106 
NATURE 
[Dec 8, 1870 


contraction, but wrong in supposing that contraction to be caused 
by an increase of temperature on the approach of the thaw, and 
that I was right in attributing the final break-up into prisms to | 
the liquefaction spreading in every direction from the lines of air 
bubbles, but wrong in speaking of them as if they had originally 
been formed in those lines. We need not resort toa hypothetical 
contraction about the melting-point; we have a vera causa in 
accesses of cold, which will give you the desired contraction, and 
me my vertical lines of air bubbles. The true explanation I take 
to be as follows. ‘ 
“After the ice has formed, andsevere cold follows, it will con- 
tract, and probably equally inall directions. There is no impe- 
diment to its shrinking perpendicularly, as the whole sheet would 
then only be somewhat reduced in thickness ; but the ice is entan- 
gled with the shores, and the whole sheet cannot contract hori- 
zontally, but relieves itself by anumber of minute cracks. These | 
| had been closed for some days, and had received a pretty thick 
coating of snow in the meantime, and, knowing that the ice must 
are easily seen on the surface of glare ice, but one can hardly 
imagine that they do not more or less spread through the whole 
substance, and in your first letter you seem to say that you have 
noticed this after severe frosts. If then an air bubble comes near 
any such crack, it would seem natural that the direction of the 
crack should be diverted towards it. In fact the air bubbles, 
being weak points, would in a great measure determine the 
direction of these small fissures. Water would insinuate itself 
into them from below, carrying the air bubbles with it, which, 
upon the whole freezing, would not necessarily be exactly where 
they were before. A repetition of this process at frequent inter- 
vals during the winter would cause a rearrangement of the air 
bubbles, which one would naturally expect to find from this cause 
in more or less vertical lines. According to this explanation the 
prismatic structure would set almost from the first formation of the 
ice, The air bubbles would from the first be the indication of 
the direction of former fissures, and the lines which would deter- 
mine that of new ones. As long as severe frosts continued, all 
other indications of the structure would be obliterated, but, as 
the temperature approached the melting-point, these lines of 
bubbles, as they formerly determined the direction of the fissures, 
would now be the weak points at which the thaw would com- 
mence pervading the whole mass. 
“This explanation seems to supply everything that is wanted, 
and, upon looking back at your original letter, I am rather sur- 
prised, from the facts you mention, that-you should have missed 
it. You say, ‘I have seen it several times—in fact, a/ter every 
severe frost.’ This I have no doubt is correct ; but why, then, 
do you say, lower down, that ‘this finer structure may be found, 
if looked for, in every tolerably gradual thaw’? Again, you say 
that on one occasion you found the structure obliterated in- 
ternally, ‘except where some vertical lines of air bubbles 
marked the position of a tube or joint.’ This exactly corre- 
sponds with what I have above supposed would be the process. 
“Still, there are several points upon which more exact obser- 
vations should be made before one can speak with any certainty 
upon the subject. 
“ty, Does ice contract on approaching 32° F. ? 
“2, Do air bubbles form from the first in vertical lines ? 
‘*3, Is there any indication, as the winter advances, of a re- 
arrangement of the bubbles, as that they run into cach other, and 
get more and more ranged in vertical lines ? 
“4, Is there any indication in the earlier stages of the ice, that, 
after a night’s hard frost, the cracks seen on the surface spread 
through its substance? And if so, to what extent do they follow 
the lines of air bubbles? 
“5. Ifa block of ice is cut early in the winter, before the pris- 
matic structure from contraction with cold has commenced in 
any noticeable degree, and when it is no longer in a position in 
shrinking to crack in one direction more than another, to what 
extent does that structure afterwards develop itself? 
