Vol. 6 3 .-] 



THE MOTION OP SUB-SURFACE WATER. 



83 



percolating water, this air is cushioned and compressed, and at 

 pressures slightly higher than the normal prevents the ingress 

 of water to the pores that it occupies. Consequently, a recharged 

 rock may be considered as an aggregation of water-charged pores 

 surrounding a nucleus of air-charged pores; while, in a fully- 

 charged rock, the water occupies uniformly all the pores. There is 

 some confirmation of this theory, in that the rocks with coarser 

 interstices are more readily recharged, and take up an initial 

 quantity of water more nearly approximating to their full capacity, 

 than do those rocks that have finer interstices. 



Table I, showing the Eate of Absorption of Water by various Stones : the 

 Water absorbed being expressed as a Percentage of the Volujie of 

 the Stone. 



(Continuous immersion in a beak 



er of water, at atmospheric pressure.) 



Number 



of hotirs 



im raersion. 



Chalk. 



Bath 



Weatherstone. 



Millstone 

 Grit. 



Remarks. 



Mottisfont. 



Micbeldever. 



Piece 

 1. 



Piece 



2. 



Piece 

 3. 



Piece 



4. 



Piece 

 1. 



Piece 

 2. 



Piece 

 1. 



Piece 

 2. 





34-65 



36-34 



11394 



187-15 



36-48 



40-12 



135-50 



68-47 



Weigh t,dry 



0-5 ... 



36-26 



35-41 



3977 



36-13 



20-29 



22-55 



12-34 



13-30 





10 ... 



36-53 



3581 



40-23 



36-49 



20-70 



2303 









4-5 ... 



36-96 



3661 



40-29 



36 99 



22-45 



23 93 









47-0 ... 



39-51 



38-16 



4239 



3888 



2397 



23-93 











45-86 





46-79 





24-18 







14-98 





Maximum. 



On account of this cushioniug action of the air, I consider that all 

 determinations of the flow of water through rock based upon 

 corresponding determinations with air, and the ratios derived from 

 the relative flow of air and water through capillary tubes, are 

 illusory, as air will not cushion air and interfere with the flow of 

 air through the interstices of a rock, while at moderate pressures 

 the cushioning air will seriously interfere with the flow of 

 the water. Therefore the two discharges will not be directly 

 comparable, as there is not a corresponding interference in the 

 capillary tubes ; and, farther, this interference will be of greater 

 magnitude in a rock with many fine pores than in one with a lesser 

 number of coarser pores — resulting in a corresponding increase in the 

 error of the calculated discharge. 



The rate of drying of saturated stone will, of course, vary with the 

 fineness of the pores, and with the temperature and velocity of the 

 air surrounding it. When the temperature is normal and the motion 

 of the air is only that of a closed room, the rate of desiccation is 



e 2 



