24

Scientific Memoirs by

      From the above figures it appears that the water took 30 days to traverse
a stratum of soil 66.c. thick, and closed, save at its extremities, by an impermeable
material. Subsequent to this slow percolation continued to occur as indicated
by the gradual accumulation of a considerable quantity of water within the
distal portion of the tube beyond the soil.

      The above experiments appear to demonstrate sufficiently clearly that the
subsoil in Calcutta is just as little "impermeable" as it is "blue clay," and
it now only remains to attempt to find an explanation for the erroneous ideas
prevalent in regard to its nature. The peculiar colour of the material, its con-
sistence when freshly excavated and moist, and its stony hardness on drying
are perhaps enough to account for the popular description of it as consisting of
"blue clay." It is, however, at first sight, difficult to understand on what the
idea of its impermeable nature was founded, seeing that it must have been
patent to the most casual observation that in its normal site it included a very
considerable amount of water content and that it parted with this with great
readiness on exposure to sunshine and dry air. The belief must apparently
have been due to the fact that, whilst really containing a very large amount of
air or water space, the soil is yet so fine as to prevent rapid transit of water
under ordinary circumstances. Excavations made in a soil of this character
will, of course, not be liable to contain conspicuous accumulations of percolated
water so long as evaporation is taking place with any considerable activity from
the exposed surfaces. The last of the experiments on the permeability of blocks
of the soil affords a direct proof of this, for in it it was only with increased
atmospheric humidity and diminished recipient power in the soil that the sur-
faces became conspicuously moist. In the laboratory in which the experiments
were conducted there was a regular daily loss of 0.125 of an inch in depth from