The important subjects of the distribution of temperature iu the atmosphere over the sea, and of the mutual relations of the temperature of the two elements, will be considered in the article TEMPERATURE, TERRESTRIAL, DISTRIBUTION OF.
Masses of ice surround the two poles. The cause of this has been thus generalised by Sir J. F. W. Herschel :—" Beyond the 56th parallels of latitude, the temperature of the water is lower at the surface, and rises as the thermometer descends, till the level of 39° [or the temperature of greatest density] is reached. The sea, then, even in deep water, becomes frozen at the surface in the winter months, or rather through all that portion of the year which intervenes between the beginning of September and the latter end of June ; July and August being, in high latitudes, the only open months There can be very little doubt that, in the winter time, the surface of the ocean at both poles is entirely frozen ; but at the North Pole it is probable, from many indications, that open water exists over a very large area of the central polar basin during a considerable portion of the wanner months." Some of these indications are then stated, but a fuller view of them will be found iu Sir John Richardson's ` Polar Regions,' pp. 222-231.
It was formerly conjectured that the mass of ice inclosiug the North Pole extended to the vicinity of 81° N. lat., because all navi gators who had attained that latitude agreed in stating that the ice there rose to a great height, and stood firm like a wall. This general opinion gave rise to the attempt of Captain Parry to reach the pole by travelling on this ice, which was supposed to be immovable. But Parry was soon aware that he was travelling ou ice which was in con tinuous motion, being carried by a current towards the south and south-west, and this circumstance occasioned the failure of the under taking. When he had advanced, according to his calculation, several miles to the northward in twenty-four hours, he found, on observing the altitude of the sun, that the motion of the ice had carried him as far southward, sod that for several days he had advanced very little nearer to his object. He was obliged to abandon the attempt, after having reached 82' 40r N. lat. Thus we have learned that the exterior parts of the great mass of ice supposed to inclose the poles consists of moving masses, which lie close together, and are only occa sionally divided from one another by narrow straits. The pieces of ice which detach themselves from this great accumulation and enter the open sea are called heary drift-ice. The larger pieces of ice of this description are a mile in length and breadth, and upwards of 30 feet in thickness; but others are of leas dimensions. The farther they advance southward, the more their dimensions are reduced by the action of the sun and of the water. But there are two other descriptions of ice. masses in the sea, which appear to have a different origin—the ice-fields and the icebergs. The term ice field is applied to sheets of ice so extensive that their limits cannot be discerned from the mast-head. They often occur of the diameter of 20 or 30 miles, and, when they are very closely united, they sometimes extend to the length of 50 or 100 miles. Their average thickness may be from 10 to 15 feet, and their surface is mostly level, except where humtnocA-s or low ice-hills occur, and then the thickness is often 40 and even 50 feet. These hummocks are produced by two fields coming into contact, when their broken edges are raised by the violent concussion, and thrown upon the fields themselves. These hummocks, therefore, are usually situated near the edges of the field. In some fields the hummocks form ridges or chains; in others they consist of isolated peaks. The smaller fields, or those whose extent can be seen from the mast-head, are called floes. The surface of these masses of ice, before July, is always covered with a bed of snow from a foot to &fathom in depth ; this snow dissolves in the end of summer, and forms extensive pools and lakes of fresh water. The great extent and the level surface of the fields show that they cannot be portions of the ice over which Parry travelled. It is there
fore supposed that they are generated in the sea which lies between Greenland and Spitzbergen, and which, though navigable during the summer, is covered with a continuous sheet of ice in the colder season. The fields appear to be the parts of this great sheet, formed by its breaking up at the approach of summer. When, on the further advance of the season, the field-ice and flees of the general surface break, the ice becomes heaped together in sheets confusedly piled on each other, into what is called peek-ice, or an ice-pack. The pellicle of ice formed over great areas of the polar seas, as the immediate effect of a general depression of temperature below th'b freezing-point of sea water, is called pancake-ice. It is, in fact, the beginniug of the pro duction of ice in the ocean ; and the possibility of its existence, as well as that of all continuous masses of ice, depends on the property of regelation, to which Faraday has recently called attention [Ice ; WATER], and which Tyndall has applied with so much success to the elucidation of the phenomena of glaciers, the parents of icebergs, to which we must now proceed. These are immense masses of ice rising to a great height above the level of the sea; some of them attain a height of 100 feet above the surface of the sea, and a few have been found which seem to be more than twice that height. Their base near the sea-level is not extensive, the larger masses generally being not more than 4000 feet in circumference, though Middleton states that ho Saw one which was from three to four miles in circuit. The most common form of the iceberg is for one side to rise perpendicularly to the very summit, the opposite side being very low, while the inter inesliate surface forms a gradual elope. Some have regular flat sur faces, but frequently they present a gnat variety In form and appear ance. Seine of them resemble palaces, or churches, or old castles, with spires, towers, windows, and arched gateways ; while others resemble pyramids and obelisks, and others are like ships, trees, animals, and human beings. When a number of them are near one another, which frequently happens, they present the appearance of a mountainous country. When seen from a abort distance, they look like huge hills of marble; and when the sun shines on them, they glitter like silver. Sometimes earth, gravel, and sand may be observed in them. Their prevailing colour in the fresh fracture is greenish gray, approaching to emerald-green. This colour resembles that of the glaciers of Switzerland [GLicrees, in NAT. llmsr. Div.], and the ice bergs are masses broken off from glaciers, or from barrier lines of ice-cliff They are rarely met with in the sea between Greenland and Spitsbergen, because in these parts only n few glaciers approach near the water's edge. But on both sides of Davis's Strait and Baffin's Bay, and lilacs on the eastern shores of Greenland as far north as 70' N. lat., glaciers cover the land, and in many places advance to the shores of the sea. In some places they terminate in a precipitous edge on the coast. It is only in the sea which these coasts that the icebergs are numerous. They seem to owo their origin to the circum stance of glaciers being in a continual state of progress. The glaciers of Greenland, which are situated on the margin of the sea, protrude their exterior parts over the ocean, and in summer, when the ice becomes brittle, the force of cohesion is overcome by the weight of the prodigious masses that overhang the sea, and they are detached from the gLacier with a dreadful crash. Thus an iceberg is formed. These icebergs, as it seems, are most common along the eastern shores of Greenland, and at the distance of 15 to 20 miles from the coast, where they occur by hundreds and thousands, forming a sort of barrier outside the drift-ice which is near the shore, and preventing its removal by en off-shore wind. Captain Graah states that this barrier of icebergs renders it impossible for vessels to approach these shores.