5. _Fragments._ Most meteors burn up high in the atmosphere. A few, if they are large enough in size (at least ten to twenty pounds) and tough enough in structure, survive to reach the earth as stony or metallic fragments. Marked differences characterize the various meteor streams. The Taurids (maximum November 12) are relatively rigid structures, unusually tough, and show little tendency to break up in their flight. The many Taurid fireballs show that fairly large bodies have survived. The Geminids (maximum December 14) are of average strength but appear to be very dense, while the Draconids (October 10) are featherlike and fragile, with low density. Some of the most brilliant fireballs may be structures of ice and frozen gases which quickly vaporize on reaching the earth and hence leave no detectable fragments. The fiery object that struck Siberia in 1908 may have been such an “icy cometoid”; although it devastated an area of hundreds of square miles and uprooted or knocked down some eighty million trees, it apparently left no physical trace[V-15]. If some of the physical body does survive to reach the earth’s surface, finding it is still a problem. Recovery is rare even when the fall occurs in daylight over well-populated country and the flight path can be charted from the accounts of reliable witnesses. When the fall occurs at night, recovery is even rarer[V-5]. After dark, even experienced observers find it difficult to judge true directions and distances, and they may plot a place of fall that is many miles from the actual point of impact. Meteoriticists know that there is small chance of finding meteorites that fall at night except in regions where most of the land is under cultivation. In the fifty years between 1898 and 1948, of forty-eight recoveries from observed meteorite falls in the United States, only seven were made from falls occurring after 8 P.M.[V-5]. Recovery depends on many factors: the number of persons who saw the event, the accuracy of their estimates of distance and direction, the size of the meteorites, the patience of the searchers, the time and money available for the search, and, most important of all, just plain luck. The Norton County fall of February 18, 1948, illustrates both the detective work and the luck required. At about 4:56 P.M. C.S.T. a brilliant detonating fireball soared over an area including Kansas, Nebraska, Colorado, Oklahoma, and Texas, and left a large white cloud that was visible for about an hour afterward. Newspapers publicized the phenomenon as a flying saucer and a few excited witnesses agreed. One man affirmed that shortly before the explosion the strange craft hovered over his yard at eye level, belching fire and showering sparks, then suddenly took off, climbing fast, and exploded.