Yol. 63.] THE MOTION OF SUB-SURFACE WATER. 105 



Prof. Watts added his testimony to the value of the paper. 

 Such maps as those plotted out by the Author, showing the actual 

 distribution of underground water, would be of great service, and 

 would help to correct the generally erroneous ideas of the public, 

 who were prone to imagine that the distribution of water was con- 

 ditioned by absolutely magical laws. 



The Author thanked the President and the Fellows for their 

 kind attention to, and their remarks upon, the paper. In reply 

 to Mr. Young, he remarked upon the indiscriminate use of the 

 word porosity to express both porosity and permeability, and 

 he also touched upon the influence of air in obstructing the flow of 

 water into and through a porous rock. 



In reply to Mr. Baldwin's query, he stated that, as he had 

 pointed out in the paper, the quantity and chemical composition of 

 the dissolved salts and the temperature of the water were factors of 

 primary importance in investigations of the rate of flow of water in 

 rocks ; and that, although this had not received the attention which 

 it deserved in previous investigations, it was most essential, in order 

 to obtain comparable results, that the salinity and temperature of an 

 underground stream should be recorded at the point of outflow, 

 and wherever else that was possible. 



Q. J. G. S. No. 250. 



