« AnteriorContinuar »
forty or fifty yards in diameter. Now, such a number of these pieces collected together in close contact, so that they cannot, from the top of the ship's mast, be seen over, are termed a pack.
When the collection of pieces can be seen across, if it assume a circular or polygonal form, the name of patch is applied; and it is called a stream when its shape is more of an oblong, how narrow soever it may be, provided the continuity of the pieces is preserved.
Pieces of very large dimensions, but smaller than fields, are called floes thus, a field may be compared to a pack, and a floe to a patch, as regards their size and external form.
Small pieces which break off, and are separated from the larger masses by the effect of attrition, are called brash-ice, and may be collected into streams or patches.
Ice is said to be loose or open, when the pieces are so far separated as to allow a ship to sail freely amongst them; this has likewise been called drift-ice.
A hummock is a protuberance raised upon any plane of ice above the common level. It is frequently produced by pressure, where one piece is squeezed upon another, often set upon its edge, and in that position cemented by the frost. Hummocks are likewise formed, by pieces of ice mutually crushing each other, the wreck being coacervated upon one or both of them. To hummocks, the ice is indebted for its variety of fanciful shapes, and its picturesque appearance. They occur in great numbers in heavy packs, on the edges and occasionally in
the middle of fields and floes. They often attain the height of thirty feet or upwards.
A calf, is a portion of ice which has been depressed by the same means as a hummock is elevated. It is kept down by some larger mass; from beneath which, it shews itself on one side. I have seen a calf so deep and broad, that the ship sailed over it without touching, when it might be observed on both sides of the vessel at the same time; this, however, is attended with considerable danger, and necessity alone warrants the experiment, as calves have not unfrequently (by a ship's touching them, or disturbing the sea near them) been called from their sub-marine situation to the surface, and with such an accelerated velocity, as to stave the planks and timbers of the ship, and in some instances, to reduce the vessel to a wreck.
Any part of the upper superficies of a piece of ice, which comes to be immersed beneath the surface of the water, obtains the name of a tongue.
A bight signifies a bay or sinuosity, on the border of any large mass or body of ice. It is supposed to be called bight from the low word bite, to take in, or entrap; because, in this situation, ships are sometimes so caught by a change of wind, that the ice cannot be cleared on either tack; and in some cases, a total loss has been the consequence.
Comparison of Ice frozen from Sea
Water and Rain-Water. When the sea freezes, the greatest part of the salt it contains is deposited, and the frozen spongy
2 M 2
mass probably contains no salt, but what is natural to the seawater filling its pores. Hence, the generality of ice affords freshwater, when dissolved. As, however, the ice frozen from seawater does not appear so solid and transparent as that procured from snow or rain-water, sailors distinguish it into two kinds, accordingly as it seems to have been formed from one or the other.
Ice frozen from Sea-Water.
What is considered as saltwater ice, is porous, white, and in a great measure opaque, (except when in very thin pieces), yet transmits the rays of light with a greenish shade. It is softer, and swims lighter than fresh-water ice, and when dissolved, produces water sometimes perfectly fresh, and sometimes saltish; this depends in a great measure on the situation from whence it is taken: such parts as are raised above the surface of the sea in the form of hummocks, appear to gain solidity by exposure to the sun and air, and are commonly fresh, whilst those pieces taken out of the sea are somewhat salt. Although it is very probable, that this retention of salt may arise from the sea-water contained in its pores, yet I have never been able to obtain, from the water of the ocean, by experiment, an ice either compact, transparent, or fresh. That the sea-water has a tendency to produce fresh ice, however, is proved from the concentration observed in a quantity exposed in an open vessel to a low temperature, by the separation of the salt from the crystals of ice,
in the progress of the freezing. Thus it is, that in the coldest weather, when a ship exposed to a tempestuous sea is washed with repeated sprays, and thereby covered with ice, that in different places obstructing the efflux of the water overboard, a portion always remains unfrozen, and which, on being tasted, is found to contain salt highly concentrated. This arises from the freezing point of water falling in a certain ratio according to the degree of saltness; thus, though pure water, of specific gravity 1.0000, freeze with a temperature of 32o, water of specific gravity 1.0263, containing about 5 oz. (avoird.) of salt in every gallon of 231 cubic inches, that is, with the degree of saltness common to the Greenland seas, freezes at 28°. Sea-water concentrated by freezing, until it obtains the specific gravity of 1.1045, requires a temperature of 133° for its congelation, having its freezing point reduced 1810 below that of pure water; and water saturated with sea-salt remains liquid, at a temperature of —4°.
