In the course of its wanderings the body comes near the earth, and within a few hundred miles of its surface begins to encounter the upper surface of the atmosphere with which the earth is enclosed. To a body moving with the appalling velocity of a meteor, a plunge into the atmosphere is usually fatal. Even though the upper layers of air are excessively attenuated, yet they suddenly check the velocity almost as a rifle bullet would be checked when fired into water. As the meteor rushes through the atmosphere the friction of the air warms its surface; gradually it becomes red-hot, then white-hot, and is finally driven off into vapour with a brilliant light, while we on the earth, one or two hundred miles below, exclaim: "Oh, look, there is a shooting star!"
We have here an experiment illustrating the mechanical theory of heat. It may seem incredible that mere friction should be sufficient to generate heat enough to produce so brilliant a display, but we must recollect two facts: first, that the velocity of the meteor is, perhaps, one hundred times that of a rifle bullet; and, second, that the efficiency of friction in developing heat is proportional to the square of the velocity. The meteor in passing through the air may therefore develop by the friction of the air about ten thousand times as much heat as the rifle bullet. We do not make an exaggerated estimate in supposing that the latter missile becomes heated ten degrees by friction; yet if this be admitted, we must grant that there is such an enormous development of heat attending the flight of the meteor that even a fraction of it would be sufficient to drive the object into vapour.
Let us first consider the circumstances in which these external bodies are manifested to us, and, for the sake of illustration, we may take a remarkable fireball which occurred on November 6th, 1869. This body was seen from many different places in England; and by combining and comparing these observations, we obtain accurate information as to the height of the object and the velocity with which it travelled.
It appears that this meteor commenced to be visible at a point ninety miles above Frome, in Somersetshire, and that it vanished twenty-seven miles over the sea, near St. Ives, in Cornwall. The path of the body, and the principal localities from which it was observed, are shown in the map (Fig. 75). The whole length of its visible course was about 170 miles, which was performed in a period of five seconds, thus giving an average velocity of thirty-four miles per second. A remarkable feature in the appearance which this fireball presented was the long persistent streak of luminous cloud, about fifty miles long and four miles wide, which remained in sight for fully fifty minutes. We have in this example an illustration of the chief features of the phenomena of a shooting star presented on a very grand scale. It is, however, to be observed that the persistent luminous streak is not a universal, nor, indeed, a very common characteristic of a shooting star.
Fig. 75.—The Path of the Fireball of November 6th, 1869.
The small objects which occasionally flash across the field of the telescope show us that there are innumerable telescopic shooting stars, too small and too faint to be visible to the unaided eye. These objects are all dissipated in the way we have described; it is, in fact, only at the moment, and during the process of their dissolution, that we become aware of their existence. Small as these missiles probably are, their velocity is so prodigious that they would render the earth uninhabitable were they permitted to rain down unimpeded on its surface. We must, therefore, among the other good qualities of our atmosphere, not forget that it constitutes a kindly screen, which shields us from a tempest of missiles, the velocity of which no artillery could equal. It is, in fact, the very fury of these missiles which is the cause of their utter destruction. Their anxiety to strike us is so great, that friction dissolves them into harmless vapour.