490 



NA TURE 



[February 24, 1910 



tory in 1894. Now, when these maps came to be 

 scrutinised for the canals, each of them failed to show 

 any such features. Nor had any observer previous 

 to 181:14 recorded them, as the observatory library of 

 the subject bore witness. Schiaparelli had never seen 

 them, nor had his predecessors or successors. This 

 determined definitely that no human eye had ever 

 looked upon them before. But, stirring- as it is to 

 know that one is the first to see a new g-eographical 

 feature on another planet, akin to the thrill of finding 

 unknown land in our own Antarctic regions, a much 

 deeper scientific interest attaches to the question 

 whether a phenomenon previously undiscovered was 

 also previously non-existent. For in that case one 

 has seen something come into being, with all that 

 such origination implies. 



It might seem to persons not versed in the subject 

 that its absence on the charts was proof that a canal 

 was itself new in the second sense because it was so 

 in the first ; but study of Mars has shown that this 

 cannot be taken off-hand for granted. Several points 

 must each be carefully considered. In the first place, 

 one must be sure that the phenomenon could have been 

 seen before, yet was not. It must be of a size which 



could not have escaped detection previously. Now, 

 the great majority of canals discovered here were be- 

 yond the hope of detection elsewhere, owing to the 

 character of the air, the improvement in instrumental 

 means, and the long acquired knowledge of the 

 observers. That they were not seen by Schiaparelli, 

 therefore, by no means implies that thev did not exist 

 in his day — or even in the earlier days of observation 

 here. We see to-day vastly more than we did in i8q4, 

 because of the experience acquired since. In the 

 present case, however, this possibility of error was 

 excluded by the size of the canals in question. They 

 were not difficult detail of the order here mentioned, 

 but, as I have said, the most conspicuous on the disc, 

 canals which no observer of any standing whatever in 

 good air could possibly pass by. They would strike 

 any skilled observer of such matters the moment he 

 looked at the planet. So far as this point went, then, 

 they could not have existed before. 



The next point to be considered was whether they 

 were results of a characteristic of the planet of vital 

 import in its cartography — the annual seasonal change 

 which affects all its features. For the world of Mars 

 is as subject to recurrent seasonal change as our own, 

 NO. 2104, VOL. 82] 



and more markedly so. This change stamps itself 

 unmistakably upon all its features, obliterating some 

 and bringing into prominence others according to 

 the time of the Martian year. Examples of this occur 

 in the study of Mars regularly at each opposition, the 

 aspect of the disc varying according to a definite law 

 dependent on the Martian season. To be sure, there- 

 fore, that a canal is itself new, the planet must have 

 been previously carefully depicted at the same season 

 of its vear, and then when these earlier drawings are 

 critically scanned the canal in question not found re- 

 corded on them. Now, the possibility of definite and 

 conclusive intercomparison of the sort is not presented 

 so frequently as one might think. Mars comes to 

 opposition each time later and later by about two 

 and a quarter of our own months. This means that 

 we meet him in a different part of his orbit at each 

 fresh approach, and so at a different season of his 

 year. Now, until Schiaparelli 's time, it was at or 

 near opposition only that his face was studied. Schia- 

 parelli extended tlie time greatly, but not until the 

 subject was taken up at Flagstaff was the period of 

 observation prolonged to six and eight months for each 

 opposition epoch, thus enabling the same Martian 

 season to be recurrently viewed by the seasonal over- 

 lapping of two or more observation periods. 



But even so, the disc is not equally well presented 

 in successive Martian years, because of the differing 

 distances the two bodies are apart, and the difficulty 

 of consequent comparison on the score of size. Still 

 another difficulty in the way of parallelism is that of 

 phase. Unless the two bodies be exactly opposite at 

 the same season of the Martian year in the two cases, 

 Mars will show a differing phase at each, and this 

 means a different slant in the illumination. This is a 

 very important distinction, because the disc shows 

 very diversely when illuminated from above or from 

 the side, so diversely that faintness of detail has often 

 been attributed to intrinsic weakness of feature when 

 illumination itself was the cause. 



In consequence, the observer can never be quite sure 

 that his data are comparable until he has himself seen 

 the Martian disc under like conditions, or nearly such, 

 which recurrent presentations demand a lapse of 

 fifteen to seventeen years. 



Furthermore, to be conclusive, the observations 

 must all have been made by the same observer, work- 

 ing under like conditions, and grown, in consequence, 

 familiar with every detail of the disc, since the per- 

 sonal equation, including by that term the site and the 

 instrumental methods and equipment, is always a 

 factor. A Martian cycle, that is, a round of about 

 sixteen years, must have been gone through by the 

 same observer before definite judgment can be pro- 

 nounced. Such a cycle now stands complete at Flag- 

 staff. 



Examining the records here we find that Mars was 

 observed four times previously at the same season of 

 the Martian year as occurred during the epoch of the 

 appearance of these two canals. The canals were seen 

 at this opposition as follows : — ■ 



.909 '"eb^i've"" ^fp,i°"^- Opposition 



chronology ^ 



Northern 

 Hemispheie 

 First appearance Sept. 30 277° Jan. 6 4° 50' Sept. 23, 



1909 

 Last observed Dec. 12 320° Feb. 17 47° 43' 



The previous occasions on which the canals should 

 have been visible, if their appearance or non- 

 appearance were a consequence of the Martian season, 

 were : — 