Thus, we are presented with a natural process for extracting salt from the sea, at least for greatly facilitating that process in a concentration of the saline particles, by the agency of frost.
When salt-water ice floats in the sea at a freezing temperature, the proportion above, to that below the surface, is as 1 to 4 nearly; and in fresh water, at the freezing point, as 10 to 69, or 1 to nearly. Hence, its specific gravity appears to be about 0.873. Of this description is all young ice as it called, which forms a considerable
considerable proportion of packed and drift ice in general; where it occurs in flat pieces commonly covered with snow, of various dimensions, but seldom exceeding fifty yards in diameter.
Fresh-water ice, is distinguished by its black appearance when floating in the sea, and its beautiful green hue and transparency when removed into the air. Large pieces may occasionally be obtained, possessing a degree of purity and transparency, equal to that of the finest glass, or most beautiful crystal; but generally, its transparency is interrupted by numerous small globular or pear-shaped air-bubbles: these frequently form continuous lines intersecting the ice in a direction apparently perpendicular to its plane of formation.
Fresh-water ice is fragile, but hard; the edges of a fractured part are frequently so keen, as to inflict a wound like glass. The homogeneous and most transparent pieces, are capable of concentrating the rays of the sun, so as to produce a considerable intensity of heat. With a lump of ice, of by no means regular convexity, I have frequently burnt wood, fired gunpowder, melted lead, and lit the sailors' pipes, to their great astonishment; all of whom, who could procure the needful articles, eagerly flocked around me, for the satisfaction of smoking a pipe ignited by such extraordinary means. Their astonishment was increased, on observing, that the ice remained firm and pellucid, whilst the solar rays emerging
therefrom were so hot, that the hand could not be kept longer in the focus, than for the space of a few seconds. In the formation of these lenses, I roughed them with a small axe, which cut the ice tolerably smooth; I then scraped them with a knife, and polished them merely by the warmth of the hand, supporting them during the operation in a woollen glove. I once procured a piece of the purest ice, so large, that a lens of sixteen inches diameter was obtained out of it; out of it; unfortunately, however, the sun became obscured before it was completed, and never made its appearance again for a fortnight, during which time, the air being mild, the lens was spoiled.
The most dense kind of ice, which is perfectly transparent, is about one-tenth specifically lighter than sea-water at a freezing temperature Plunged into pure water, of temperature 32°, the proportion floating above to that below the surface, is as 1 to 15, and placed in boiling fresh water, it barely floats. Its specific gravity is about 0.937.
Fields, bergs, and other large masses, chiefly consist of this kind of ice. Brash-ice likewise affords pieces of it, the surfaces of which are always found crowded with conchoidal excavations when taken out of the sea.
quantities that are dissolved and dissipated by the power of the waves, and the warmth of the climate into which it drifts. It has frequently been urged, that the vicinity of land is indispensable for its formation. Whether this may be the case or not, the following facts may possibly determine.
