98 PHYSICAL GEOGRAPHY. of cyclones. Tornadoes appear to be due to ro- tary motion of the air occurring above the earth’s surface, which results in a rapid sucking up of the warmer surface air. 259. Water-spouts.— When tornadoes or whirl- winds occur on the water they cause a water-spout. A rapid condensation of vapor takes place, both from the different temperature of the winds and from the rarefaction produced at the centre of the revolving mass of air. Portions ofthe clouds are sometimes drawn down from above and whirled around in the form of an immense funnel-shaped mass; finally the whirl reaches the water, and a column of spray is thrown up, which unites with the mass above and moves over the surface of the water ‘as an immense pillar. Though of formidable ap- pearance, water-spouts have never been known seriously to damage large vessels. Similar phenomena are noticed on the land when tornadoes occur. Here, however, only the cloud cone is observed. 260. The North-Easters and other Storms of the United States—The following important facts have been discovered in regard to the extended storms which occur in the United States: (1.) All our great storms are attended by an immense whirling of the wind, and are, in fact, species of cyclones. (2.) The great north-east storms of our eastern sea-board usually originate in the west, in an area of low barometer, somewhere between Texas and Minnesota. In the front and rear of this area the barometer is high. (8.) The calm centre of the storm, or the area of low barometer, usually moves toward the north- east. The shape of the calm centre is longer from north to south than from east to west. (4.) The storms begin by the winds blowing toward the area of low barometer. (5.) During the prevalence of the storm the winds are north-east, east, or south-east; toward the end, north-west, west, and south-west. 261. Sailing Routes.—A knowledge of the di- rections of the winds and ocean currents has ma- terially diminished the time required by sailing vessels to go from one port to another. Opposing. winds and currents often render it advisable for the vessel to begin its journey in a direction consider- ably out of the direct line of the desired port. Hurope—America.—The Gulf Stream and prevailing westerly winds render the passage across the ocean from east to west considerably longer than from west to east. The general route, in either direction, varies with the season of the year. : New York—San Francisco.— After leaving New York the course is considerably to the east, in order to clear the South American coast in the region of the trades. After doubling Cape Horn the course is westward. The zone of the north-east trades is entered about 118° W. long. America—India—Australia.—In sailing from Amer- ica to India or Australia the vessel takes the same route as between Eastern America and San Francisco. About opposite Rio Janeiro, however, the routes diverge. On entering the Indian Ocean the direction is dependent on that of the prevailing monsoon. Europe—India—Australia.—The vessels either pass through the Mediterranean Sea and the Suez Canal, or around the Cape of Good Hope. The broad expanse of ocean in the southern hemisphere, in the zone of the vari- ables, renders the westerly winds very steady. Vessels sail- ing from Atlantic ports of America or Europe generally find it preferable to go by the eastward route, around the Cape of Good Hope, and return by the westward route, around Cape Horn, thus circumnavigating the globe. California—Japan.—The southerly route, from east to west, is aided by the north-east trades and the north equa- torial current of the Pacific; the northerly route, from west to east, is necessary in order to avoid the trade winds. ; The general sailing routes hetween some of the most important ports are traced on the map of the winds. 2 PIA SYLLABUS. 0028300 — Atmospheric air is composed mainly of a mixture of ni- trogen and oxygen, in the proportion, by weight, of about 77 parts of nitrogen to 23 of oxygen in every hundred parts. The atmosphere also contains small quantities of carbonic acid and the vapor of water. The oxygen of the air is necessary to combustion and respiration; the carbonic acid and the vapor of water, to plant-life. At the level of the sea the atmosphere presses on every square inch of the earth’s surface with a force of about 15 pounds. The upper limit of the atmosphere has been variously estimated at from 50 to 200 miles above the level of the sea. A barometer is used for measuring the pressure of the at- mosphere; a thermometer, for measuring its temperature. The vertical rays of the sun are warmer than the oblique rays—l. Because they are spread overa smaller area of the earth. 2. Because they pass through a thinner stratum of air, and consequently lose less of their heat by absorption. 3. Because they strike the earth more directly, and there- fore produce more heat. .