 

 contraction of the tank itself, of the water in the tank, and of 

 the air in the chamber above the water. The combined result is 

 change in the vapor pressure with consequent influence upon 

 effective regulation. 



Water in the glass tube will fall with an increase in tem- 

 perature within the Mariotte tank, and readings on the scale, 

 taken at this time, will be erroneous. A test of the effect of 

 temperature on scale readings showed that an increase of 30 F. 

 in outside air temperature caused a drop of 1 centimeter of the 

 water level in the glass gage. As the temperature returned to 

 the starting point, water in the gage came back to its initial 

 position. Early morning is a better time for observations than 

 later in the day when temperatures are higher. 



Float Valves 



A simple device for maintaining a definite depth of water 

 above the soil surface in tule tanks is an ordinary float valve 

 connected by a feed pipe to a supply tank. The float valve is 

 adjusted to operate at the required water surface. As the water 

 is used the float drops to open a needle valve and admit water 

 from the supply tank. A gage on the side of the tank permits 

 readings of water levels, and the quantity of water released be- 

 tween observations is equal to the consumptive use by tank growth. 

 This equipment has the advantages of fitting into a small space 

 and of maintaining a constant depth of water. It is easily in- 

 stalled and gives satisfactory results. A water-stage recorder 

 with float in the supply tank may be attached to obtain contin- 

 uous records of consumptive use. 



