=Nourishment of continental glaciers.=—Explorations upon and about the glaciers of Greenland and Antarctica have shown that the circulation of air above these vast ice shields conforms to a quite simple and symmetrical model subject to spasmodic pulsations of a very pronounced type. Each great ice mass with its atmospheric cover constitutes a sort of refrigerating air engine and plays an important part in the wind system of the globe. (See Fig. 291, p. 263). Both the domed surface and the low temperature of the glacier are essential to the continuation of this pulsating movement within the atmosphere (Fig. 312). The air layer in contact with the ice is during a period of calm cooled, contracted, and rendered heavier, so that it begins to slide downward and outward upon the domed surface in all directions. The extreme flatness of the greater portion of the glacier surface—a fraction only of one degree—makes the engine extremely slow in starting, but like all bodies which slide upon inclined planes, the velocity of its movement is rapidly accelerated, until a blizzard is developed whose vigor is unsurpassed by any elsewhere experienced. [Illustration: FIG. 312.—Diagram to show the nature of the fixed glacial anticyclone above continental glaciers and the process by which their surface is shaped.] The effect of such centrifugal air currents above the glacier is to suck down the air of the upper currents in order to supply the void which soon tends to develop over the central portion of the glacier dome. This downward vortex, fed as it is by inward-blowing, high-level currents, and drained by outwardly directed surface currents, is what is known as an _anticyclone_, here fixed in position by the central embossment of the dome. The air which descends in the central column is warmed by compression, or adiabatically, just as air is warmed which is forced into a rubber tire by the use of a pump. The moisture congealed in the cirrus clouds floating in the uppermost layer of the convective zone, is carried down in this vortex and first melted and in turn evaporated, due to the adiabatic effect. This fusion and evaporation of the ice by its transformation of latent, to sensible, heat, in a measure counteracts, and so retards, the adiabatic elevation of temperature within the column. Eventually the warm air now charged with water vapor reaches the ice surface, is at once chilled, and its burden of moisture precipitated in the form of fine snow needles, the so-called “frost snow”, which in accompaniment to the sudden elevation of temperature is precipitated at the termination of a blizzard.