THE STRANGE MARKINGS ON MARS. 51 



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 ap- 

 pearance of the canals can be ascribed to similar causes is a ques- 

 tion 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 observa- 

 tion. Moreover, he finds here a striking appearance of connection 

 with the seasons. The epoch most favorable for the disappear- 

 ance 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 un- 

 certain manner, by a slight shading which irregularly extends it- 

 self 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 mid- 

 dle, 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 de- 

 fined. 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 con- 

 dition, " 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 



