per second. At its present rate of speed—and if Vega is really a "fixed" planet—then our sun would reach it in 320,000 years. However, it is a known certainty that the quantity of matter that is invisible is so much greater than the visible that the visible may be ignored. There may, too, be hundreds of millions of dark bodies, extinct constellations far larger than our own sun. Any one of these could approach our solar system and annihilate it with its impact for, in passing the orbit of the earth on their way around the sun, they attain a regular velocity of 26½ miles per second. If one of these dark comets should overtake the earth and strike it, the velocity of impact would be about eight miles per second; but if it should meet the earth in a head-on collision, the speed, when it struck, would be forty-five miles per second, a momentum beyond the power of the brain to fathom—indeed, man can not think of sixty miles per minute. Let a solid nucleous collide with the earth and imagination would reel at the result.
The earth moves over 18½ miles every second, and this added to or subtracted from 26½ makes 45 or 8. If a comet should strike at right angles to the direction of the earth's motion the speed of collision would be 26½ miles. But 8, 26½ or even 15 would hurl destruction if large enough.
A visible change is taking place in the giant sun Betel-