4 o 



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 screw pump in fig. 13 is available. This consists 

 essentially of a thick screw 5 playing into the brass tube cc, which is closed 

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

 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. 1 1 a. The brass 

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

 turn of the screw S raises the level ww in fig. 1 1 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. 1 1 a and that of the screw S in fig. 13 

 must be known. In the figure the ratio of the areas is about ten to one. 





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Fig. 11 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 c often more than a centimeter and with considerable 

 force. Direct and reflected waves are recorded in this way. 



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 