‘As there has been this one point in the behaviour of ice in 
which we have both taken an interest, I am induced to mention 
some other peculiarities which have come under my notice, with 
regard to some of which I have not even attempted to suggest an 
explanation. Several years ago I lived on the shores of a lake 
in the backwoods, and as in those carly days the ice in winter 
and a canoe in summer formed one’s only means of locomotion, 
ne naturally thought a good deal about ice in its various stages 
of formation and decomposition, I lived about two miles below 
the head of the Jake, where a river fell into it, upon which was a 
considcrable fell, and which connected it with another much 
dee) er lake, about a mile above. The effect of the stream was 
felt for a ccnsiderab!e distance into this lower lake, which was 


narrow at its head, and the ice was never safe there; indeed, 
excepting in the very severest frosts, there was as it were a bay of 
open water extending into the ice almost as far down as my 
house. But when the ice first formed in the fall it invariably 
took over the whole lake, even on that part whicl was after- 
wards open water during almost the whole winter. Indeed, upon 
one occasion, upon getting up in the morning I was astonished 
to see ice formed exactly on that part which was usually open 
water, whilst the rest of the lake, which was usually closed, 
had no ice upon it at all. Very soon after the sun rose that ice 
disappeared and the lake did not freeze over for a week after. 
The first ice, which always formed over the whole lake, would 
generally remain till there was a fall of snow on it, soon after 
which it would disappear where the effect of the stream was felt. 
The first winter I was there I nearly suffered from ignorance of 
this habit of the ice. I had occasion to cross the lake, which 
be pretty thin, I took the precaution to wear snow shoes. After 
a while I felt a peculiar sinking of the snow shoes, and observed 
that the track filled with water, and upon feeling with a stick I 
carried, I found no resistance of ice at all. I was, in fact, walk- 
ing upon little more than a cake of snow. You may imagine 
that I at once madea little circuit and did not stop till my stick 
| encountered good ice, and an hour or two afterwards that part of 
the Jake was open water. Now I think I can explain this 
peculiarity of the early formation of ice where no ice remained 
during much colder weather in the winter. I take it that the 
whole surface, at least, of the water in my lake, which was 
| rather a shallow one, had been reduced to 32° F., or nearly so, 
and that on a very cold night the water from above, being 
thoroughly exposed to the cold in coming over the fall, had been 
reduced even lower, so that when it reached the comparative quiet 
of the lake, where it naturally floated on the surface, it became 
ice for a short time that morning I spoke of, though it could not 
long maintain that condition. So also on ordinary occasions, 
when all the lower lake was ready to freeze, the water, thoroughly 
cooled at the fall, would freeze also, although the lake above had 
not yet been frozen over. At the footof the upper lake, im- 
mediately above the fall, was a very shallow bar, so that the 
only part of the upper water which would come over would be 
the coldest layer on the top. But after the uy per lake had frozen 
over also, and had received its coating of snow, very little more 
cold would penetrate to reduce the temperature of the surface, 
and the lake being very deep, and receiving fresh accessions of 
heat from below, the water would soon get considerably above 
the freezing-point, and with the aid of the friction of the stream 
would thaw away the snow-covered ice below with which it first 
came in contact. One is inclined to ask why the complete ex- 
posure of the water to the cold in coming over the fall, which IT 
have supposed to reduce the temperature so much in the begin- 
ning of winter, had not the same effect in the severer cold after- 
wards ; but the water which came from under the snow-covered 
ice would probably be much warmer than that which formerly 
came from the surface of the open water, and moreover the 
spray soon formed ice, which gradually crept over the shallower 
parts of the fall at its edges, and the exposure and its cooling 
effect may not have been as complete as at first. I cannot say 
that I am altogether satisfied with my explanation of the curious 
anomaly that a part of the lake would freeze over at a tempera- 
ture of 20°, which would remain open when it was far below 
zero; but such are undoubtedly the facts. 
“Tf there was this anomaly in the first formation of the ice, its 
sudden disappearance in the spring, which I mentioned in my 
former letter as giving rise to the popular prejudice that it 
sinks, was almost equally astonishing. Upon one occasion the 
ice was evidently in the last stage ot decomposition, and I had 
got my canoe ready for a journey in the morning, when I fully 
expected the lake would be open ; but before starting, I wanted 
to go to the two or three houses at the fall, which we dignified 
with the name ofa village. Although the lake at my landing 
was an unbroken sheet of honeycombed ice, which had even 
borne me early in the morning, the open water extended to a 
point about half a mile above me, and I determined to carry my 
canoe so far through the woods. I cannot have been a quarter 
of an hour in doing sc, but when I launched my canoe beyond 
the point, there was not a vestige of ice down as far as my 
landing, though I still saw it across the whole lake a little farther 
down. Being anxious to see the process of the actual disappear- 
ance, I turned my canoe down the lake, and paddled as fast as 
eee 
“es 
