Page:Popular Science Monthly Volume 79.djvu/196

192 by a meteor belonging to the swarm known as the Leonids, and appeared at first, as is always the case, a narrow glowing streak. The train soon after its appearance was sixteen to eighteen miles in length and at an altitude above the earth of fifty-six miles as determined by triangulation from Sidmouth and Cardiff, which are fifty miles apart. It appeared lance-like for a few moments and then was seen to be bending like a long floating ribbon, slowly curving about as shown in sketch B. The train also gradually expanded in size. The enlargement of these luminous clouds is unquestionably due to the diffusion of the particles of which they are composed. A careful study of the observations of a large number of trains has proved conclusively that the distortions, such as those shown in the illustrations, are due to the different wind currents that are in the atmosphere at great heights, even as far distant from the earth as sixty-five miles. It is also evident that these currents may vary both in direction and in velocity at different levels at one time, quite similar in fact to the drifts of the atmosphere in the cloud region near the surface of the earth.

Meteor trains have been observed more frequently during the periodic meteor showers than at other times, and apparently the meteors which have produced the greatest number of trains are the so called Leonids and Perseids. The former, it may be remembered, appear every year about November 14, but produce what is known as a meteoric shower every thirty-three or thirty-four years, the actual period being