The phenomenon of the production of ice at the bottoms of rivers
has been repeatedly noticed, but I am not aware that any
satisfactory solution of the cause has hitherto been given. In
Nicholson's "Dictionary of Chemistry," several different
hypotheses are enumerated, which I shall not stop now to examine,
since it may be safely asserted that they neither accord with the
established principles of chemistry, nor with the facts for which
they endeavour to account. The most recent theory with which I am
acquainted is that of Mr. A. Knight, who in a paper lately
published in the "Philosophical Transactions," seems to consider
the particles of ice as originally formed at the surface, and
afterwards absorbed by the eddies of streams to the bottom. He
states, in support of this idea, that he did not observe any
similar phenomenon in still water. I shall advert to this
hypothesis in the sequel, and at present it may suffice to remark
of it and all others which I have hitherto seen, that supposing
any of them to be correct, the same effects ought regularly to be
produced whenever the atmosphere is at a similar temperature, or
in other words, that whenever the frost is so intense as
materially to affect the water of a river, we may then expect to
find ice at the bottom. Now this is certainly not the case, since
the appearance we are treating of never occurs but under peculiar
_atmospherical_ circumstances, and rivers are frequently frozen
over, and remain so for a length of time without a particle of ice
being visible at the bottom of their streams. I do not now profess
to have developed this mystery, but merely intend to state the
circumstances under which the phenomenon takes place, as well as a
few particulars connected with it, which are perhaps not generally
known, and which may hereafter be serviceable as data for
investigating the cause.
It is well known to meteorologists that a severe frost in winter
does not always commence in a uniform manner. Sometimes it begins
with a gentle wind from the E. or N.E., and is at first
comparatively mild in its operations, but afterwards gradually
increases in intensity. Frosts of this kind are generally more
lasting than others, and during such, I have not observed that any
ice is generated at the bottoms of streams; though the deep and
still parts of rivers are often frozen over to a considerable
extent. At other times, during the continuance of the violent
south-westerly gales which are so prevalent in this country in the
winter months, the wind frequently shifts on a sudden from S.W. to
N.W., commonly about an hour before sunset, and blows with great
impetuosity in the latter direction, attended with a severe frost,
and sometimes with a heavy fall of snow. The effects of this
frost, in places exposed to the wind, are extremely rapid, so as
to render the ground impenetrably hard in about a couple of hours
from its commencement. Situations that are not so much exposed
seem comparatively little affected--at least, I have repeatedly
observed that a small sheltered pond in a field was nearly free
from ice, while the current of a large and rapid river at no great
distance was nearly choked up by it. I believe that the phenomenon
under consideration seldom occurs except during such frosts as
these, and the following are the principal circumstances connected
with it which I am able to state from my own observation.
It may here be premised that ice of this description is seldom
seen adhering to anything beside rock, stone, or gravel, and that
it is more abundantly produced in proportion to the greater
magnitude and number of the stones composing the bed of the river,
combined (as will be further noticed) with the velocity of the
current. I have been informed by a friend that he has occasionally
seen it attached to solid wooden piles at a considerable depth
below the surface of the water, but I never saw or heard of any on
earth, mud, or clay. It is not easy to ascertain the precise time
at which the process begins to take place. It appears, however,
almost invariably to commence during the first night of the frost,
and probably within a few hours after sunset. On the ensuing
morning the first thing which strikes an observer is an immense
quantity of detached plates of ice floating down the stream. Mr.
Knight naturally enough supposed these to have been formed at the
surface by the influence of the freezing atmosphere, and
afterwards absorbed by the current; but I think that a minute
inspection would have led him to form a different conclusion--
viz., that they are first formed in the bed of the river, and
afterwards rise to the surface. It is true that none are to be
seen in situations where there is no sensible current, and that
they abound most in rough and rapid places; but on closely
examining any stream of moderate velocity, yet smooth, equable,
and free from all appearance of eddy or rippling, a great number
of these plates of ice will be found adhering to the rock, stone,
or gravel at the bottom. If they are watched with attention, they
will be observed to rapidly increase in bulk, till at last, on
account of their inferior specific gravity, aided, perhaps, by the
action of the current, they detach themselves from the substances
to which they first adhered, and rise to the surface of the water.
The form of these pieces of ice is very irregular, depending in a
great measure on the size and shape of the stones or other
substances to which they were originally attached. Most of them
seem to be of an oblong or circular figure; they are generally
convex on the upper surface, and have a number of laminae and
spiculae shooting from them in various directions, especially from
their circumference. Sometimes when those floating pieces or
plates meet with any obstruction in the channel of the river, they
accumulate in such quantities as to cover the surface of the
water, and become frozen together in one large sheet, but this
kind of ice may be always readily distinguished from that produced
in the usual way by the action of the cold air on the surface,
which is smooth, transparent, and of an uniform texture; on the
contrary, one of these conglomerated fields or sheets is opaque,
uneven, full of asperities, and the form of each separate plate
composing it may be distinctly traced. In this situation, they
generally assume the shape of irregular polygons, with angles
somewhat rounded; a form apparently caused by the lateral pressure
of the contiguous pieces.
