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 ASTRONOMY the observers, due to the fact that the limb of the planet nearest the sun was so much brighter than the rest of the disc that the observers of the apparent centre set very nearly on the limb, or from the assumed diameter being too small, as supposed by Mr Lowell. Schiaparelli made his well-known studies upon Venus contemporaneously with those upon Mercury, and his Venus conclusions were summed up in five brief notes read to the Milan Academy during the year 1890. The first four of these are devoted to a very full and exhaustive history of all existing telescopic observations upon the supposed spots of the planet from the time of Bianchini (1728), the result being that no positive conclusions can be drawn from any of the observations, either as to permanent markings on the planet or its time of rotation. Herschel had devoted special attention to the subject, and, as is well known, could find no evidence of any well-defined period of rotation. The first definite clue to a conclusion which Schiaparelli could obtain, was suggested by the long continuance of two faint spots near the southern cusp of the planet. It is true that these spots were not always visible; yet, under the best conditions, they could be seen at some hour every day for weeks at a time, and sometimes during several hours on the same day. He agreed with Herschel and others of the best astronomers in concluding that these spots were not really permanent, but disappeared at the end of a few weeks at most. At the same time the faint temporary spots which he succeeded in discerning remained immovable for several hours in succession, thus precluding the idea of previous observers that the planet rotated in a period not very different from twenty-four hours. Moreover, their continued observation for several weeks seemed to show that their rotation must be very slow, and their period several months, perhaps equal to that of the revolution of the planet round the sun. Lowell reached a similar but more definite conclusion, maintaining that the planet really did always present the same face to the sun. But in the opinion of the most conservative astronomers, the whole subject of the rotation, both of Mercury and Venus, is still in doubt. The supposed spots are so indefinite, that it is not safe to rest a positive conclusion on them. The most recent and most definite result is that of Belopolsky at Pulkowa, who determined by the spectrographic method the difference of the velocities with which various points of the circumference of the disk of Venus were moving to or from us. The mean of a great number of measures gave a velocity of rotation of 0‘9 km. per second, corresponding to a period of twelve hours. So short a period seems incompatible with the absence of sensible ellipticity in the figure of the planet, yet the observations greatly weaken Schiaparelli’s conclusion (Ast. Nach. No. 3641). Conclusive evidence that Venus is surrounded by a transparent, or at least a translucent atmosphere, was gathered at the transit of Venus in 1882, and scarcely less conclusive was the evidence that this atmosphere bears a great volume of clouds, which probably would completely obscure the surface of the planet. After onehalf of the dark body of the planet had entered upon the disc of the sun, that portion of its outline without the sun became visible as a thin line of light extending around its border. This line can be attributed to no other cause than the refraction of the sun’s rays produced by an atmosphere surrounding the planet. Had this atmosphere been quite transparent, we should have expected the line of light to make its first appearance at the most distant point A (see Fig. 1) of the circumference of the planet, owing to the fact that before the planet had half entered, the only rays that could be refracted to us would necessarily pass near this region.

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From this point, A, the line of light should have extended in both directions, until, after more than half the planet was on the solar disc, it would have been seen complete. Instead of this the light first showed itself along a small arc at the point B. A few minutes later, small portions of it began to glimmer here and there around other parts A

of the dark circumference, but it was not until the planet was almost entirely on the disc that the arc of light became plainly continuous. It is difficult to attribute this phenomenon to any other cause than clouds floating in the lower region of the atmosphere, and possibly an irregularly translucent condition of the latter rather than entire transparency. There is a class of telescopic observations which, at first sight, seem incompatible with those made about the time of a transit. At intervals of eight years before and after a transit, Venus passes very near the sun. On such an occasion, in 1866, Lyman, at New Haven, saw the entire outline of the planet as an exceedingly fine ring of light, and on 1st and 5th December 1890 Barnard saw the entire outline except about 20°. An atmosphere refracting rays so strongly that the illumination of the entire planet would be visible under such circumstances, ought to show a bright arc of light on the side farthest from the sun, whereas, in fact, this was always the darkest portion of the outline. A very elaborate discussion of all the observations of this class has been made by H. N. Bussell (Astrophysical Journal, ix. 284), who proposes the explanation that the visible outline which we see is not the illuminated surface of the planet but the upper illuminated region of the atmosphere, bearing suspended particles of dust or fog. He also concludes that the atmosphere of Venus is decidedly rarer than that of the earth. This seems the best explanation of the seeming paradox that has yet been offered. The earlier telescopic observers had no difficulty in discerning bright and obscure regions on the surface of Mars, and in concluding from the varying Mars aspects of these regions that it rotated on its axis in a period of a little more than twenty-four hours. The only important additions to our knowledge of the surface between the time of Herschel and Schroeter, and the opposition of 1877, were embodied in maps of the planet, which agreed in the main features, but differed greatly in details. At the opposition of 1877, the planet was unusually near the earth; and although its south declination was not favourable to observations in the northern hemisphere, Schiaparelli instituted a series of studies which, continued through several subsequent oppositions, have given rise to a large mass of literature on the subject. Observers had long seen, or fancied they could see, a remarkable resemblance between the surface of Mars and that of the earth. The brighter regions were supposed to be continents; the darker ones oceans. Schiaparelli’s great discovery was that these supposed oceans were connected by comparatively narrow streaks, to which he applied the term channels (canale), a word which, through an unfortunate misconception, has been translated into English as “ canal,” thus giving rise to S. I. — 92