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 count