and the centrifugal reaction due to the rapid diurnal rotation of the earth on its axis. The solar radiation maintains the temperature of the equatorial regions. The cold air of the polar region is both by gravity and by centrifugal force driven toward the equator. Thus the general currents are maintained moving from the poles toward the tropics and return. They are most intense in the Northern Hemisphere in January, when the sun is farthest south or over the Tropic of Capricorn, because at that time and subsequently the difference of temperature between the equator and the North Pole is greatest, and the reverse holds good in June, when the sun is north of the equator. The general circulation is greatly modified by the difference in temperature and moisture of the air over the land and the ocean, so that in summertime the tendency of the air to flow inward toward a continent or mountain is very decided. The general circulation is also greatly modified by the presence of snow, ice, mountains, plateaus, clouds, forest, etc. The winds, when once formed by differences of temperature and moisture, are themselves affected by the rotation of the earth. No matter in what direction they may be moving they are at once deflected from their polar path; in the Northern Hemisphere they turn to the right; in the Southern Hemisphere to the left. Therefore those flowing toward the equator become the northeast and southeast trade winds and those flowing toward the poles, or the upper return trade winds, become the westerly winds of the north and south temperate zones.
The differences in temperature between the continents and the ocean give rise to the so-called monsoon winds. The general centrifugal action of the winds produces a low pressure in the regions about which the winds rotate, namely, a low pressure in the Arctic and Antarctic regions; a low pressure on the left of the winds blowing around a storm centre, and on the right hand side of these same winds considered as blowing around an adjacent region of high pressure; a low pressure at the equator between the northeast and southeast trades. The reaction of the easterly winds near the equator and the westerly winds farther north also produces a similar area of high pressure between these two systems of wind corresponding to the high pressure under the tropics of Cancer and Capricorn.
A full exposition on these and other theorems by Prof. William Ferrel will be found in his Treatise on the Winds (New York, 1893). The results of later researches are presented in Prof. F. H. Bigelow's report on international cloud observations (Washington, 1900), and his Report on Barometry (Washington, 1902), but these are written for purely technical and mathematical readers. A general résumé of the laws of atmospheric motion is given in the appendix to Hann, Lehrbuch der Meteorologie (Leipzig, 1901). An elementary presentation of the subject, especially adapted to those who are beginning the study of meteorology, will be found in Davis, Elementary Meteorology (Boston, 1894); and in Ward, Practical Exercises in Elementary Meteorology (Boston, 1899). For the history of practical meteorology in the United States, see Weather Bureau.
Some details as to the instruments used in meteorology will be found under the topics: Actinometer; Anemometer; Barometer; Pyrheliometer; Nephoscope; Rain Guage; and Thermometer. Some of the results of observation will be found treated under the topics: Atmosphere; Atmospheric Electricity; Aurora Borealis; Blizzard: Climate; Clouds; Dark Day; Dew; Doldrums; Dust; Equinoctial Storm; Fog; Frost; Hail; Halo; Humidity; Indian Summer; Isobarometric Lines; Isothermal Lines; Lightning; Monsoon; Polarization of Skylight; Scintillation; Simoom; Snow; Snow Line; Storms; Heat; Typhoons; Weather; Whirlwinds; Wind.
METEORS (OF. meteore, Fr. météore, from Gk. μετέωρον, meteōron, meteor, from μετέωρος meteōros, on high, from μετά, meta, beyond + αείρειν, aeirein, to lift). A term now applied by astronomers to those shooting stars that flash into view without detonation or explosion. As thus characterized, meteors form a class of bodies distinct from the aërolites (q.v.). Sometimes those meteors of which fragments are not known to reach the earth are called bolides. The phenomena are exactly the same except that the fragments are not actually found.
The brilliant display of November 13, 1866, gave a vigorous impulse to astronomical investigation of shooting stars, leading to the discovery that the November meteors move in an orbit round the sun, and that in all probability this orbit forms a ring or belt of innumerable small fragments of matter, distributed with very variable density of grouping along it, thus corresponding so far to the planetoid (q.v.) group between Mars and Jupiter. It is also known that the motion of this meteor ring round the sun is retrograde; that the earth's orbit at that point where she is situated on November 13-14 intersects this ring; and that, probably in 1799, 1833-34, and 1866-67, it is the same group of meteors which has been observed. The last-mentioned hypothesis has been made the foundation of a calculation of the probable orbit and periodic time of this meteor ring. The fact that a November star-shower may occur for two years in succession, and then recur at an interval of 32 or 33 years, seems to indicate that though the earth may pass through the meteor-orbit every year, the meteors are so grouped at intervals along the ring and their periodic time differs so much from that of the earth that it requires 32-33 years before this accumulating difference amounts to a complete revolution of either the earth or the ring, and a repetition of the star-shower becomes possible.
Professor Newton of Yale, who entered into an elaborate investigation of the subject, concluded that there were five possible periodic times for the meteor ring: 33¼ years, 376 days, 354 days, 188 days, 177 days. The English astronomer Adams then showed that of these the 33¼-year period was the only one actually consistent with known facts, and this is therefore now accepted as the time required by the November meteors to complete a revolution around the sun.
That there is an intimate relation between meteors and comets is an ascertained fact of much interest. There is a great similarity between the orbits of some of the more important showers and certain of the comets, a similarity so close as to establish some kind of mysterious relationship beyond the possibility of mere coincidence.
Popular interest has been very keen in the mat-
