248 ANNUAL OF SCIENTIFIC DISCOVERY. 



derived from springs, for these occur distinct from them, in the same 

 formations, and present peculiar characters of their own, often bub- 

 bling out from the surface of the sand, even below the line of high- 

 water on beaches. It might be supposed that they are the result, in 

 part, of a filtration of the salt water through the sand. To test this, 

 Mr. Cabot poured a quantity of salt water through sand, and found 

 that it lost two per cent, of its specific gravity ; a curious and unex- 

 pected result, but not sufficient fully to explain the case. On the 

 whole, he was inclined to accept the opinion of Mr. Mather, that these 

 supplies of fresh water are derived from rains, and are prevented from 

 oozing out laterally, by the pressure of the neighbouring salt water. 

 As this advances, it recedes, and its level rises ; as the tide goes out, 

 it follows, and its level is depressed. The practical result from these 

 investigations is, that it will undoubtedly be found that, in all deposits 

 of sand like those examined by Mr. Cabot, an abundant supply of 

 fresh water may be obtained at all times, a fact of great importance 

 to mariners. 



These observations induced some discussion, in the course of which 

 Mr. Ayres said, that he knew an instance of a sand-bank, 8 feet high, 

 formed within his recollection, in which fresh water might be obtained 

 at the depth of eighteen inches. At Sag Harbour there is a well 

 about 40 rods from the tide, in which the water rises and falls 4 feet a 

 little after the tide. A little farther from the shore is another, which 

 rises and falls 2 feet, while another varies 1 foot, and one still farther 

 from the shore is not sensibly affected. 



Dr. Pickering mentioned that, in the coral islands of the Pacific, 

 the natives obtain fresh water by a slight excavation. The President 

 said, that in Boston there are some wells situated so high, that it is 

 impossible to account for their water by the supply afforded by rains 

 alone. It would seem, therefore, that it must have been brought by 

 underground currents, perhaps from a great distance, following the 

 course of an impervious underlying stratum. 



At a later meeting, Mr. Cabot mentioned some experiments made 

 to ascertain the cause of the non-intermingling of salt water with 

 fresh, in dune sand. Having nearly filled a vessel with salt water, 

 he immersed in it a large sponge saturated with fresh water, con- 

 taining, imbedded in it, perpendicular tin tubes, with perforated 

 sides. The external pressure caused water to appear in these tubes, 

 and to rise to the level of the surrounding fluid. After standing 

 some hours, the water in these tubes was found to be fresh. On 

 reversing the experiment, placing the sponge full of fresh water first 

 in the vessel, and gradually filling the surrounding space with salt 

 water, the same result followed. Capillary attraction seemed to be 

 the force which kept the different fluids apart. With regard to the 

 difference between fresh water in dune sands and springs, Mr. Cabot 

 said that he did not consider it an essential one. In the former case, 

 the water formed, so to speak, a homogeneous spring ; in the latter, 

 underlying strata and lateral boundaries limited it, and gave it the 

 character of a current. Proc. Boston Nat. Hist. Society. 



