ILLUSION PERFECT.

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Mirages are caused in this way:-The density of the air generally diminishes with the height; rays of light that proceed obliquely from an object then become more and more horizontal, but generally pass away into space. When the density of the air diminishes with the height with unusual rapidity, as when the air is cooler, the nearer it is to the earth, then the ascending rays may become quite horizontal, and then bend downward toward the earth, reaching the earth at a far distant point from the object reflected.

The observer at that point sees distant objects at an unusual elevation, or sees above the true horizon erect images of objects which may or may not be beyond the horizon. If the layer of air near the earth be uniformly dense, as in the cold air over a frozen sea, and a warmer stratum lie above it in which the density rapidly diminishes, so that the rays are brought back to the earth, the rays cross one another in the hot stratum, and the observer sees objects upside down.

In the desert of Sahara and other arid deserts the conditions are reversed, for the air is hottest near the hot sand. Skylight rays descending become bent upward. The mirage is not inverted and the illusion is often perfect.

CHAPTER XXI.

THE GLACIERS.

Wonders of the northern territory—The great ice fields—The formation and action of glaciers-What is known of the remarkable Malaspina glacier—Some freaks of nature which man studies with intense interest-Some mysteries in the frozen land which he cannot solve-The Muir, Guyot, Seward and other glaciers.

PER

ERHAPS no single feature in Alaska, aside from its gold treasure, has excited so much human interest and investigation as its glaciers. The Malaspina, Muir, and other less well-known Alaskan glaciers are regarded in the scientific world as among the most remarkable works of nature of this class on earth.

The name glacier is one given to a mass of ice, having its origin in the hollows of great mountains where perpetual snow accumulates but which makes its way down. toward the lower valleys, where it gradually melts, until it terminates exactly where the melting, due to the contact with the warm air, earth, and rain of the valley compensates for the bodily descent of the ice from the snow reservoirs of the higher mountains. Of the manner in which glaciers are formed and moved and disappear much has been learned by the scientist in a general way, but much of the story of their work in the ages which are gone, of the stupendous force which they exert on the earth's surface, is yet to be learned.

A recent report to the government on the Mt. St.

Elias district gives some interesting statistics of the great Malaspina glacier which may be regarded as the type of these ice bodies called Piedmont glaciers. They are so called because formed at the foot of mountains by the union and expansion of ice streams from the valleys of adjacent highlands. The glaciers flowing south from the great neve fields on the mountains of the St. Elias system, for full one hundred miles west of Yakutat Bay, expand on reaching the flat lands between the base of the mountains and the sea, and unite to form a vast lake of ice, which has been named in honor of Malaspina.

The glacier extends with unbroken continuity from Yakutat Bay, seventy miles westward, and has an average breadth of from twenty to twenty-five miles. Many of the glaciers are vastly greater in dimensions, but the formation and movements of this one, as known to man, will serve to show the general laws. The area is about that of the State of Delaware, or a little larger. It is a vast, nearly horizontal plateau of ice. The general elevation of its surface, at some distance from its outer border, is fifteen hundred feet. The central portion is free from moraines, or dirt of any kind, but it is rough and broken by thousands of crevasses. Its surface is a broad, desolate prairie, not unlike the rolling lands of the Western plains.

The Malaspina consists of three principal lobes, each one formed by the expansion of a large tributary ice stream. The largest has an eastward flow toward Yakutat Bay, and is supplied mainly by another smaller

glacier, known as the Seward. The next lobe to the west is the result of the Agassiz glacier. Its current is toward the southwest. Still a third lobe lies between the Chaix and Robinson hills, and is supplied by the Tyndall and Guyot glaciers. Its central current is southward.

The Seward lobe melts away before reaching Yakutat Bay, but its southern margin has been eaten into by the ocean, forming the Sitkagi bluffs. The Agassiz lobe is complete, and is fringed in all its extremity by wood moraines. The other lobe pushes boldly out into the ocean, where it breaks suddenly, forming the wellknown Icy Cape. The waves undermine these great ice cliffs and piece after piece is deposited in the ocean to sail away in the form of bergs. This is the only instance known in Alaska where a glacier advances into the open ocean. The ice cliff at its extremity is one of the finest specimens of its kind to be seen in the world, and furnishes to the tourist one of the most beautiful sights on the Pacific coast.

On the northern border of the Malaspina glacier, but below the line of perpetual snow, where the great plateau has a gentle slope, the melting surface gives the origin to hundreds of rivulets, which course along in channels of clear ice, until they reach a crevasse, where they plunge down to the drainage beneath. On a summer day, when the sun is well above the horizon, and where the surface of the glacier is inclined, the rush of the water may be heard constantly, but as soon as the shadows of evening fall the flow ceases. These streams