720 JUPITER AMMON dieted phenomena of the satellites occurred earlier when Jupiter was in opposition than when he was in quadrature, and that in fact the further Jupiter was from opposition up to the time when he was so near conjunction that his satellites could no longer he observed, the later these phenomena occurred. It was at length suggested by Romer that the discrepancy was due to the increase of distance, the light which brings to our earth information of the phenomena taking longer in reaching the earth when the planet is further away. Repeated observations confirmed this theory, which at first astronomers of repute ridiculed as too fanciful for serious consideration. Bradley's discovery of the aberration of light placed the theory beyond the possibility of question. The appearance of Jupiter's disk is such as to sug- gest the idea that the planet is enveloped in a deep vaporous atmosphere, heavily laden with cloud masses. A series of broad bands or belts, alternately dark and light, and differing in color, lie across the disk, agreeing generally in position with the latitude parallels of the planet. On a close study with telescopic power, these belts are found to present pecu- liarities of structure exceedingly interesting. Rounded clouds appear to float separately with- in the deep atmosphere, and from time to time changes of shape and of color are noticed which seem to imply the action of forees of great intensity. Theoretical investigations applied to the subject of an atmosphere of great depth, attracted by the strong gravity of Jupiter, sug- gest that conditions of pressure would exist in- compatible with the gaseity of the envelope. And the known small density of the planet, combined with the result just mentioned, sug- gests that in the case of Jupiter, as in that of the sun, the increase of pressure and therefore of density, which we should expect from the mere mass of the planet, is counteracted by the expansive effects of intense heat. This view of the planet's condition has been adopted recently by Prof. Benjamin Peirce on inde- pendent mathematical grounds, and may be regarded as altogether more probable than the old-fashioned but quite unsupported opinion that the planet's condition resembles generally that of our own earth. JUPITER 1SDI01V. See AMMOJT. JURA, an island off the W. coast of Argyle- shire, Scotland, one of the inner Hebrides ; area, about 85 sq. m. ; pop. in 1871, 761. It is separated from the mainland on the east by the sound of Jura, about 5 m. wide, from the island of Islay on the southwest by a narrow strait If m. wide, and from the island of Scar- ba on the north by the gulf and strait of Oor- ryvrecken. Its length is 27 m., and its greatest breadth 7 m. On the west the coast line is broken by a narrow deep indentation, called Loch Tarbert, which nearly cuts the island in two, penetrating to within a mile of Tarbert bay on the east. The western shores are bleak and rugged, but the eastern are more pleasing, JURA having green slopes and a belt of plain. A ridge of rugged mountains traverses the entire length of the island, rising at three points into high conical peaks, called the Paps of Jura, the highest of which is 2,566 ft. There is little arable land, only 600 acres being under cultivation. Oats, barley, potatoes, and flax are raised, and large flocks of sheep and goats are fed upon the mountains. From 1,000 to 1,200 head of cattle are exported annually. There are excellent slate quarries and a very fine sand for glass making. The island is famous for its red deer, and for remarkable caverns on its E. coast. With some adjacent islands it forma the united parish of Jura and Colonsay. .11 KA, a range of mountains between Switz- erland and France, extending about 180 m. in length, from the waters of the Rh6ne in the department of Ain on the S. W. to those of the upper Rhine in a N. E. direction. The great valley of Switzerland and the lake of Neufchatel lie along its S. E. base, and over these from its summits may be seen Mont Blanc and the principal peaks of the Alpine chain. The Jura, like the Appalachian chain of the United States, consists of parallel ridges including narrow longitudinal valleys, along which the rivers flow in one or the other di- rection, occasionally passing through a break in the mountains into the next valley. In their external form, and the wave-like arrange- ment of the stratified rocks of which they are composed, the resemblance is still more stri- king. They occupy a belt of country averaging about 30 m. in width ; and the highest summits, which are mostly in the S. part of the range, attain nearly the same elevation as the White mountains in New Hampshire. The principal summits are Cret de la Neige, Reculet de Toiry, Mont Tendre, Dole, Pie de Marmiers, Chasse- ron, Chasseral, Oredoz, and Colomby ; the first named of which is 5,653 ft., and the last 5,200 ft. high. The principal strata are limestones of the oolite formation, named the Jura from their abundance in this range, and with them are associated shales and sandstones, including beds of gypsum. The highest summits of the Jura lose their snowy winter covering in the summer, and are then green with dense forests of fir. The growth below is in great part of walnut, groves of which surround almost every village. In the valleys are found some of the richest pasture lands in Switzerland, where are produced the Gruyfire and other cheeses famed throughout Europe. Great numbers of cattle are reared and fed on the mountains. The Jura and the intermediate undulating coun- try abound in wooded hills, among which rocky masses project at intervals above the fertile slopes, which by the aid of irrigation yield three crops of grass annually. The most picturesque scenery is presented by the Val Moutiers, or Munster Thai, between Basel and Bienne, the pass of Klus at the foot of the Ober-Hauenstein, and the lac de Joux. The name Jura has also a wider application than
