On the Water which permeates through Strata. 175 



February, to about 1000 gallons per acre per diem. Again, on 

 the clay lands, the fall from October till 12th December 1856 

 was about 160 gallons per diem per acre ; while on the 9th 

 January 1857, the outlets ran 125 gallons per diem ; but on 

 the 10th the discharge w r as increased from 125 to 5150 gal- 

 lons per diem per acre. Such sudden accessions of water 

 upon places of the rocks already subjected to pressure, but 

 not at other times sufficient to effect a dislocation, maybe the 

 immediate cause of many a landslip, or of such a threatened 

 calamity as was lately impending on Greenside from the 

 Calton Hill. 



In such a rock as the Calton Hill, the surface rain-water 

 from the crown of the hill will penetrate down to a certain 

 limit, which, adopting the phraseology used in describing 

 sections of chalk hills, we shall call the water-line. This 

 line will be situated at no great depth from the surface of 

 the hill, probably not much below the stone steps which aid 

 the traveller to enter its precincts. In estimating the pres- 

 sure of the rain-fall contained in the sectional area above the 

 water-line, the undulating contour of the hill must be con- 

 sidered. We shall thus obtain a section, the height of which 

 indeed is limited, but the horizontal area of which is con- 

 siderable, and, as we have already seen, the pressure on a 

 spot of the section immediately abutting on the water-line 

 will be equal in depth to an imaginary section represented 

 by a straight line drawn from the highest point, wdiere the 

 w T ater is absorbed, to a perpendicular drawn from the point 

 of pressure. 



The line of saturation in chalk districts has been ascer- 

 tained to be generally about 1 in 400, or 13 feet per mile ; 

 while in other parts of the chalk districts the inclination of 

 this line is probably not less than 40 feet a mile. No such 

 observations have been made on this line in trap rocks ; 

 but their absorptive pow r er is well known. And especially 

 through rocks containing so many ash-beds as the Calton 

 Hill much water must permeate. 



The author next proceeded to show that hydraulic pres- 

 sure, such as had been attempted to be shown existed in 

 the rocks, would be best developed in the basins of the 



