Everyone has seen the so-called shooting-stars.  They break out with a sudden brilliancy, shoot a few degrees with quiet speed, and are gone before we can say, “See there!” The cause of their appearance, the [Page 120] conversion of force into heat by their contact with our atmosphere, has been already explained.  Other facts remain to be studied.  They are found to appear about seventy-three miles above the earth, and to disappear about twenty miles nearer the surface.  Their average velocity, thirty-five, sometimes rises to one hundred miles a second.  They exhibit different colors, according to their different chemical substances, which are consumed.  The number of them to be seen on different nights is exceedingly variable; sometimes not more [Page 121] than five or six an hour, and sometimes so many that a man cannot count those appearing in a small section of sky.  This variability is found to be periodic.  There are everywhere in space little meteoric masses of matter, from the weight of a grain to a ton, and from the density of gas to rock.  The earth meets 7,500,000 little bodies every day—­there is collision—­the little meteoroid gives out its lightning sign of extinction, and, consumed in fervent heat, drops to the earth as gas or dust.  If we add the number light enough to be seen by a telescope, they cannot be less than 400,000,000 a day.  Everywhere we go, in a space as large as that occupied by the earth and its atmosphere, there must be at least 13,000 bodies—­one in 20,000,000 cubic miles—­large enough to make a light visible to the naked eye, and forty times that number capable of revealing themselves to telescopic vision.  Professor Peirce is about to publish, as the startling result of his investigations, “that the heat which the earth receives directly from meteors is the same in amount which it receives from the sun by radiation, and that the sun receives five-sixths of its heat from the meteors that fall upon it.”

[Illustration:  Fig. 49.—­Bolides.]

[Page 121] In 1783 Dr. Schmidt was fortunate enough to have a telescopic view of a system of bodies which had turned into meteors.  These were two larger bodies followed by several smaller ones, going in parallel lines till they were extinguished.  They probably had been revolving about each other as worlds and satellites before entering our atmosphere.  It is more than probable that the earth has many such bodies, too small to be visible, revolving around it as moons.

[Illustration:  Fig. 50.—­Santa Rosa Aerolite.]

Aerolites.

Sometimes the bodies are large enough to bear the heat, and the unconsumed centre comes to the earth. [Page 123] Their velocity has been lessened by the resisting air, and the excessive heat diminished.  Still, if found soon after their descent, they are too hot to be handled.  These are called aerolites or air-stones.  There was a fall in Iowa, in February, 1875, from which fragments amounting to five hundred