40 



THE DIFFUSION OF GASES THROUGH 



To keep the water level at the proper height or to raise or lower it by a 

 definite amount, the screv; pump in fig. 13 is available. This consists 

 essentially of a thick screw 6' playing into the brass tube cc, which is closed 

 at the top by the stuffing box a compressing the ring of soft material hh. 

 The bottom of the tube cc ends in the tubulure i, to be joined by appro- 

 priate tubing with the corresponding tubulure i in fig. 11 a. The brass 

 tube cc, fig. 13, is quite filled with water to the exclusion of air, so that ever}' 

 turn of the screw S raises the level ww in fig. 11 a by a definite amount. 

 The screw 5 is also provided with a graduated head (not shown), so that the 

 whole turns and fractions of a turn may be read off, and the ratio of the 

 diameter of the water level in fig. 11 a and that of the screw S in fig. 13 



must be known. 



In the figaire the ratio of the areas is about ten to one. 



d 



^wik /'f 



V} 



d 



ff 



B 





Fig. 1 1 A. — Absoluteelectrometer with disk carried by tubular hydrometer of aluminum. 

 B. — Absolute ^electrometer with glass U-tube for adjusting levels. 



It is interesting to note in passing that on sudden advance or retreat of 

 the screw 5 an impulsive wave passes through the liquid, suddenly raising 

 and lowering the disk e often more than a centimeter and with considerable 

 force. Direct and reflected waves are recorded in this v/ay. 



In many of the experiments it was found sufficient to coat the screw (hot) 

 with an adhesive layer of resin and beeswax. A well-fitting brass screw 

 0.75 inch in diameter and 20 threads to the inch was used. A socket of 

 indurated fiber is even preferable, there being no appreciable leakage for 

 some time. This avoids the complication of a stuffing box, but of course it 



