Page:Popular Science Monthly Volume 51.djvu/655

Rh another group of phenomena, those of meteorology, which are of interest to the whole earth, and are especially remarkable in the polar regions. An interchange of great wind currents between the equator and the poles is constantly going on, upon which the movements of the atmosphere and the pressure in the intermediate regions are ultimately dependent, and the study of the atmospheric phenomena of the polar regions is indispensable to our proper knowledge of them.

The excess of heat at the equator forces masses of air into the highest regions of the atmosphere; the congestion at the pole, the necessary consequence of accumulation there, forces them back to the earth. On their way through the higher regions these masses are attenuated and cooled, so that, even when condensed at their sinking, they can not overcome the polar cold; and as they bring little moisture, and consequently little cloudiness, the radiation of heat goes on continuously during the long polar night; the more so because snow and ice are extremely good radiators. Hence the extreme cold which Nansen found in Greenland, and which makes that interior a second pole of cold along with that in the interior of Siberia, is fully explained.

Yet the winds contribute to the warming of the polar sea. They drive the waters from warmer regions in wide superficial currents into the higher latitudes, where, being heavier in consequence of their greater content of salt than the fresher water resulting from the melting of the glaciers and the ice and from the outpour of the great Siberian rivers, they sink beneath them to the bottom and keep the temperature of the sea constantly above the freezing point. The colder, lighter water has to give way to these under-sea currents, and flows into the Atlantic Ocean, cooling the American coasts. At the south pole currents flow in from all the seas, and superficial waters spread into all the oceans.

How shall we account for the masses of polar ice, for the immense icebergs, and the glaciation of Greenland? The snowfall of the polar regions is light. The air is nowhere drier than over the cold glacier ice, as is proved every day in SwitzterlandSwitzerland [sic] by the quickness with which clothes dry when hung over it. At the same time the ice is covered with extremely fine, hardly visible snow crystals. If we boil water in a retort which is connected with another vessel containing a piece of ice, all the steam will pass over on to the ice and deposit itself as ice upon it. The same takes place in a larger degree on the earth, where the retort is the warm evaporating water of the tropical regions, the connecting pipe is the upper atmosphere, and the thickening ice is at the pole. Thus, without any rain or snow falling, all the moisture and all the vapor is withdrawn from the atmosphere by this ice