Astronomers had long examined the neighbourhood of Mars with very powerful telescopes, in the hope of discovering Martian moons. But the hope had so thoroughly been abandoned for many years that the planet had come to be known as "moonless Mars." The construction, however, of the fine telescope which has been mounted at Washington, with an object-glass twenty-six inches in diameter, caused at least American astronomers to hope that after all a Martian moon or two might be discovered. Taking advantage of the exceptionally favourable opportunity presented during the planet's close approach to our earth in the autumn of 1877, Prof. Asaph Hall, of the Washington Observatory, paid special attention to the search for Martian moons. At last, on August 16, 1877, he detected close by the planet a faint point of light, which he was unable to examine further at the time (to see if it behaved as a satellite, or as one of the fixed stars). But on the 18th he saw it again, and determined its nature. He also saw another still fainter point of light closer to the planet; and subsequent observations shewed that this object also was a satellite. During the next few weeks both the moons were observed as closely as possible, in fact, whenever weather permitted, and the result is that we now know the true nature of their paths. In fig. 23 these paths are shown as they appeared in 1877. Of course the paths themselves are not seen; but if the satellites left behind them a fine train or wake of light, the shape of this train would be as shown in fig. 23. The satellites themselves could not be shown at all in a picture on so small a scale--the diameter of either would certainly be less than the cross-breadth of the fine elliptical line representing its track. The size of the planet is correctly indicated, and the true pose of the planet in 1877 is shown in the figure, his southern pole being somewhat bowed towards the earth. This is the uppermost pole; for the figure represents the planet and his satellites' orbits as they would appear in an astronomical telescope, which inverts objects. [Illustration: Fig. 23.--Mars and the paths of the Martian satellites as at present situated.] The outer satellite is probably not more than ten miles or so in diameter, the inner one, perhaps, the same; but neither can be measured. In the most powerful telescopes they appear as mere points of light. Nor is it easy to determine, from their lustre, or rather from their faintness, their true dimensions; for we cannot compare them directly in this respect with objects of known size, because their visibility is partly affected by the proximity of the planet, whose overpowering light dims their feeble rays. This remark applies with special force to the inner satellite.