throughout the entire length of the visible path of the meteor. These observations are substantiated by the others more recently made. In Fig. 9, the tracks of seventeen trains are shown. The lengths of the paths with respect to altitude are drawn only. The paths of the meteors had different slants and hence differed in length very much more than shown in the drawing. The location of the persistent train is indicated with
Fig. 9. Chart giving Vertical Projections of the Paths of Seventeen Meteors, showing the altitudes of the trains. In a number of cases the mean altitude of the train has been measured only.
respect to the length of the entire track or trail, and it is evident from the chart that the production of the long enduring glow has something to do with the altitude of the meteor above the earth. As already stated, the upper and lower limits of the zone in the atmosphere where the trains are formed appear to be usually about sixty and fifty miles respectively from the earth's surface. If the glow is considered to be a phosphorescence of the rarefied air in the meteor's track, the conclusion that must be drawn, based on laboratory experiments, is that the barometric pressure at these heights is not far from two-tenths of a millimeter of mercury, or, in the neighborhood of from one two-thousandth to one four-thousandth of the pressure at the surface of the earth. Any information relating to the density of the atmosphere at great altitudes would be of value, making this part of the research an important one.