THE CHALK WATER SUPPLY OF YORKSHIRE. 33 
stood at 270 feet above ordnance datum, being 108 feet above that 
of Langtoft well.* The extent of this subzrial water-containing 
chalk, measured from its outer margin toa line along the coast, is 
about 780 square miles. ‘The average depth may be taken 
at 350 feet, giving about 282,687,000,000 cubic yards of water- 
bearing chalk. 
The average annual rainfall on the chalk wolds is about 
27% inches, of which I should say about one-half is added to 
the subterranean water, whilst the remainder is intercepted 
by plants, etc., and evaporation. In 1872, the rainfall at Little 
Driffield was 40°19 inches, causing in the autumn of that 
year and the beginning of 1873 a copious flow of the three Wold 
gypsey races, and producing a maximum water-line nearly equal 
to the elevation in the second week of January, in the year 1877. 
In 1873 and 1874 the fall of rain at the same place was only 
21°46 and 23°19 inches respectively. These two small annual 
rainfalls happening together led to great loss and inconvenience to 
the inhabitants in the Wold districts, during the summer and 
autumn of 1874, by the drying up of numerous springs and 
of nearly all ponds and surface water; and the lowering of 
the water-line so as to leave quite dry the bottom of many wells 
on the Wolds. A great number of wells in this district were then 
deepened. 
In my opinion the great change of level in the water-line after 
an increase or diminution of the rainfall is mainly due to the 
two following causes. First, to the chalk situated immediately 
above the water-line having its pores always charged to saturation, 
so that it cannot hold more water when immersed in it, the 
interstices of the rock being the only spaces left for the descending 
rain on approaching the water-line ; and secondly, to the mass of 
rock traversed by the rise or fall of the water-line being in 
the shape of a wedge and to the area of elevation being limited to 
the outer edge of the reservoir or zone of outcrop. Hence 
the rapid and seemingly disproportionate rise of the water-line 
after prolonged rain. 
When the water-line in the Mid Wolds moves upwards or down- 
wards to the extent of roo feet, its rise or fall at Driffield, just 
* The angle of inclination to horizon or level line is accordingly 28 minutes 
8 seconds of arc. The greatest I have observed in Berkshire was 28 minutes 
3 seconds. This was on Jan. 7th, 1778, when the water in Dean well (not far 
from Loughborough, the highest point of the Chalk Downs) stood 35 ft. 7 in. 
she the water in Upper Farm well, further down the slope, and 4365 ft. 
istant. 
On Aug. toth, 1898, the water-line at Dunswell stood 54 ft. above that of 
Cottingham, 7431 feet distant. In this case the inclination of water-line to 
horizon is 24 min. 59 sec.—H.P.S. 
