2. _Speed and trajectory._ Meteors vary widely in their velocities and flight paths. They plunge from space into the earth’s atmosphere at speeds estimated to range from seven to forty-five miles a second relative to the earth--from 25,000 to more than 150,000 miles per hour. Members of a particular meteor stream usually show a characteristic velocity. The Perseids, for example, travel at high speed, some thirty-six miles a second, while the Geminids saunter in at a mere twenty-one miles a second. Most of these “falling stars” become visible to us when they have descended to around sixty or seventy miles above the earth. Flashing down in a steep path, they usually burn up and vanish by the time they have fallen to around fifty or forty miles. The larger the meteor’s body, the longer its life and the lower its point of disappearance. Most meteors maintain a straight course as they descend toward earth. A typical path is that photographed by Smithsonian astronomers in New Mexico on the night of November 23, 1960 (see Plate IIIa). Some fireballs have been reported to change course after exploding. More probably, the witness is actually observing the shifting pattern of the smoke cloud left by the meteor. The Puerto Rico fireball of January 12, 1947, left an erratic trail of this type, which was photographed ten to twenty minutes after the meteor had disappeared (see Plate IIIb). The original entrance velocity, angle of entry, size, and chemical structure all influence the shape of a meteor’s path and its time of survival. The apparent angle of descent as seen by the observer depends on the distance and the direction the object is moving relative to the observer. When the meteor travels parallel to the observer’s line of sight, it seems much slower than when it passes the line of sight at right angles. The greater the distance between the observer and the meteor, the slower its apparent motion[V-13]. Some meteors move very slowly; traveling at an almost leisurely rate, they soar through the sky on a long, level path almost parallel with the earth. The slow fireballs in the great meteor procession of 1913 maintained a horizontal course over a distance of several thousand miles, from western Canada to Brazil[V-14]. Astronomical records show that green meteors are usually slow. Some 230 persons reported to the American Meteor Society that on November 28, 1953, at 6:30 P.M., a fireball moved slowly through the sky from Massachusetts to Pennsylvania. Described as blue-white-green, changing to orange-yellow-red, it was huge, disk-shaped, and vanished silently without depositing fragments [V-1, p. 273]. On May 15, 1954, at 11:22 P.M., more than 100 persons observed (and reported) a slow-moving fireball, blue-green changing to red, of luminosity so great that it woke sleeping people. Toward the end of its course it seemed to stop, spiraled a couple of times, and then simply vanished without leaving fragments [V-8, p. 336].