The number of bergs that become detached from the glacier every twelve hours varies with tide and weather, but generally a new one is thus fresh born every six or seven minutes, and so massive that the discharge may be heard like thunder or cannonading two or more miles away. When one of the fissured masses falls there is first a heavy, plunging crash, then a deep, deliberate, long-drawn-out thundering roar, followed by clashing, grating sounds from the agitated bergs set in motion by the new arrival, and the swash of waves along the beach. All the very large bergs rise from the bottom with a still grander commotion, rearing aloft in the air nearly to the top of the wall, with tons of water pouring down their sides, heaving and plunging again and again ere they settle and sail away as blue crystal islands; free at last after being held rigid as part of the slow-crawling glacier for centuries. And strange it seems that ice formed from snow on the mountains two and three hundred years ago, should after all its toil and travel in grinding down and fashioning the face of the landscape still remain so lovely in color and so pure. The rate of motion of the glacier as has been determined by Professor Reid is, near the front, about from five to ten feet per day. This one glacier is made up of about 200 tributary glaciers, which drain an area of about a thousand square miles, and contains more ice than all the eleven hundred glaciers of the Alps combined. The distance from the front back to the head of the farthest tributary is about fifty miles, and the width of the trunk below the confluence of the main tributaries is twenty miles or more. Next to the Muir, the largest of the glaciers enters the bay at its extreme northwestern extension. Its broad, majestic current, fed by unnumbered tributaries, is divided at the front by an island, and from its long, blue wall the icebergs plunge and roar in one eternal storm, sounding on day and night, winter and summer, and from century to century. Five or six glaciers of the first class discharge into the bay, the number varying as the several outlets of the ice-fields are regarded as distinct glaciers, or one. About an equal number of the second class descend with broad, imposing currents to the level of the bay without entering it to discharge bergs; while the tributaries of these and the smaller glaciers are innumerable. [Illustration: FACE OF MUIR GLACIER, ALASKA.]