We first seek the velocity of light. In Fig. 4 the earth is 92,500,000 miles from the sun at E; Jupiter is 480,000,000 miles from the sun at J. It has four moons: the inner one goes around the central body in forty-two hours, and is eclipsed at every revolution. The light that went out from the sun to M ceases to be reflected back to the earth by the intervention of the planet Jupiter. We know to a second when these eclipses take place, and they can be seen with a small telescope. But when the earth is on the opposite side of the sun [Page 23] from Jupiter, at E', these eclipses at J' take place sixteen and a half minutes too late. What is the reason? Is the celestial chronometry getting deranged? No, indeed; these great worlds swing never an inch out of place, nor a second out of time. By going to the other side of the sun the earth is 184,000,000 miles farther from Jupiter, and the light that brings the intelligence of that eclipse consumes the extra time in going over the extra distance. Divide one by the other and we get the velocity, 185,000 miles per second. That is probably correct to within a thousand miles. Methods of measurement by the toothed wheel of Fizeau confirm this result. Suppose the wheel, Fig. 5, to have one thousand teeth, making five revolutions to the second. Five thousand flashes of light each second will dart out. Let each flash travel nine miles to a mirror and return. If it goes that distance in 1/10000 of a second, or at the rate of 180,000 miles a second, the next tooth will have arrived before the eye, and each returning ray be cut off. Hasten the revolutions a little, and the next notch will then admit the ray, on its return, that went out of each previous notch: the eighteen miles having been traversed meanwhile. The method of measuring by means of a revolving mirror, used by Faucault, is held to be even more accurate. [Illustration: Fig. 5.--Measuring the Velocity of Light.] When we take instantaneous photographs by the exposure [Page 24] of the sensitive plate 1/20000 part of a second, a stream of light nine miles long dashes in upon the plate in that very brief period of time. The highest velocity we can give a rifle-ball is 2000 feet a second, the next second it is only 1500 feet, and soon it comes to rest. We cannot compact force enough behind a bit of lead to keep it flying. But light flies unweariedly and without diminution of speed. When it has come from the sun in eight minutes, Alpha Centauri in three years, Polaris in forty-five years, other stars in one thousand, its wings are in nowise fatigued, nor is the rapidity of its flight slackened in the least. It is not the transactions of to-day that we read in the heavens, but it is history, some of it older than the time of Adam. Those stars may have been smitten out of existence decades of centuries ago, but their poured-out light is yet flooding the heavens.