The principal obstruction and danger in arctic navigation arises from the ice; fields of which often occur of twenty or thirty miles in diameter, and ten or fifteen feet in thickness. From these crystal plains rise sometimes isolated, sometimes in groups, elevations of thirty or more feet in height, called _hummocks_. Dr. Scoresby once saw a field so free from hummocks and fissures that a coach might have traversed it for leagues in a straight direction, without obstruction. In May or June these fields begin to drift along in solemn procession to the southwestward, in which direction they hold their steady course, whether in calm or in spite of adverse winds. When these floating continents emerge from the drift ice which had hitherto protected them, they are shattered and broken up by the long, deep swell of the ocean. A ground-swell, hardly perceptible in the open sea, will break up a field in a few hours. These fields sometimes acquire a rotary motion, which gives their circumference a velocity of several miles an hour, producing a tremendous shock when one impinges upon another. "A body of more than ten thousand millions of tons in weight," says Dr. Scoresby, "meeting with resistance when in motion, produces consequences scarcely possible to conceive. The strongest ship is but an insignificant impediment between two fields in motion."--Mr. Snow gives the following account of TAKING THE FIRST ICE. "We had come so quickly and unexpectedly upon this "stream" (not having seen it, owing to the thick weather, until close aboard of it), that promptitude of decision and movement was absolutely necessary. It was one of those moments when the _seaman_ comes forward, and by boldly acting, either in the one way or the other, shows what he is made of. In the present case the question instantly arose as to whether the vessel should at once run through the ice now before her, or wait until clearer and milder weather came. The mate, as ice-master, was asked by the captain which, in his opinion, was best. He advised _heaving to, to windward of it, and waiting_. The second mate was then asked; and he, without knowing the other's opinion, strongly urged the necessity of _running through at once_. Captain Forsyth, using his own judgment, very wisely decided upon the latter, and accordingly run the ship on. And a pretty sight, too, it was, as the "Prince Albert" under easy and working sail, in a moment or two more entered the intricate channels that were presented to her between numerous bergs and pieces of ice, rough and smooth, large and small, new and old, dark and white. It was hazy weather, snowing and raining at the time; and all hands having been summoned on deck, were wrapped in their oil-skin dresses and waterproof overcoats. Standing on the topsail-yard was the second mate conning the ship; half-way up the weather rigging clung the captain, watching and directing as necessary; while aft, on the raised counter near the wheel,