Page:Popular Science Monthly Volume 79.djvu/195

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HE nature of the luminous cloud occasionally left glowing in the wake of large meteors and called the "persistent" streak or train has long been regarded as a mystery by astronomers. Many of these trains have been observed which have remained visible to the naked eye for quite as long as fifteen or thirty minutes after the disappearance of the burning metorite itself. In numerous instances trains have lasted for more than one hour, floating in the cold upper atmosphere, a luminous mist-like cloud projected against the dark night sky.

Meteors are usually visible but for a few seconds in their rapid flight through the upper regions of the atmosphere at a velocity of from twenty to thirty or more miles a second. Their track is almost always marked by a bright streak of fast-fading luminosity which also disappears from view in a second or two. Occasionally, however, the streak remains for many minutes brightly glowing, continually expanding in size, and drifting with the moving atmosphere. This is the phenomenon which has been called the "persistent train."

Meteors which leave these trains are a very small proportion of the total number that are seen, yet authentic and definite facts concerning the trains have been recorded at various astronomical observatories in all parts of the world, hence the chief characteristics of this remarkable phenomenon are known.

A serious study of the subject has been made only recently, but it is now recognized as being of considerable importance because it teaches important facts concerning the upper atmosphere. Most of the observations of meteor trains which from time to time have been made by different astronomers have been incidental in the course of the usual investigations of the heavens. Perhaps the best way to show the extraordinary features of the self-luminous meteor train is by drawings of a bright and long-enduring train carefully observed in England many years ago. The drawing marked A in the first illustration is a sketch made by one of the observers of the train as it appeared only a few seconds after the meteor nucleus had disappeared. This train, which was seen by many observers in the vicinity of both Sidmouth and Cardiff in England at the time of the great meteor shower of 1866, was formed