One of the most difficult problems to explain is the cause of the luminosity of the meteors. Many of them start into brilliancy at the enormous heights of eighty or ninety miles above the Earth and, after dashing at planetary speed across a distance of perhaps a hundred miles or more, disappear at heights of thirty or forty miles from the surface. Compared with the rapidity of their motion the quickest bullet is practically at rest. The explanation most widely accepted is that the friction of the meteorite in passing through our atmosphere at such an enormous speed ignites it and rapidly destroys it. Objection has been raised to this theory on the ground that the atmosphere at great heights is exceedingly rare and that it is difficult to believe it could offer enough resistance. Another problem has hitherto proved quite insoluble; i. e., the long persistence of the train of luminous particles which remain drifting in the upper air after the disappearance of the explosive bolides. For instance, on February 22, 1909, such a luminous train was seen for several hours drifting across the sky at high speed. Its height was so great that it was visible over a large part of England and France. Why these sparks do not go out instantly, in the same manner as those which follow the ordinary shooting-stars, is an unsolved mystery. The only thing that is well established about meteor showers is that most of them are periodic and come from well-defined quarters of the heavens. From the study of the directions from which these streams come, it has been calculated that they travel round the sun in long elliptical orbits, and are members of his family. An orbit of thirty-three years has been computed for the famous November meteors. They probably extend about as far as the planet Neptune on one side of the Sun. The wonderful displays of November meteors seen in 1833 and 1866, which astonished the whole world, were probably caused by the passing of the Earth through a particularly dense portion of the stream. In 1866 we met the same portion that we had encountered in 1833. It was again looked for in 1899, thirty-three years later, but, to the surprise of the astronomers, there was but a very ordinary display. Many reasons have been offered for this, but no one really knows enough to explain it satisfactorily. A few of the meteoric streams follow the tracks of comets, and it is supposed that they may be the disintegrated remains of comets, particularly in the cases where the latter have faded away. There are many other peculiarities in the behavior of meteorites and of the meteoric streams which are quite incomprehensible, but enough has been said to show that the problem is full of interest to inquiring minds. * * * * *