Fune 3, 1886] 
NATURE 
105 
reminding one of the “canals” of Schiaparelli. This 
special marking, which is not included in Mr. Green’s 
map, may be identical with the network of dark narrow 
streaks figured in this region by Schiaparelli in his chart 
for January and February, 1882. It is also more or less 
definitely shown in some other drawings, notably on one 
by Schmidt, which forms No. 17 in Dr. Terby’s 
Areography. 
As to the Kaiser Sea, it appears very faint and narrow, 
if not really broken, in the region some 10° or 15° south 
of its north extremity. This peculiarity is well drawn in 
Herr Boeddicker’s drawings of December 27, November 
19, and December 26, 1881 (Nos. 11, 13, and 14) in the 
scientific 7yazsactions of the Royal Dublin Society for 
December, 1882. Consulting other drawings I cannot 
find that this feature is sufficiently indicated. It is obvious, 
however, that it would only be well detected when placed 
near the apparent centre of the disk as during the recent 
opposition. 
Mr. Knobel’s drawings in 1873 (JZonthly Notéces, vol. 
xxxili., facing p. 476) agree generally with mine far closer 
than those he has published in the Memzozrs, vol. xlviil. 
part ii., 1884. I always see Knobel Sea on Green’s chart 
separated on its south side from the fainter curving band 
running east, as in the sketches Nos. 6, 7, 8, and 9, 1873. 
This break is not depicted in the subsequent drawings of 
1884, so that the appearance has either been subject to 
actual variation of aspect or the difference of inclination 
has originated the want of uniformity. Probably the 
latter is the real cause, for the inclination of Mars in 
April and May, 1873, was nearly identical with that of 
March and April, 1886, and it is for these periods the 
drawings are so nearly alike in their more conspicuous 
forms. Isee the northern boundary of Knobel Sea dis- 
tinctly separate from the dark longitudinal strip imme- 
diately contiguous to the north polar cap. The drawing 
No. 12, May 19, 1873, by Mr. Knobel, portrays the leading 
features of this region much as I have more recently ob- 
served them. In 1884, Mr. Knobel delineated the whole 
mass of shading outlying the north pole as blended uninter- 
ruptedly, but these differences are unquestionably due to 
the changes of inclination, which must necessarily intro- 
duce such discordances into the apparent forms of the 
markings as observed at different epochs. 
As to the canal-shaped features of Schiaparelli, first 
seen in 1877 and 1878, and subsequently confirmed, I 
have distinguished a large number of appearances highly 
suggestive of such a configuration, but the Italian 
drawings made during the three months from Oc- 
tober, 1881, to February, 1882, give them a definite cha- 
racter, and (apart from their duplication) a straightness 
of direction and general uniformity of tone which my 
observations do not confirm. The more delicate and 
complex markings on the planet appear to my eye, under 
the best circumstances, as extremely faint, linear shadings 
with evident gradations in tone and irregularities oc- 
casioning breaks and condensations here and there. If 
they existed under the same aspect and with the same 
boldness as delineated by Schiaparelli, they would have 
been readily detected here whenever the seeing was fairly 
good, for these objects are referred to as readily observed 
in the 8-inch refractor of the Milan Observatory in Feb- 
ruary, 1882, when the planet’s diameter was only 13’. The 
duplication of these lines was also traceable under the 
same unfavourable conditions. The wonder is, not that 
the eminent Italian astronomer has discovered such a 
marvellous extent of curious detail on this planet, because 
this detail unquestionably exists, though scarcely in the 
form and character under which it is represented, but that 
he should have observed its more complex and difficult 
configuration at the very period when Mars was so very 
unfavourably situated for observations of this critical 
nature. 
The surface markings of this planet are so numerous 
and varied that they are far from being adequately repre- 
sented on existing charts. In certain regions the disk is 
so variegated as to give a mottled appearance. Dark 
lines, and spots, and bright spaces are so thickly inter- 
spersed, and so difficult to observe with sufficient steadi- 
ness to estimate their positions and forms, that I found 
it impossible to make thoroughly satisfactory drawings. 
An observer has to be content with endeavours to depict 
the more prominent marks only, and even in connection 
with these there is always some element of uncertainty. 
The rotation-period of the planet is, however, so slow, 
the hourly rate being only 14°°6 in comparison with 36°°7 
in the case of Jupiter, that plenty of time is afforded for 
drawing the leading markings before they show a dis- 
placement obvious to the eye. In addition to this a 
drawing of Mars may be made to rest on several succes- 
sive evenings of observation if the observer comes 37°4 
min. later to the telescope on each occasion. In regard 
to Jupiter the difficulty of suitably drawing the details is 
far greater, though they admit of more ready observation. 
The rapid rotation of this planet displaces objects in a 
few minutes, and makes it imperative that the work both 
of observing and charting should be very hastily per- 
formed ; and it is not feasible in this case to basea sketch 
on observations of following nights, because the markings 
are influenced by different velocities, and suffer large 
relative displacements even at short intervals of time. 
During the past few months the north polar cap of 
Mars has been very bright, sometimes offering a startling 
contrast to those regions of the surface more feebly re- 
flective. Some of the other parts were also notably 
brilliant. These luminous regions of Mars require at 
least as much careful investigation as the darker parts, 
for it is probably in connection with them that physical 
changes (if at present operating on the planet’s surface) 
may be definitely observed. In many previous drawings 
and descriptions of Mars sufficient weight has not been 
accorded to these white spots. 
Many of our leading treatises on astronomy attribute a 
dense atmosphere to Mars, but nothing has been observed 
during my recent observations to corroborate this theory. 
It seems to me far more plausible to assume that the at- 
mosphere of this planet is extremely attenuated. The 
chief spots are invariably visible, and the phenomena 
occasionally observed are rather to be imputed to the 
vagaries of our own atmosphere than to that of Mars. 
Jupiter and Saturn are doubtless enveloped in dense 
vapours shrouding their real surfaces from terrestrial 
eyes. Their markings are atmospheric, though in some 
cases very durable, and constantly undergoing changes of 
aspect and displacements of position by longitudinal 
currents. On Mars a totally different nature of things 
prevails. Here the appearances descried are absolute 
surface markings displaying none of the variations which 
are so conspicuously displayed in the markings on Jupiter. 
It is probable that many, if not all, the changes supposed 
to have occurred in the features of Mars are simply at- 
tributable to the constantly varying conditions under 
which the planet has necessarily to be observed. Were 
the circumstances of observation more equable there 
would be much greater unanimity amongst observers of 
this interesting object. It seems to me that the very 
pronounced character of the markings and their great 
permanency are quite opposed to the idea that the planet 
is surrounded by a dense cloud-laden atmosphere. 
W. F. DENNING 
M. CORNU ON THE HYDROGEN FUNCTION 
OF CERTAIN METALS* 
HEN we examine on different photographs those 
groups of lines which reappear periodically with 
a particular regularity, we find that these groups belong 
* Translation from an article in the Journal de Physzque. 
