October 29, 1896] 



NA TURE 



62s 



MARS AS SEEN A T THE OPPOSITION IN 1894. 



SOME three years ago, M. Camilla Flanimarions 

 classical book on the " Planet Mars" was noticed 

 in these columns i'Xatuke, vol. xlvii. p. 553). This 

 work was a compilation of all the observations made up 

 to that period from the very earliest record, and a 

 thorough discussion of them was given, as was to be 

 expected, in a masterly way. Since that time, how- 

 ever, the planet's surface has been studied by observers 

 in numerous parts of the world, and their oljservations 

 have been published n various journals and in different 

 languages. Perhaps the most important, or at any rate 

 the most consecutive series, of such observations hails 

 rom Flagstaff, Arizona, Mr. Percival Lowell having, at 

 ^reat expense, equipped himself with some fine instru- 

 ments, and set out for that region to make a systematic 

 study of the surface markings during the opposition of 

 the planet in the year 1S94. 



It may at first be asked why an observer should 

 choose a place so far away, when most excellent 

 mstruments of large aperture are at work nearer home. 

 This question may be answered in a few words. For 

 a study of planetary details, a steady atmosphere is the 

 most essential thing to be secured ; the size of the 

 instrument, as Mr. Lowell says, being a matter of quite 

 secondary importance. To convince ourselves we have 

 only ti) recall the fact how .Schiaparelli, with quite a 

 moderate aperture, made numerous discoveries as regards 

 the canals and their doubling, when no one, even with 

 apertures double that of his telescope, could detect the 

 delicate tracery that he saw. It is well known among 

 astronomers that this observer has a marvellously keen 

 eye for observation, but e\en this would not account for 

 these great differences. 



Mr. Lowell, however, wished to set up his instrument 

 under the very best conditions obtainable ; and this is why 

 he finally settled upon .'\rizona, as not only was the planet 

 observable near the zenith there, but observation showed 

 that the air was as pure and as still as he could find 

 anywhere. The result has been that he was able to 

 work from .May 24, 1S94, to .April 3, 1895, practically 

 without a break of any importance during the whole of 

 this period, and the result of his labour will be found in 

 the first volume of the Annals of his observatory. This 

 volume, as one would naturally suppose, contains the 

 original data set out in great detail, but practically too 

 technically for the general reader. 



For a more general account of the observations, and 

 the results to be drawn from them, wc are indebted to 

 him in his book ".Mars,''' which has been recently 

 published, and of which we propose to give some account 

 n the present article. It must be remembered, before 

 proceeding, that in this volume the observations are 

 confined to those made at Flagstaff' by Mr. Lowell, 

 and associated with him Prof W. H. Pickering and Mr. 

 A. E. Douglass. 



It may be thought at first that any book on Mars, to 

 take a high place in the literature on planetary astronomy, 

 must refer to a great extent to the previous work done 

 by other observers. This, certainly, should generally be 

 the case, but there may occasionally be exceptions, and 

 this is one of them, when such a treatment would divert 

 the aim of the book in question. What Mr. Lowell here 

 does is to discuss his own beautiful series of observations 

 (a series quite unique as regards the number of con- 

 secutive days of observation), and to make, if possible, 

 plausible deductions from them. Personal equation seems 

 to play a very important role in the observation of 

 planetary detail, so the more this element is eliminated 

 by dealing with observations made by one man with one 

 instrument, the more should our knowledge o changes, if 

 they occur, be advanced. 



' "Mars," by Percival Lowell. (London: Longm-ins, Green, and Co.. 

 1896.) 



NO. 1409, VOL. 54] 



The subject-matter of this book is divided into six 

 subheads, and we cannot do better than consider each 

 separately. First, then, as regards the general character- 

 istics of the planet's disc. Here wc shall limit ourselves, 

 and only refer to the shape of the planet, as an interesting 

 discovery has been made with regard to it. The disc of 

 Mars generally appears perfectly round, but nevertheless 

 it is to some extent flattened at the poles. Nearly all the 

 measures of it have resulted in giving a rather larger 

 value for this flattening than theory seemed to allow. 

 The reason underlying this apparent discrepancy was 

 first noticed after a careful series of measurements 

 of the polar and equatorial diameters. The explana- 

 tion given, which seems to agree with the facts very 

 well, is that at the edge of the disc there is a fringe 

 of twilight, which affects unequally the equatorial and 

 polar diameters. The equatorial diameter is apparently 

 always too large, and suffers variations due to the 

 different positions of the sun ; while in the case of the 

 polar diameter the variations are much less. LInder 

 " Atmosphere," the title of the second chapter, this 

 point is again referred to : that we are dealing with 

 an effect of the air, and not one due to mountains, is 

 accounted for by the systematic changes the measured 

 diameters show, which are functions of the sun's position. 

 Calculation shows that the minimum arc of twilight 

 amounts on Mars to 10°. 



That Mars possesses an atmosphere has long been 

 known, and indeed it would be difficult to account for the 

 changes that take place on his surface without the inter- 

 vention of such a medium. This atmosphere is further 

 described as being remarkably free from clouds, a cloud 

 being " a rare and unusual phenomenon." This result is 

 somewhat out of harmony with previous observations, 

 clouds, or what looked very much like them, having been 

 recorded as being distinctly seen passing over and 

 blotting out, locally and temporarily, from view the 

 surface markings. 



Mr. Lowell, however, does not say that clouds do not 

 exist there, but that they simply, during the whole time of 

 observation, never blotted out any markings. He admits, 

 however, that the planet's disc has appeared at times un- 

 accountably bright, and that small bright spots have been 

 observed, but nothing in the shape of moving masses in 

 the atmosphere has attracted attention. That there are 

 clouds in the atmosphere he deduces from certain pheno- 

 mena visible only at the terminator, and observed by Mr. 

 Douglass. During the opposition no less than 736 

 irregularities on the terminator were observed. The 

 peculiarities of these lie in their shape and distribution : 

 some are projections, others depressions. That they 

 are due to mountains seems, according to Mr. Lowell, to 

 be very improbable when all the facts concerning this 

 planet are taken into account ; but that they may be due 

 to clouds, seems more possible. Mr. Lowell discusses 

 this point at some length, and finally considers that these 

 irregularities must be produced by the presence of the 

 latter. 



It is perhaps on this point that Mr. Lowell difters 

 most from other observers of Mars. The bright lights 

 seen on the terminator since 1890 seem to indicate the 

 presence of mountains on the Martian surface, so that 

 deformations at the terminator would seem to be more 

 probably due to these than to the assumption of cloud- 

 banks. 



We come now to the third chapter of the book, the 

 question of water, and under this heading the polar cap, 

 areography, and seas are discussed. .About the first 

 there is little to note. The whole polar area was watched 

 minutely, and found to disappear entirely, an occurrence 

 never before chronicled. During these observations there 

 was always seen a broad blue belt following the cap as it 

 retreated towards the pole, showing that water was 

 actually being formed from the melting of the snow, and 



