On the Water ivhich '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 was 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, may be 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 wdiich is con- 
siderable, and, as we have already seen, the pressure on a 
spot of the section immediately abutting on the vv^ater-line 
will be equal in depth to an imaginary section represented 
by a straight line drawn from the highest point, where the 
water 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 tins line in trap rocks ; 
but their absorptive power 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 
