Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/193

Rh AND CIRQUES IN NORWAY AND GREENLAND. 155

once only in three days. The " calving " was accompanied by a ter- rific crash, and by white clouds of spray (or comminuted ice ?) hurled into the air. Simultaneously an enormous jagged mass of ice, forming part of the terminal wall of the glacier, was seen to turn over, bringing its edge, as it rotated, high above the level of the glacier. As it rose, huge towering pinnacles were shattered, and came crashing down in fragments, like a shower of gravel. Tho " calving," which had commenced at a place near the middle of the glacier, was continued in another. A second large mass of the solid glacier broke loose, and at first moved almost horizontally, at tho rate of perhaps a metre in a second. This, like the glacier itself, was cleft into ice pinnacles, and the one group in moving past the other produced a curious effect. How many bergs were produced in this " calving" I cannot say, as they were formed simultaneously in several places ; further, white clouds of spray partly concealed the glacier, and the old bergs in front of it were set in motion ; there was an indescribable confusion and a continuous crashing noise, which lasted for about half an hour, then things became once more quiet. The height of one of the bergs formed by the " calving " was by measurement 89 metres.

In order to estimate the quantity of ice discharged by a fjord, we must know the thickness, breadth, and rate of motion of the glacier. The first cannot be directly measured, except so far as it rises above the sea; but if we know the proportional parts of a berg above and below water, we can form an idea of the thickness of the glacier. Taking the specific gravity of water as unity and that of ice 0918 (according to Brunner), the volume above the water is O082, or about one twelfth of the whole. In the case of the bergs, however, there is not so much submerged, because of the saltness of the fjord water and the lower specific gravity of the ice, owing to the included air-bubbles. To determine this specific gravity, I took five pieces of ice from the bergs and put them into water. Into this I continued to pour alcohol until three of the pieces sank, the others still float- ing, and observed the specific gravity with an alcoholometer. I then placed in the mixture a piece of solid ice, and added water until it would neither sink nor rise. The difference between the specific gravity of these two mixtures is about equal to the difference between the specific gravities of the ice with bubbles and the solid ice*. From these experiments (on ice from bergs in Disko Bay) I found that the specific gravity of the bergs is decreased by 0*032 on account of the air-bubbles, or from 0*918 to 0*886. The specific gravity of the water in the ice-fjord varies; it was 1*0228 in Jakobshavn Fjord, near to the mouth of Tasiussak. From these ex- periments it results that 0*86 of a berg is under, and 0*14 is above

equal to the specific gravity of the mixture ; for the fragments partly dissolve and are surrounded by water, and so sink in a niixl are of higher specific gravity than that of the ice itself: the difference, however, between the specific gravity of the mixtures is nearly, though not exactly, equal to the specific gravity of the ice-pieces.
 * The specific gravity of the ice when it neither sinks nor rises is not exactly