I have noticed the process of freezing from the first appearance of crystals, until the ice had obtained a thickness of more than a foot, and did not find that the land afforded any assistance or even shelter, which could not have been dispensed with during the operation. It is true, that the land was the cause of the vacancy or space free from ice, where this new ice was generated; the ice of older formation had been driven off by easterly winds, assisted perhaps by a current; yet this new ice lay at the distance of twenty leagues from Spitzbergen. But I have also seen ice grow to a consistence capable of stopping the progress of a ship with a brisk wind, even when exposed to the waves of the North Sea and Western Ocean, on the south aspect of the main body of the Greenland ice, in about the seventy-second degree of north latitude. In this situation, the process of freezing is accomplished under peculiar disadvantages. I shall attempt to describe its progress from the
sludge, and resembles snow when cast into water that is too cold to dissolve it. This smooths the ruffled sea, and produces an effect like oil in stilling the breaking surface. These crystals soon unite, and would form a continuous sheet, but, by the motion of the waves, they are broken into very small pieces, scarcely three inches in diameter. As they strengthen, many of them coalesce, and form a larger mass. The undulations of the sea still continuing, these enlarged pieces strike each other on every side, whereby they become rounded, and their edges turned up, whence they obtain the name of pancakes: several of these again unite, and thereby continue to increase, forming larger pancakes, until they become perhaps a foot in thickness, and many yards in circumference.
When the sea is perfectly smooth, the freezing process goes on more regularly, and perhaps more rapidly. The commencement is similar to that just described; it is afterwards continued by constant additions to its under surface. During twenty-four hours keen frost, it will have become two or three inches thick, and in less than forty-eight hours time, capable of sustaining the weight of a man. This is termed bay-ice, whilst that of older formation is
Freezing of the Ocean in a rough distinguished into light and heavy
The first appearance of ice whilst in the state of detached crystals, is called by the sailors
ice; the former being from a foot to about a yard in thickness, and the latter from about a yard upwards.
It is generally allowed, that all thas
that is necessary in low temperatures for the formation of ice, is still water here then, it is obtained. In every opening of the ice at a distance from the sea, the water is always as smooth as that of a harbour; and as I have observed the growth of ice up to a foot in thickness in such a situation, during one month's frost, the effect of many years we might deem to be sufficient for the formation of the most ponderous fields.
There is no doubt, but a large quantity of ice is annually generated in the bays, and amidst the islands of Spitzbergen: which bays, towards the end of summer, are commonly emptied of their contents, from the thawing of the snow on the mountains causing a current outwards. But this will not account for the immense fields which are so abundant in Greenland. These evidently come from the northward, and have their origin between Spitzbergen and the Pole.
On the Generation of Fields. As strong winds are known to possess great influence in drifting off the ice, where it meets with the least resistance, may they not form openings in the ice far to the north, as well as in latitudes within our observation? Notwithstanding the degree in which this cause may prevail is uncertain, yet of this we are assured, that the ice on the west coast of Spitzbergen, has always a tendency to drift, and actually does advance in a surprising manner to the south or southwest; whence some vacancy must assuredly be left in the place which it formerly occupied.
These openings, therefore, may be readily frozen over, whatever be their extent, and the ice may in time acquire all the characters of a massy field.
It must, however, be confessed, that from the density and transparency of the ice of fields, and the purity of the water obtained therefrom, it is difficult to conceive that it could possess such characters if frozen entirely from the water of the ocean;-particularly as young ice is generally found to be porous and opaque, and does not afford a pure solution. The succeeding theory, therefore, is perhaps more consonant to appearances; and although it may not be established, has at least probability to recommend it.
It appears from what has been advanced, that openings must oc casionally occur in the ice between Spitzbergen and the Pole, and that these openings will in all probability, be again frozen over. Allowing, therefore, a thin field or a field of bay-ice to be therein formed, a superstructure may probably be added by the following process. The frost, which constantly prevails during nine months of the year, relaxes towards the end of June or the beginning of July, whereby the covering of snow, annually deposited to the depth of two or three feet on the ice, dissolves. Now, as this field is supposed to arise amidst the older and heavier ice, it may readily occupy the whole interval, and be cemented to the old ice on every side; whence the melted snow has no means of escape. Or, whatever be the means of its retention on the surface of the young field, whether