_;" Darwin
equally concedes that to keep the rain fresh when banked in, as he
assumes, by the sea, the mass of madrepore must be "sufficiently thick
to prevent mechanical admixture; and where the land consists of loose
blocks of coral with open interstices, the water, if a well be dug, is
brackish." Conditions analogous to all these particularised, present
themselves at Jaffna, and seem to indicate that the extent to which
fresh water is found there, is directly connected with percolation from
the sea. The quantity of rain which annually falls is less than in
England, being but thirty inches; whilst the average heat is highest in
Ceylon, and the evaporation great in proportion. Throughout the
peninsula, I am informed by Mr. Byrne, the Government surveyor of the
district, that as a general rule "_all the wells are below the sea
level_." It would be useless to sink them in the higher ground, where
they could only catch surface water. The November rains fill them at
once to the brim, but the water quickly subsides as the season becomes
dry, and "_sinks to the uniform level, at which it remains fixed for the
next nine or ten months_, unless when slightly affected by showers."
"_No well below the sea level becomes dry of itself_," even in seasons
of extreme and continued drought. But the contents do not vary with the
tides, the rise of which is so trifling that the distance from the
ocean, and the slowness of filtration, renders its fluctuations
imperceptible.

On the other hand, the well of Potoor, the phenomena of which indicate
its direct connection with the sea, by means of a fissure or a channel
beneath the arch of magnesian limestone, rises and falls a few inches in
the course of every twelve hours. Another well at Navokeiry, a short
distance from it, does the same, whilst the well at Tillipalli is
entirely unaffected as to its level by any rains, and exhibits no
alteration of its depths on either monsoon. ADMIRAL FITZROY, in his
_Narrative of the Surveying Voyages of the Adventure and Beagle_, the
expedition to which Mr. Darwin was attached, adverts to the phenomenon
in connection with the fresh water found in the Coral Islands, and the
rise and fall of the wells, and the flow and ebb of the tide. He
advances the theory propounded by Darwin of the retention of the
river-water, which he says, "does not mix with the salt water which
surrounds it except at the edges of the land. The flowing tide pushes on
every side, th