Besides these phenomena, which prove that Mars is surrounded by an atmosphere having a great deal of similarity to our own, a further proof is afforded by the fact that the dark spots, which appear sharply defined and black when they are seen near the centre, become less and less visible as they advance towards the limb, and are totally invisible before they reach it. Moreover, the spectroscope also indicates the existence of an atmosphere, and even the presence of watery vapor in it. A very curious state of the Martial atmosphere is revealed by my observations of 1877-78. During eight consecutive weeks, from December 12th to February 6th, a whole hemisphere of the planet--precisely that represented on Plate VIII.--was completely covered by dense vapors, or a thick fog which barely allowed the dark spots to be seen through it, even when they were in the centre of the disk. The opposite hemisphere of Mars appeared just as clear and calm as possible; there all the spots and their minutest details could be seen, and when the planet was observed at the proper time, the line separating the foggy from the clear side was plainly visible. The reddish tint observed on the continents of Mars has been supposed by some astronomers to be the real color of the atmosphere of this planet. But, for many reasons, this explanation is not acceptable. Besides the fact that the border of the planet appears white, while it should be more red than the other part, owing to the greater depth of atmosphere there presented to us, the polar spots, the white bands along the sea-coasts, and the cloud-like forms appear perfectly white, not the slightest tint of red being visible on them, as would be the case if these objects were seen through an atmosphere tinted red. Other astronomers have supposed that the vegetation of this planet has a reddish color; but this is not supported by observation. It has been again supposed, with much more probability, that the surface of Mars is composed of an ochreous material which gives the planet its predominant ruddy color. Until lately Mars was supposed to be without a satellite, but in August, 1877, Professor Hall, of the Washington Observatory, made one of the most remarkable discoveries of the time, and found two satellites revolving around this planet. These satellites are among the smallest known heavenly bodies, their diameter having been estimated at from 6 to 10 miles for the outer satellite, and from 10 to 40 miles for the inner one.