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EXPLANATION OE PLATE V. 



Hydrological map of Dorset, Wiltshire, and Hampshire, on the scale 

 of 4 miles to the inch. 



Discussion. 



Sir John Evans said that they were very much indebted to the 

 Author for his observations. With regard to the Chalk, permeability 

 depended not so much on the nature of the Chalk itself as on the 

 fissures by which it was traversed. The angle of friction determined 

 the flow of streams above ground as well as below; thus, in the case 

 of the Gade and the Bulbourne, the inclination of the stream was 

 125 feet to the mile near Watford, but in other localities nearer the 

 base of the Chalk, where it became more clayey, the inclination was 

 19-5 feet to the mile, and near Dover 40 feet to the mile. 



Mr. W. Whitaker commented on the great practical as well as 

 theoretical value of such papers as that brought forward by the 

 Author. 



Mr. Cpnningham-Craig said that he could fully endorse the 

 previous speakers remarks as to the usefulness of information of 

 this kind, especially in connexion with water-supply in the tropics. 

 If the Author's points were more fully and frequently present to 

 the minds of geologists, the provision of water for small communities 

 in tropical regions would be greatly facilitated. 



Mr. GL W. Young enquired the precise meaning attached by the 

 Author to the word porosity. Was it the total amount that the 

 rock would absorb, or the amount absorbed within some given 

 time ? 



Mr. W. Baldwin said that he would like to ask the Author 

 whether he had made any experiments showing how variation of 

 pressure would affect the porosity of various rocks. 



