_Anchor Frosts._
A correspondent of the "Magazine of Natural History," in
endeavouring to explain the causes why water freezes at the bottom
in rapid streams, says this unusual phenomenon may be rationally
accounted for by anyone who has attended to it; that the streams
in which anchor frosts occur generally are those which contain
water of different temperatures--viz., surface-drainage and land
springs and main springs, the first being always colder than the
latter, in winter these never being less than 40 degrees, even in
severe frosts.
These colder globules being first frozen, float on the surface of
the water individually, being prevented from coalescing by the
intermediate main-spring-water, and where the water passes in a
shallow stream over the pebbles the crystals are intercepted by
the interstices of the stones, and then become heaped together in
thick beds.
The fact of the crystals of ice (which are specifically lighter
than the water) sinking below the surface, is a circumstance
requiring explanation. They do not sink from their specific
gravity, but in the commotion of the current they are occasionally
submerged, and while so are stopped by any obstruction, when they
commence and compose the aggregation.
Thinking this was an erroneous view of the matter, I replied as
follows:--
J. M., in his remarks on anchor frosts, appears to me to have
fallen into several errors in endeavouring to account for them
(they are called bottom frosts in Yorkshire); for, admitting that
main springs are of the temperature stated (40 degrees) when they
issue from the earth, I am by no means prepared to believe that
they keep that temperature long, or that the water issuing from
them does not mingle intimately and immediately with the water of
the river into which it flows; especially in the situations where
anchor frosts are most common, which are rough and rapid streams.
From J. M.'s statement it would appear that globules of water of
different temperatures mix together without the one imparting its
excess of caloric to the other, which is contrary to the
experience of everyone; it is true, that in still places there
will be different temperatures in the same body of water, but it
is not owing to the main springs of which J. M. speaks, but to the
peculiar way in which water is affected by cold. It is well known
that water increases in density down to 40 degrees, below which
temperature it begins to expand, and this expansion continues
until it reaches the freezing-point, so that in severe frosts
there will be strata of different temperatures from 32 degrees to
40 degrees. Again, he says that "the crystals of ice are
intercepted by the interstices of the stones, and then become
heaped together in thick beds;" but if my observations are
correct, these depositions begin first round the large stones,
which are not likely to stop small spiculae any more than are the
water-gates of mills, where, he says, the accumulations also take
place.
Anchor frosts are most common in the rapid streams occurring below
deeps in rivers, and I have seen a weir on the river Wharfe which
had a wall of ice four feet high formed upon it in a single night
by a sharp north wind. In my opinion a sufficient explanation of
this freezing at the bottom of rivers is to be found in the fact
that water when kept still may be cooled down below the freezing-
point without being congealed; but if the vessel in which it is
kept be shaken, a portion of it will be converted into a porous,
spongy ice, and the temperature immediately rises to 32 degrees.
In the deeps of rivers the same cooling below the freezing-point
takes place without congelation, but as soon as this water reaches
the stream below, the agitation immediately converts a portion of
it into ice, which collects round the large stones at the bottom
in the same way that crystallization commences in a solution of
salt or sugar around a piece of thread or other substance which
may be suspended in it. If a severe frost is followed by a bright
day, thousands of these detached pieces of spongy ice may be seen
rising from the stones which have served as nuclei for them; which
proves that the detention of them is not merely mechanical, but
that precipitation (if I may be allowed to call it so) takes place
in the first instance, the stone serving as a nucleus, and that
this adhesion is destroyed by the action of the sun's rays.
I have never seen any attempt to explain the phenomenon of bottom-
frosts before this of J. M.'s, and I am not philosopher enough to
speak positively on the subject; but the above is the way in which
I have always endeavoured to account for it. Perhaps some of your
scientific readers may be able to give much better reasons for it
than have been offered either by J. M. or myself. (January 17th,
1832.)
Another writer (J. Carr, of Alnwick,) says that anchor frosts are
merely long and severe ones where long masses of ice are frozen to
the stones at the bottom of rapid streams, and this is simply
owing to these stones acquiring a degree of cold far below the
freezing-point, and the water in contact with them freezing and
spreading into large sheets 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, and to whom few would refuse to submit. (May
2nd, 1832.)
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