Page:The Solar System - Six Lectures - Lowell.djvu/27



It is a conceptual dividing line between ellipses and hyperbolas, the paling between the sheep and the goats.

Now the planets all move in ellipses. They are therefore under the Sun's control and form part of his system. Occasionally stones fall out of the sky on to the earth. Suddenly a flash occurs overhead, a detonation follows, and then if the observer be near enough, a mass of stone or iron is seen to bury itself in the ground. This is a meteorite, aerolite, or bolide, a far wanderer come at last to rest. The flash, the report, and the fused exterior of the mass found are due to the meteor's striking against our air. The bodies enter the upper atmosphere at speeds of from ten to forty miles a second, and such speeds are equivalent to immersing them in a blow-pipe flame of a temperature of many thousands of degrees. For the temperature of a gas is as the mean velocity-square of its molecules, and the rush of the meteor produces the same effect as if the molecules of the air were moving and the air therefore very hot. Its outward condition is a consequence of the last stage in its journey, but its inner state at times continues to bear witness to a previous