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22
the method of observation pursued, affords a clear explanation.
This will be seen in the following table:

London is situated just at the point of the river,
where the ascending tidal-wave and down-freshet meet
in conflict. In the winter the freshet has the superior
volume: in the summer the tidal-wave beats back the
feeble freshet up the river.
The explanation of the remarkable variations
observed, is this: on the 23rd March, the river still
swollen with the winter freshets kept back the tidalwave
to a point below Greenwich. Hence the quantity
of organic and mineral admixture both in high and low
water was moderate. On the 10th August, when by
evaporation and scanty rain-fall, the down-freshets had
been reduced to a low point, the bed of the river was
filled by the ascending salt-wave now powerful enough
to pass Greenwich. The rise of the salt-wave at this
period was further increased by the high tides. About
this time the water—although never so black as it
appeared last June—was at times offensive at low water.