s of ice, which are sometimes torn up
and carry away the gravel adhering to the under surface.

Thinking that this was an error, I again wrote to the "Magazine of
Natural History" as below:--

I perceive that others beside myself have endeavoured to account
for anchor frosts. Mr. Carr says they never occur except in long
and severe frosts, and that the adhesion of the ice to the stones
at the bottom is owing to their acquiring a degree of cold far
below the freezing-point. He is in error when he says they never
occur except in long-continued frosts, as the walls of ice which
are sometimes raised on the crowns of weirs are invariably (so far
as my observations have extended) deposited there _before_ the
water in the reservoir above is frozen over, which proves that the
frost has not been of long continuance, although it may have been
severe. As to what he says about the stones acquiring a degree of
cold far below the freezing-point, and imparting that coldness to
the water, I would just ask how it is that a stone at the bottom
of a river acquires this excess of cold, and if it is not more
probable that the stones impart warmth to the surrounding water? I
can easily conceive how the stones may, by the action of the sun's
rays upon them, warm the surrounding water; but I do not see how
they can impart cold, or, in other words, how their temperature
can be reduced below that of the water by which they are
surrounded. Stones certainly impart warmth to the water they are
in, in bright weather, as the rays of the sun do not give much
warmth in passing through any transparent medium; but on coming in
contact with any opaque bodies, the heat is absorbed or reflected
as the case may be, and in this way transparent media such as air
and water acquire a warmth by contact which they would not
otherwise possess. Thus, if an anchor frost is followed by a
bright day, the rays of the sun impart so much warmth to the
stones at the bottom of the river as is sufficient to liberate the
ice from them, and on such days thousands of pieces of ice may be
seen rising from the bottom and floating down the streams.

Since my former observations were written I have had the
satisfaction of finding my views on the subject confirmed by a
very eminent chemist, [15] and if the discussions in your Magazine
were to be settled by authority, and not by argument (which I
trust will never be the case), he is one to whom many would be
inclined to appeal,