POLARIZATION plementary color. This effect is obtained by placing the principal section of the small cen- tral disk 22i from that of the other films of selenite which form the star. The rule to as- certain the time by this instrument is as fol- lows : the tube must be turned round by the hand of the observer until the color of the star entirely disappears, while the disk in the cen- tre remains red ; the hand will then point ex- actly to the hour. The accuracy with which the solar time may be indicated by this means will depend on the exactness with which the plane of polarization can be determined ; one degree of change in the plane corresponds with four minutes of solar time." " The advantages a polar clock possesses over a sun dial are : 1. The polar clock being constantly directed to the same point of the sky, there is no locality in which it cannot be employed ; whereas in order that the indications of a sun dial should be observed during the whole day, no obstacle must exist at any time between the dial and the places of the sun, and therefore it cannot be applied in any confined situation. The polar clock is consequently applicable in places where the sun dial would be of no avail ; on the north side of a mountain or of a lofty building, for instance. 2. It will continue to indicate the time after sunset and before sunrise ; in fact, so long as any portion of the rays of the sun is reflected from the atmosphere. 3. It will also indicate the time, but with less accuracy, when the sky is overcast, if the clouds do not exceed a certain density. The plane of polari- zation of the north pole of the sky moves in the same direction as that of the hands of a watch ; it is more convenient, therefore, -to have the hour graduated on the lower semi- circle, for the figures will then be read in their direct order, whereas they would be read back- ward on an upper semicircle. In the south- ern hemisphere the upper semicircle should be employed, for the plane of polarization of the south pole of the sky changes in the same direction as the hands of a watch. If both upper and lower semicircles be graduated, the same instrument will serve equally well for both hemispheres." The inventor devised sev- eral forms of the instrument, but the one given in the engraving illustrates the principle. POLARIZATION. See LIGHT, vol. x., p. 445. POLAR SEAS, the waters surrounding the north and south poles. I. ARCTIC OCEAN. This is properly only the termination of the Atlantic, and is bounded by the northern coasts of America, Europe, and Asia. The Arctic circle includes nearly the whole basin with the exception of some of the border seas, such as Hudson bay and a part of the White sea. It communicates with the Pacific through Behring strait, and with the Atlantic through Davis strait, besides its wide connec- tion between Greenland and Norway. (See ARCTIC DISCOVERY.) In its known parts isl- ands are numerous and some of them are large. N. of America they form an archipel- 672 VOL. xiii. 42 POLAR SEAS 653 ago ; N. of Europe are Spitzbergen, the new- ly discovered Francis Joseph Land, and Nova Zembla ; N. of Asia the New Siberian islands and the imperfectly known Wrangell Land. There are probably more in the unexplored central region, as land has been seen to the northward of the furthest points reached by the American and Austrian expeditions; but the absence of icebergs from the drift ice N. of Spitzbergen indicates the absence in that direction of land extensive enough to support glaciers. There are border seas and bays of considerable size, forming dependencies of the Arctic ocean, such as the White sea, the Kara sea, the gulf of Obi, Hudson bay, and Baffin bay. The drainage of N. E. Europe, the whole of N. Asia, and a large portion of North Amer- ica is poured into the arctic basin by rivers of considerable size and length. The unex- plored part of this sea is about as large as the continent of Australia ; as the ice is the only impediment to navigation in it, a knowledge of its different stages and of the names they have received is of importance. Icebergs are the largest masses, which on account of the slow- ness of their melting descend furthest toward the equator. They are all formed on land in the shape of glaciers. (See GLACIER.) Of these the largest have been observed in Green- land, where they cover a great part of the in- terior, and abut on the sea either by arms de- scending into the bays or fiords, or by a bold front many miles in extent, as in the Hum- boldt glacier in Smith sound. If the glacier terminates on a cliff fronting the sea, large masses of ice gradually overhang and finally break off by their own weight ; but if the shore is low and the end of the glacier dips into the sea, it will push its front part forward until it reaches water deep enough to float the mass, which is then broken off by the force of buoyancy. The process of breaking off has been called by the whalers "calving." The icebergs are then carried away by the current, and by irregular melting are made to assume the most fanciful forms. Being originally formed by snow, the ice is perfectly fresh, and it is often melted down by navigators to renew their water supply. The sea water freezes in winter to the depth of 6 to 10 ft., and when it breaks up it forms ice fields, often many miles in extent. Smaller ice fields are called floes. By collision the edges of fields and floes be- come broken and piled up into hummocks. Fields and floes much broken up, partly open and partly refrozen, are called the pack. Drift ice and brash ice are parts of the pack scatter- ed and in motion, the latter term being used when the pieces are much reduced. A margin of ice which adheres to the shores and is formed by the tides is called the ice belt or ice foot. Navigators can judge of the extent of ice beyond the horizon by a peculiar glistening of the atmosphere known as the ice blink ; over open water the sky looks dark and is known as water sky. The nearest approaches to the