On the river Wharfe, near Otley, in the West Riding of Yorkshire,
is a weir or milldam where this phenomenon is sometimes manifested
in a striking manner. This structure is of hewn stone, forming a
plane inclined at an angle of from 35 degrees to 50 degrees,
fronting the north and extending from west to east, to the length
of 250 or 300 yards. When one of the above-mentioned frosts
occurs, the stone which composes the weir soon becomes incrusted
with ice, which increases so rapidly in thickness as in a short
time to impede the course of the stream, which falls over it in a
tolerably uniform sheet, and with considerable velocity; at the
same time, the wind blowing strongly from the north-west,
contributes to repel the water and freeze such as adheres to the
crest of the weir when its surface comes nearly in contact with
the air. The consequence is that in a short time the current is
entirely obstructed, and the superincumbent water forced to a
higher level. But as the above-mentioned causes continue to act,
the ice is also elevated by a perpetual aggregation of particles,
till by a series of similar operations an icy mound or barrier is
formed, so high as to force the water over the opposite bank, and
thus produce an apparent inundation. But in a short time the
accumulated weight of a great many thousand cubic feet of water
presses so strongly against the barrier as to burst a passage
through some weak part, through which the water escapes and
subsides to its former level, leaving the singular appearance of a
wall or rampart of ice three or four feet high, and about two feet
in thickness, along the greatest part of the upper edge of the
weir. The ice composing this barrier where it adheres to the
stone, is of solid consistency, but the upper part consists of a
multitude of thin laminae or layers resting upon each other in a
confused manner, and at different degrees of inclination, their
interstices being occupied by innumerable icy spiculae, diverging
and crossing each other in all directions. The whole mass much
resembles the white and porous ice which may be seen at the edge
of a pond or small rill where the water has subsided during a
frost.
It may be further observed that a frost of this kind is very
limited in its duration, seldom lasting more than thirty-six or
forty hours. On the morning of the second day after its
commencement, a visible relaxation takes place in the temperature
of the atmosphere. Usually before noon, the wind on a sudden
shifts to the south-west, and a rapid thaw comes on, frequently
attended with rain. What appears somewhat remarkable is, that
during several hours after the commencement of the thaw, the
production of ice at the bottom of rivers seems to go on without
abatement, and upon examining a rapid stream, the stones over
which it flows will be found at this period completely incrusted
with the above description of icy plates. It seems evident from
this that the bed of the river, which has been reduced below the
freezing temperature, is not for some time affected by the change
of the atmosphere. This may be in some measure illustrated by the
well-known fact, that rain which falls upon a rock or stone wall,
is frequently converted into ice, though the air and the ground
are evidently in a state of thaw. Before the following morning,
the ice of which we have been speaking generally disappears, being
carried away by the current or dissolved by the thaw.
The last time that I remarked this phenomenon, was in a stream of
the river Aire, near Bradford, in Yorkshire, on the 1st of
January, 1814. This instance did not precisely accord with what I
have stated to be the usual circumstances of the case, as the
frost then had existed several days without any previous
appearance of this kind; but there were several indications of
approaching change of temperature, and the day following there was
a partial thaw attended with rain, the wind having veered from
north-west to south-west. This thaw, however, did not continue
long, and was succeeded by a frost which surpassed all within my
recollection in severity and duration. Yet during the whole of the
period, though the thermometer often stood below 18 degrees
Fahrenheit, and the estuary of the Tees several miles below
Stockton, where the spring-tides rise from twelve to eighteen
feet, was for two months frozen over, so as to allow the passage
of a loaded waggon, I could never perceive a particle of ice
adhering to the rock or gravel, in the bed of the small and rapid
river Leven in Cleveland, where I then resided. This circumstance
seems decisively to prove that the phenomenon does not merely
depend on an intensity of cold.
I confess I am unable to frame any hypotheses respecting the
above-mentioned facts which would not be liable to numerous and
formidable objections. The immediate cause of the formation of the
ice seems to be a rapid diminution of the temperature in the stone
or gravel in the bed of the river, connected with the sudden
changes in the state of the atmosphere, but it does not seem very
easy to explain the precise nature of this connection.
We may easily conceive that by a sudden change from a state of
thaw to an intense frost attended by a strong wind, the whole body
of water in a river may become quickly cooled, and consequently
diminish the temperature of the stone or gravel over which it
flows; but to suppose that water which is not itself at freezing-
point is capable of reducing the substances in contact with it by
means of a continual application of successive particles so far
_beneath_ that temperature as in process of time to convert the
contiguous water to ice, seems not to accord very well with the
usually received theory of the equilibrium of caloric. However,
the fact that the quantity of ice thus produced is always greater
in proportion to the superior velocity of the stream, little or
none being found where there is no sensible current, seems in some
degree to countenance the above idea.
I cannot learn that any experiments have ever been instituted on
this subject, though it seems that they might easily be made by a
person conveniently situated and possessed of the necessary
instruments. A careful examination by properly contrived
thermometers of the relative temperatures of the air, the water,
and the bed of the river and of the changes undergone by them
during the above process, would probably go a great way towards
solving the problem. I know no one better qualified for this
undertaking than Mr. Knight, if he should at any future time have
leisure and opportunity to direct towards it the same acuteness of
observation and accuracy of investigation which have enabled him
to make such important discoveries in the economy of the vegetable
kingdom, and if the explanation of this phenomenon should ever
lead to results of any importance to the cause of science, I shall
feel sufficiently satisfied if it be deemed that I have been of
any service in pointing out the way.