stretches of time much longer still. We know from the testimony of the rocks that our earth has been the abode of living creatures for periods which it seems impossible to express in less than thousands of thousands of years. It therefore seems an interesting question to investigate the possible amount of transformation which the heavens have undergone in, let us say, a million of years.
The line of reasoning we shall employ suffices at all events to show how mighty is the transformation which has occurred within geological periods. The changes in the heavens are as profound as the changes in the earth. Let us consider the case of a star, or other celestial body, which moves through space at the constant rate of 20 miles a second. I have adopted this particular velocity as fairly typical of sidereal motions generally. It is rather larger than the speed with which the earth moves in its orbit. The velocities of many of the stars are, however, known to be quite as great as that which we have assumed. Indeed, in the case of many stars the speed is greatly in excess of 20 miles a second. There are several stars certainly known to be moving twice as fast; nor are speeds considerably higher than even this unobserved. We may, for instance, mention the famous star known to astronomers as Groombridge No. 1830, which hurries along at the rate of at least 200 miles a second. Indeed, in some cases stellar velocities are attained which appear to be even greater than that just mentioned. We do not therefore make any extravagant supposition in adopting a speed of 20 miles per second, as the basis of our calculation. This being granted it is now a simple problem to discover with sufficient approximation the change in apparent visibility which such a
