Page:Popular Science Monthly Volume 79.djvu/203

Rh wrong, was about two miles in thickness and fifteen miles in length. About the same time several persistent trains were seen at Dunmore, Northwest Territory, Canada, one of which remained visible twenty minutes, and several others were reported from the Mount Lowe Observatory, California, by Professor F. L. Larkin.

It is evident that meteor trains that are observed at night occur at a very definite altitude, and furthermore various facts indicate that the formation of the train is due rather to the state of the earth's atmosphere where the train is formed than to the constitution, size or condition of the meteor itself. Also that the train is a glow phenomenon of the phosphorescent type and probably of electrical origin.

When meteors penetrate into the atmosphere of the earth they blaze forth at various heights, some even at as great an altitude as 100 miles, but the average height at which they appear has been found to be about 80 miles. The usual height at disappearance is about 40 miles. These altitudes are based on many observations. There is strong evidence in favor of the hypothesis that there is a certain density of the air which is favorable for the formation of the persistent train, because the altitude above the surface of the earth at which meteor trains occur when seen at night is usually confined to definite limits between forty-five and sixty-five miles, and the height which appears to be most favorable for longest visible duration is about fifty-five miles. Thirteen trains carefully observed at two or more stations, the altitudes of which have been determined by triangulation by well known astronomers, give a height of fifty-four miles as the mean altitude of the middle portions of the trains. The region where these self-luminous streaks of meteors occur extending from a little above sixty to a little below fifty miles altitude may well be called the meteor train zone of the atmosphere.

Trains of meteors which fall in daylight or twilight are not infrequently seen. They appear as thin smoke trains illuminated by the light of the sun, and according to triangulation observations which are possible only when the train is seen from two stations, they occur as low as twenty-five miles altitude, but seldom above forty miles. They are thus as a rule at a much lower altitude than the trains seen at night, which are usually, if not always, above forty-five miles. This fact would seem to indicate that in the upper levels of the atmosphere the glow does not mainly arise from light reflected from fine meteoric dust, but is a luminosity of the gas in the meteor's track.

The colors of the meteor trains show a good deal of variation according to the records of observations of different trains. The colors of trains observed at night include orange-yellow, emerald-green, bluish-green, silver and also white. In numerous cases green trains changed gradually to white, and in one instance from greenish to a