Page:Popular Science Monthly Volume 37.djvu/318

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THE POPULAR SCIENCE MONTHLY.

the parent glacier, the form of its bed, and the depth of the water at its mouth. The larger fragments originate for the most part along that remarkable break which is presented in the normal formation of the coast-line between Egedesminde and the Svartenhuk Peninsula. Rink enumerates not more than thirty Greenland glaciers which discharge really large icebergs, and of this number only six or eight yield blocks of the first magnitude.

The average velocity of the congealed masses is about fifty feet in the twenty-four hours, but in some places a much greater speed has been recorded, though still varying considerably with the seasons. A branch of the Augpadlartok glacier, north of Upernavik, moves at the rate of one hundred feet a day, the highest yet measured. But how enormous must be the pressure of the inland ice-fields to discharge into the sea the vast quantities of icebergs which are yearly sent adrift along the Greenland seaboard! Estimated in a single block the annual discharge from each of the five best-known glaciers would represent a mass of about seventeen billion cubic feet in capacity, and fifty-six hundred feet in height, depth, and thickness. Reduced to a liquid state this mass would be equivalent to a stream discharging seaward five hundred cubic feet per second, or 15,500,000 a year.

The formation of this drift ice, or floating icebergs, is one of those phenomena which were discussed long before the seaboard had been studied, or before the breaking away of the frozen masses had actually been witnessed. Wherever the glaciers discharge through a broad valley preserving a uniform width and depth for a considerable space, and advancing seaward through a fiord of like dimensions, and with gently sloping bed, the ice may progress without any of those accidents caused by the inequalities of more rugged channels. Under such conditions the compact mass glides smoothly forward over its rocky bed without developing any rents or fissures. But as it moves down like a ship on its keel, it tends to rise, being at least one twentieth lighter than the displaced water. It is also left without support by the sudden fall of its bed beyond the normal coast-line. Nevertheless, it still continues its onward movement through the waters to a point where its weight prevails over its force of cohesion with the frozen stream thrusting it forward. At this point it snaps off suddenly with a tremendous crash, and the iceberg, enveloped in¬Ľa thousand fragments projected into space, plunges into the abyss and whirls round and round to find its center of gravity amid the troubled waters. On recovering from the bewilderment caused by all this tumult and chaos, the spectator finds that the glacier has apparently receded a long way toward the head of the bay, in the middle of which a crystal peak is seen slowly drifting away with the current. In this he recognizes the huge fragment detached