So much on the character of that dark little round spot, the passage of which across the sun hundreds of astronomers, with all manner of telescopes, spectroscopes, and photographic instruments, will watch, examine, measure, and record, as they see it sweeping on in its course on the 8th of next December. What will be the special purpose animating observers as they view the transits of 2004 and 2012—if, despite the prophetic and apocalyptic Dr. Cumming, the world lasts till then—no one can now tell. Astronomy by that time may be advanced as far beyond the present state of the science as the present state surpasses the state of two centuries ago. It is probable that new and, to that generation, most interesting questions may have then arisen, which they will strive to solve by their observations of the transits—questions now perhaps undreamed of. But at present our astronomical world is deeply impressed with the advantage and necessity of definitely ascertaining the distance of the earth from the sun. This is the paramount, though by no means the only, purpose of all this expenditure of time and skill and money in preparing for, in making the observations, and afterwards in laboriously working out the results. How, by merely looking never so attentively at an object whose distance you do not know, as it stands in a line with, and perhaps far in front of, another, likewise of unknown distance, you can tell how far off that second object is, may seem as difficult as the king’s requirement of the prophet first to tell him the dream he had forgotten, and then to explain its meaning. It might seem almost an impossibility; but a few words will explain how the difficulty is turned by availing ourselves of other data. When two planets, as is the case with the earth and Venus, both revolve in elliptical orbits around the sun, in virtue of the law of gravitation, then their respective times of orbital revolution are to each other as the cubes of their respective mean distances from the sun. This is one of the laws of Kepler. It was announced by him as the wonderful result of seventeen long years of calculations. He took the data given by the observations of Tycho Brahe and of others, and those made by himself. He tried, by every imaginable form of arithmetical supposition, to combine them together somehow, and under the form of some mathematical law. This was his last result, perhaps the most surprising result of hard plodding, long‐continued labor in the field of science. All honor to his memory. There are few discoveries in the mathematics of astronomy to be compared to this and the other laws of Kepler. He established them as experimental facts. The mathematical reason of them he did not learn.