100 BURTON EDWARD LIVINGSTON 



only through the stopper (vial D) and extends about 3 cm 

 laterally, where it ends free (the diagram shows it bent downward, 

 but this is unnecessary). Tube E is supplied with a loosely 

 fitting cork stopper, slipping upward and downward upon the 

 tube. The reservoir G is treated as in the absorbing mounting 

 ulready described, a file mark on its neck serving as zero point 

 in rilling. A metal support for the vials, tubes and cup and a 

 protecting apron of waterproof cloth to prevent rain-water from 

 entering the reservoir around the tube, are provided (not shown in 

 the diagram) . 



Vials C and D are supplied with clean mercury to a height of 

 6 or 8 mm. and their stoppers tightly inserted. The stopper 

 of the reservoir is raised, the bottle slipped to one side as far as 

 the tube will allow, and filling occurs as in the absorbing form. 

 A rubber tube is attached to the free end of tube F and suction 

 is applied, causing water to rise from the reservoir, successively 

 filling vial C, the porous cup and vial D, and lastly escaping into 

 the rubber tube. Suction is continued till no more air bubbles 

 enter this tube, when the latter is removed. The instrument is 

 now ready for operation. 



As water evaporates from the cup mercury rises in tube B 

 and falls in tube E, until water from the reservoir passes freely 

 from E to C, between the mercury and the end of the glass tube. 

 The lost water is now supplied as usual from the reservoir, the 

 column of mercury in the lower end of tube B remaining at a 

 height of a few millimeters (depending on the height of the cup 

 above the water level in the reservoir) . Water cannot enter the 

 cup from vial D and this vial remains filled, unless it is even- 

 tually lowered by slow evaporation through tube F. 



In time of heavy rain evaporation from the cup ceases, water 

 is absorbed from without, and mercury rises in tube E, simul- 

 taneously falling in tube B. Practically no water can enter the 

 reservoir from the cup. Whether water flows from the cup 

 through vial D to the outside is of no consequence. 



The purpose of the second mercury valve is simply for use in 

 filling a newly placed cup, as above described, since the cup and 

 tubes cannot readily be inverted as with simpler absorbing 



