palms, although these islands rest on a bed of sand. (Cosmas Ind. ed.  Thevenot, vol. i. p. 3, 20).  It is remarkable that in the little island of Ramisseram, one of the chain which connects Adam’s Bridge with the Indian continent, fresh water is found freely on sinking for it in the sand.  But this is not the case in the adjacent island of Manaar, which participates in the geologic character of the interior of Ceylon.  The fresh water in the Laccadive wells always fluctuates with the rise and fall of the tides.  In some rare instances, as on the little island of Bitra, which is the smallest inhabited spot in the group, the water, though abundant, is brackish, but this is susceptible of an explanation quite consistent with the experiments of Mr. Witt, which require that the process of percolation shall be continued “during long periods and through great masses of porous strata;” 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