82 



KNOWLEDGE. 



[Apbil, 1902. 



latitude (iO° at least, no tremor from the soil oominp to 

 "unlock the molecular embraces " of crystallization. Hut 

 1 hough enfeebled l\v distance, the efficiency of solar radia- 

 tion in melting or vaporizing snow has been observed to be 

 so powerful on Mars, that we should se.ircely exi>ect it to have 

 no other effect on the phenomena presented -by the planet. 



Experience agrees well with this anticipation of 

 seasonal change. Were all the dark areas of the planet 

 lo be unfreezing seas, it would, perhaps, be difficult to find 

 how seasonal change would affect their colour. B\it a few 

 of the most prominent " Maria" (like the Mare Acidalium, 

 for instauce) have been observed dark in spring, discoloured 

 in summer, and faiut iu autumn, thus suggesting change 

 subordinated to the planet's seasons, and due, according 

 to Prof. W. H. Pickering, to some sort of vegetation. 



It seems thus highly probable that part of the grey 

 markings of Mars are vast vegetation tracks. Others 

 might be water areas. At any rate, both vegetation and 

 water would present nearly kindred phenomena at the 

 great distance w-e ai-e viewing the planet. 



Now, the detection of delicate detail on Mars is by far the 

 most trying and exhausting of all astronomical observations. 



FiQ. 1. — Mars in 1900-1901. Longitude of the centre of the disc =0". 



Sustained by an enthusiasm whose reward is an inward ap- 

 proval only, the astronomer keeps his eye to the telescope 

 for hours together, sometimes seeing very little indeed, but 

 recording all fugitive impressions, objective as well as 

 .subjective. This is almost transcending ordinary experience 

 and treading into somewhat dangerous ground. An in- 

 structive instance of the manner in which illusion creeps 

 into the Avork of eminent observers is furnished by the 

 observations of Venus made at Flagstaff, iu America. 

 " The markings (canals) of Ven\is," says Mr. Lowell, " are 

 as distinct really as those of the Moon." It would be 

 useless to insist on the fallacy of this statement — a fallacy 

 obvious to the beginner in star gazing. But the remark 

 is exceedingly valuable, as establishing beyond doubt the 

 existence of what might be called the canaliform illusion. 

 And, if subjective canals can be seen as harsh as the 

 markings on the Moon, it would not be illogical to believe 

 them to assume all shades of intermediate intensitv, 



beginning with the greatest brightness of planetary discs 

 in order to culminate in the above climax of darkness. 



Nothing is easier then, in appearance, with the view of 

 avoiding the canal deadlock, than to take the obvious step, 



Frs 2.— Mars in 1900-1901. Long. = 90°. 



and brand with illusion the whole canal network of Mars, 

 the argument being that imaginary canals have been seen 

 on Mercury, Venus, and the satellites of Jupiter, and that, 

 after all, canal digging could not reasonably be the leading 



Fio. 3.— Mars in 1900-1901 



= ISO". 



preoccupation of our neighbours in space. But a more 

 careful examination of the matter will infallibly lead to 

 the fact that, contrary to those of Mars, the canals of 



