252 



KNOWLEDGE. 



[November 1, 1694. 



readily make it indistinguishable from one of the " con- 

 tinents," or from one of the " seas," as the case may be ; 

 cloud on Mars necessarily taliing the form of our lightest 

 and highest terrestrial clouds, rather than that of the 

 densest and heaviest. ^Yhen the difference of tone in 

 two contiguous markings is but small, but a little defect 

 in the transparency and steadiness of our own atmosphere 

 will be sufficient to render them indistinguishable. 



It is easy to see, if these causes are the principal 

 reasons for the apparent changes on the planet, that 

 different markings will have two or three forms under 

 which they present themselves, but will not pass through 

 an indefinite number of changes. This is actually the 

 case in more than one locality. To take perhaps the best 

 authenticated case, the marking to which reference has 

 akeady been made, Herschel II. Strait is sometimes closed 

 at its western end, and sometimes open, its southern shore 

 being then a cigar-shaped island. The first sketch in 

 Fig. 8, from Mr. Green's fine series of drawings, made at 

 Madeira, in 1877, illustrates the latter phase ; so do also 

 the first, third, sixth, and seventh of M. Guillaume's 

 sketches in Fig. 4. But more frequently the Phillips 

 Island of Green's chart is seen, not as an island, but as 

 an elbow-shaped promontory — the DeucaUonis Regio of 

 Schiaparelli. 



The rough little experiments to which I have alluded 

 may, I think, throw some light on the "canal system." 

 It must not, of course, be imagined that a power of 100 

 on Mars when 20" in diameter will show it with equal dis- 

 tinctness to the moon as seen w^ithout telescopic assistance ; 

 nor, if a line 8" in breadth be visible to the naked eye, 

 will a power of 400 show a line 02" in breadth on Mars, 

 even in the steadiest air. To begin with, all the contrasts 

 on Mars are subdued. Then, the gain by increasing the 

 power of the eyepiece is always less than the numerical 

 ratio of the magnification, till a point is reached when it 

 vanishes, either on account of optical limitations, defects 

 of the instrument, or atmospheric conditions. But a 

 narrow dark line run be seen when its breadth is far less 

 than the diameter of the smallest visible dot. Further, a 

 line of detached dots will produce the impression of a 

 continuous line, if the dots be too small or too close 

 together for separate vision. There are some intimations 

 that this may be the next phase of the " canal " question, 

 Mr. Gale, of Paddington, New South Wales, having broken 

 up one " canal " into a chain of " lakes " on a night of 

 superb definition, Mars being near the zenith, and Prof. 

 W. H. Pickering, at Arequipa, having under equally 

 favom'able circumstances detected a vast number of small 

 " lakes" in the general structure of the " canal system." 



If this be so, if the canals are, generally speaking, 

 beyond the limit of distinct definite vision, but producing 

 the impression of lines, it will be readily understood how 

 it is that different observers differ so widely as to their 

 breadth and as to their character, whether diffused or 

 sharp. If they are too narrow to be dejinedh/ seen, and 

 yet broad and dark enough to make their presence felt, 

 this is precisely what would ensue. The apparent breadth 

 of the " canal " would vary with the definition each 

 observer enjoyed, and with his persanal idiosyncrasies. 

 Indeed, one man might see the veritable " canal " itself 

 as a hard, sharp, well-defined line — if that be its actual 

 character — whilst to another, less fortunate in bis climate, 

 his telescope, or his sight, it would only be a diffused 

 shade. 



That the "canals" are actually very narrow may be 

 inferred from another circumstance. When Dawes dis- 

 covered his "Forked Bay," he looked particularly to see 

 if he could detect any rivers flowing into these two seeming 



estuaries, but found none. Now I found that I required 

 to take the distance between the points of the " Fork " on 

 a drawing, not as 8", but as more than twenty times 

 as much, as fully B' of arc in order to see the " bay " as 

 Dawes described it. (This may serve to show how great is 

 the difference between defining an object and merely 

 discerning it.) The actual distance of the points in 

 Martian longitude is 8^ It is practically certain that if the 

 " canals " which flow into the " Fork " had had a breadth 



Jia. 4. — Drawings of Mars, 

 Flammarion's 



by Guillaume, in 1S90. 

 " Mars," p. 477. 



From 



of one-sixteenth of that distance, or Oo" of a Martian 

 great circle, Dawes .could not have overlooked them, 

 provided the " canal " was as dark as the " bay." I am 

 inclined to think that the actual breadth of the rank and 

 file of the "canals" cannot exceed this hmit, if they are 

 truly continuous lines, and as dark in tint as the " seas." 

 If they are fainter than the " seas," or discontinuous, then 

 they must make up in breadth what they lack in darkness 

 or continuity. Their angular breadth in a favourable 

 opposition would then be about 0-1", and they would be 

 beyond the limits of defined vision, except on the rarest 

 occasions. 



The disappearance, reappearance, and duplication of the 

 " canals " would easily explain themselves. If my theory 

 is correct they are all always present, both the " canals " 

 and their duplicates, but being so close to the limit of 

 vision, a trifling circumstance will bring them within it, 

 or remove them without it. Suppose we take the popular 

 opinion that they are watercourses, then an increase in 

 breadth no greater than our own rivers frequently show, 

 an increase of turbidity, or a greater transparency in the 

 atmosphere abo%'e a " canal," will bring one ordinarily 

 invisiljle into view. 



Whether this be so or not, I should like once again to 

 emphasize what I feel to be an important point, not only 

 with regard to the sun and to Mars, but to all the 

 planets. W^e cannot assume that what we are able to 

 discern is really the ultimate structure of the body we are 

 examining. 



