Lambert regards _five hundred millions_ as a very moderate estimate![1] [1. Guillemin, "The Heavens," p. 251.] {p. 83} And this does not include the monstrous fiery wanderers who may come to visit us, bringing their relations ### ORBITS OF THE PERIODIC COMETS. along, from outside the solar system--a sort of celestial immigrants whom no anti-Chinese legislation can keep away. Says Guillemin: "Leaving mere re-appearances out of the question, _new comets are constantly found to arrive from the depths of space_, describing around the sun orbits which testify to the attractive power of that radiant body; and, for the {p. 84} most part, going away for centuries, to return again from afar after their immense revolutions."[1] But do these comets come anywhere near the orbit of the earth? Look at the map on the preceding page, from Amédée Guillemin's great work, "The Heavens," page 244, and you can answer the question for yourself. Here you see the orbit of the earth overwhelmed in a complication of comet-orbits. The earth, here, is like a lost child in the midst of a forest full of wild beasts. And this diagram represents the orbits of only six comets out of those seventeen millions or five hundred millions! It is a celestial game of ten-pins, with the solar system for a bowling-alley, and the earth waiting for a ten-strike. In 1832 the earth and Biela's comet, as I will show more particularly hereafter, were both making for the same spot, moving with celestial rapidity, but the comet reached the point of junction one month before the earth did; and, as the comet was not polite enough to wait for us to come up, this generation missed a revelation. "In the year 1779 Lexell's comet approached so near to the earth that it would have increased the length of the sidereal year by three hours if its mass had been equal to the earth's."[2] And this same comet did strike our fellow-planet, Jupiter. [1. "The Heavens," p. 251. 2. "Edinburgh Review," October, 1874, p. 205.] {p. 85} In the years 1767 and 1779 Lexell's comet passed though the midst of Jupiter's satellites, and became entangled temporarily among them. But not one of the satellites altered its movements to the extent of a hair's breadth, or of a tenth of an instant."[1] But it must be remembered that we had no glasses then, and have none now, that could tell us what were the effects of this visitation upon the surface of Jupiter or its moons. The comet might have covered Jupiter one hundred feet--yes, one hundred miles--thick with gravel and clay, and formed clouds of its seas five miles in thickness, without our knowing anything about it. Even our best telescopes can only perceive on the moon's surface--which is, comparatively speaking, but a few miles distant from us--objects of very great size, while Jupiter is sixteen hundred times farther away from us than the moon.