apparently hidden under a veil, the gradual withdrawal of which has again revealed their well-known contours, and in such cases the conclusion has seemed irresistible that what had been observed was the formation and subsequent dissipation of vast cloud areas concealing or obscuring the outlines of continents and seas beneath them. So the indistinctness near the edges of the disk and the altered appearance of the planetary features there have been partially ascribed to atmospheric influences as well as to the effects of perspective. Whether the observed changes in the appearance of the canals can be ascribed to similar causes is a question which can not yet be solved. Schiaparelli appears to think that they are principally due to something which occurs on the surface of the planet itself, and which something, in its turn, depends upon the changes of the seasons.
In order to form any opinion whatever upon this question it is necessary to examine a little more closely the varying aspects of the canals. Their discoverer has noted these four points:
1. A canal may remain invisible for a longer or shorter time. This invisibility, he insists, is an actual disappearance of the canal, and is not due simply to unfavorable circumstances of observation. Moreover, he finds here a striking appearance of connection with the seasons. The epoch most favorable for the disappearance of the canals is near the time of the southern solstice of Mars, which, as we have seen, occurs when the planet is nearest to the sun.
2. In many cases, according to Schiaparelli, the presence of a canal begins to manifest itself to the eye in a very vague and uncertain manner, by a slight shading which irregularly extends itself in the direction of its length. This phenomenon is so delicate that, he says, it marks, as it were, the limit between visibility and invisibility.
3. Very often the canals present the appearance of a gray band fading out on each side and having the deepest shade in the middle, which may be dark enough to suggest the appearance of a more or less clearly marked line. Sometimes, but rarely, one side of the band alone is nebulous or indistinct, the other being clearly defined. Various other anomalous appearances have been observed.
4. The most perfect type of the canals, and that which their discoverer says he regards as the expression of their normal condition, "is a dark line, sometimes quite black and well defined, looking as if it had been traced with a pen on the yellow surface of the planet." When the canals appear in this form they are very uniform throughout their length, and Schiaparelli says, on the rare occasions when he has been able to clearly distinguish the two edges, one from the other, he has discerned slight sinuosities or scollopings on the borders. He adds that the width of a canal may change with time from a thread, barely perceptible in the
