The remarkable paper from which this excerpt is taken contains a summary of Schiaparelli's views, and a complete reading of it is to be recommended.
Since Schiaparelli's time a number of astronomers both in Europe and America have been industrious observers of the ca nals, but it seems questionable whether the ground gained has been commensurate with the toll of labour its winning has exacted.
The surface of Mars is believed to be freer from topographical irregularities than that either of the earth or the Moon. This is an inference from the observed smoothness of the terminator, which would quite certainly present a more or less ragged appear ance were irregularities comparable with terrestrial or lunar moun tains upon or near it. It is, however, unreasonable to suppose that substantial differences in level do not exist, and the polar eccentricity and irregularity of outline of the southern cap, when shrinking, find a plausible explanation in the assumption that they are caused by local differences in the level of the ground. The relatively high temperatures of the dark areas, reported by Co blentz and Lampland, presently to be referred to, have led those observers to suggest that these areas lie at a lower level than the brighter ones. This is, however, an aspect of the Martian problem, which is still in the speculative stage.
Atmosphere.—To a celestial observer of the earth the detec tion of its atmosphere would offer no difficulty whatever, since the clouds, which are a part of it, would continually be seen. The discovery of the Martian atmosphere was not so easy a matter, for clouds, in the ordinary sense of the word, are rare, and few evidences of the obstructive effects of an atmosphere are encoun tered in observing the planet with a telescope. Nevertheless, that Mars has an atmosphere of some kind has long been known from the phenomena of the polar caps ; for while it is easy to under stand how a cap could disappear simply by melting, its formation must quite certainly be the result of precipitation, and it was con sequently inferred that there necessarily exists on the planet a considerable amount of vapour, the cap material, which would itself, even in the absence of other gases, constitute an atmosphere. Furthermore, projections from the terminator of Mars have occasionally been seen, for which the most satisfactory explanation known is that they are clouds at elevations of several miles. A cloud at such an altitude, illuminated by the rising or setting sun, would be seen in projection against an unilluminated part of the planet, a circumstance specially favourable for its observation. On other occasions temporary obscuration, partial or complete, of familiar surface markings has been interpreted in terms of ob scuring mists, while, infrequently, very striking patches, whose mobility has left little doubt of their cloud-like nature, have been seen and photographed upon the planet. Thus, while the meteor ological phenomena of Mars are not so conspicuous as those of the earth, their observation years ago gave ample proof of Mars's pos session of an atmosphere. During the oppositions of 1924 and 1926 a considerable amount of photographic evidence was obtained which appears to relate to the planet's meteorology.
Attempts have not been lacking to analyze the Martian atmos phere through spectroscopic examination of the planet's light. Since this light comes originally from the sun, and is merely de flected in our direction by the planet, a casual examination of the spectrum reveals merely the composition of the sun. However, the light, or more strictly speaking that part of it by which we see the planet's surface, must of necessity have traversed the planet's atmosphere both on entering and leaving, and some modification of the Fraunhofer, or dark line, spectrum might reasonably be anticipated as a consequence of this circumstance. Such modifica tion is in fact known to be effected by the earth's atmosphere which adds to the numerous lines belonging properly to the spectrum of the sun a number of so-called "telluric" lines due to its own constituent gases. The presence of these lines of atmos pheric origin adds enormously to the difficulty of a spectroscopic study of a planet's atmosphere, for they are precisely the lines that one would first seek in the planet's spectrum. Observations undertaken for the purpose of detecting on Mars any of the con stituent gases of our own atmosphere therefore present peculiar difficulties, which have only partially been overcome by the adop tion of very elaborate precautions. The observations of Huggins (1867) were for many years taken to indicate the existence of water vapour in considerable quantities in the atmosphere of Mars, and among modern investigations those carried out under the auspices of the Lowell Observatory by V. M. Slipher and F. W. Very are to the same effect. The latter observers estimated the Martian atmosphere to contain approximately 1.75 times as much water vapour, area for area, as existed in the atmosphere above their observing station at Flagstaff, Arizona, at the time of ob servation, that is to say 14 mm. of precipitable water for the Martian atmosphere. W. W. Campbell at the Lick Observatory was, however, unable to detect evidence of water vapour, and concluded that if any is present it does not exceed one fifth of the amount in the earth's atmosphere. The most recent observa tions are those made with powerful apparatus at Mount Wilson by W. S. Adams and C. E. St. John, who estimate the amount to be 6%, or 0.4 mm. of precipitable water, a quantity which they regard as comparable with the probable error of their determina tion. Their result is therefore confirmatory of that of Campbell, and the two sets of observations thus agree in assigning to the planet's atmosphere a low absolute content of water vapour. The Mt. Wilson observers find evidence of oxygen to the amount of 16% of that above their station, or about two thirds of the oxygen content of the air above Mt. Everest. The earlier estimate of the Lowell observers is approximately half of this amount. Oxygen and water are the only substances as yet identified in the atmos phere of Mars, and the latter has not been identified conclusively.