DE RANCE : UNDERGROUND WATERS IN LINCOLNSHIRE. 
23 
9 years it fell below 16 inches, viz., in 1884 and 1887. In 22 years 
at Navenby the rainfall has been 20 inches, or less, no fewer than 
6 times, on 2 of which it was below 17 inches. At Sleaford in 15 
years, the rainfall fell below 20 inches on three occasions, viz., 1884, 
1887, and 1890. At Pode Hole, Spalding, the rainfall in 21 years, 
was 20 inches, or less, in six different years, viz., 1870, 1873, 1874, 
1884, 1887, 1890. Of these four were below 17 inches, and one 
below 16 inches ; that of the Jubilee year, 1887, in which only 1513 
inches fell. 
An inspection of an Ordnance one-inch map, on which no geo- 
logical features are coloured at once discloses the permeable or im- 
permeable nature of the rocks and soils of which the area may be 
composed, by the absence or presence of stream lines indicated on 
the map, on any particular area that may be selected for observation. 
For convenience it is found useful to have " one-inch squares traced 
on tracing-cloth, each of which when placed over the map, covers an 
area of one square mile. If the area so covered contains no stream 
lines engraved, it may be safely assumed that, after deducting the 
proportion of the annual rainfall evaporated, the whole of the balance 
left will find its way into the underground stores of water below. If 
the stream lines are above a mile in linear extent, about a third of 
the rainfall will be evaporated, a third run off in floods, and a third 
absorbed. If numerous stream lines traverse the square inch in all 
directions, two-thirds of the rainfall will be run off in floods, and be 
only available in catchment reservoirs of large extent, capable of 
storing the rainfall of several months. The porous rocks vary much 
in degree of porosity, and still more in the rate at which they dis- 
charge water absorbed. The minimum supply of underground water 
in the county of Lincoln is therefore 5 inches per annum, or 200,000 
gallons of water per square mile per day. 
One inch of rainfall per annum gives 62 gallons of water per day 
per acre, or a supply for three persons from each acre were it all 
available, assuming evaporation to take a third of this quantity. 
The balance left for human consumption of the 5 inches percolating 
depends on the amount sucked up by vegetation ; this is an exceed- 
ingly variable quantity. After a long drought the first rainfall never 
