‘APRIL 20, 1897 | 
WATORE 
617 
Tue PLaNer MERcuRY.—The length of the period of rota- 
tion and the physical features of the inferior planet Mercury 
have formed the subject of a recent investigation by Mr. 
Percival Lowell, the results of which are contributed to the 
Astr. Nach. (No. 3417). The conclusion he has arrived at, 
from a considerable amount of research, is that this planet 
rotates once on his axis in the course of its circuit round the 
sun. As regards the markings on the planet’s disc, he de- 
scribes them as being perfectly distinct, absolutely defined, and 
always visible. They are narrow dark lines, but not so fine as 
they appear to be. Their relative positions are permanent. 
Their positions from hour to hour do not change, thus elimi- 
nating any question of a short period of rotation. The per- 
manency of the markings indicates the absence of clouds and 
other obscurations over the surface of the planet. With regard 
to an atmosphere, we are informed that there are no positive 
signs, either direct or indirect. There is, however, negative 
evidence against the presence of any perceptible atmosphere— 
namely, the low albedo of the planet, and the great contrast of the 
surface markings. Unlike Mars, the planet’s surface is colour- 
less, and there seems, further, to be no change in the markings, 
which might indicate presence of seasonal effects. The observ- 
ations made at the Lowell Observatory are thus in accord with 
what would be expected, considering the isochronism of the 
orbital and axial rotations, and the small inclination of the axis 
to the plane of the orbit. Without an atmosphere, and there- 
fore waterless, the visible surface of the planet may be looked 
upon, as Mr. Lowell remarks, as ‘‘ one vast desert.” 
It may be remarked that M. Leo Brenner, who has also made 
some recent observations of this planet’s surface markings, has, 
in a recent number of the Astr. Mach, come to a somewhat 
different conclusion. The spots, he says, are remarkably clear, 
and a comparison of drawings has indicated that the period of 
rotation of the planet is comprised between thirty-three and 
thirty-five hours, or in the mean thirty-four hours. It does not 
appear to be possible to admit a duration of eighty-eight days 
for the rotation period. 
Nova Avuric#.—Prof. W. W. Campbell publishes some 
spectroscopic notes in the Astrophysical Fournal (No. 4, vol. v.); 
among which are some relative to the new star that appeared in 
1892 in Auriga. The observed intensities of six of the prin- 
cipal bright lines up to October 6, 1896, were as follows :-— 
Hy A4360 HB Agg60 A5010 A5750 
1892 Aug. and Seft. ol o's I 3 10 I 
1894 May 8 ol 03 I 3 10 04 
1894 Sept. 7 ol 02 I 3 ite) o'4 
1894 Nov. 28 ol o'r I 3 10 o'3 
1896 Aug. 15 — a I 3 10 o'r 
1896 Oct. 6 — —_— I 3 10 o'r 
It will be noticed that the bright lines 4360 and 5750 have 
gradually diminished in intensity, the latter being scarcely visible 
at the last observation. These lines, it may be remarked, 
were strong in the Nova, but faint in the old nebule. Their 
gradual disappearance indicates that the return of this Nova to 
the nebula stage has been reached, ‘‘it is now of the ordinary 
type of nebular spectrum, save that the lines remain broad, as 
they have always been described.” When the question as to the 
actual visibility of the nebula after the spectroscopic evidence of 
its truly nebular character came to be inquired into, several 
observers found that its appearance was not like that of a star of 
the same magnitude ; while Prof. Barnard announced that the 
object was “really a bright nebula with a 1oth magnitude 
nucleus.” Prof. Campbell, estimating the magnitude of the 
Nova by comparing the length of its spectrum with that of a 
star of equal magnitude, came to the conclusion that ‘* the focal 
image of Nova Aurigze is stellar.” 
THE SIMPLON TUNNEL. 
"THE project of a tunnel through the Simplon has been so 
much discussed and so frequently abandoned, that one 
might almost doubt whether the scheme last suggested will ulti- 
mately prove effective. But the present plan has been brought 
to a more forward stage than on any previous occasion; the 
requirements of interested Governments have been met, the 
pecuniary difficulties seem to have been overcome, and, finally, 
contracts have been signed with an eminent firm of engineers, 
whose name is a guarantee that what is undertaken will be per- 
NO. 1435, VOL. 55] 
formed. We may, therefore, confidently expect in a short time 
to hear that this arduous work has been commenced ; and it is 
scarcely premature to glance at the various problems, physical, 
mechanical and economical, that have had to be solved, or to 
express a hope that the solutions which have been offered, based 
as they are on varied experience, will prove adequate to cope 
with the many difficulties that will arise. 
The economical problem can be easily dismissed. The advan- 
tages of the Simplon route are so patent, that the perforation of 
the mountain was suggested in the early days of railway enter- 
prise, at a time when the difficulties that the Alps presented to 
continuous traffic were more fully apprehended than were the 
means by which those difficulties were to be surmounted. It is 
a very easy thing to put down on paper the number of miles that 
separate. say, London from Brindisi, and show that the train 
mileage is less by way of Simplon than either by Mont Cenis or 
Saint Gothard ; and yet both these routes have been opened to 
traffic before the most advantageous line has been begun. If 
we select some station, as Plaisance, on the eastern side of the 
Alps, and directly on the Brindisi route, we have the following 
distances, according to the course followed :— 
From London, Calais, Paris, Cexzzs to Plaisance, 
1438 km. = 894 miles. 
From London, Calais, Bale, Gothard to Plaisance, 
1375 km. = 854 miles. 
From London, Calais, Reims, Szzp/oz to Plaisance, 
1253 km. = 779 miles. 
But this saving of distance, amounting as it does to near 
S per cent., does not express the whole of the advantag 
that a route through the Simplon would offer. This last tunn 
being at a much less elevation above the sea-level than eithe 
of the others, the speed of the trains would be greater, and n 
special precautions needed to ensure the safety in passing ov 
inclines rather steeper than are usually experienced on frictio 
railways. 
The maximum height of the Simplon tunnel is 706 m. 
” 3 an Gothard An DLS Gus 
an Aa ne Mont Cenis ,, T2905 "es 
The Arlberg railway is the highest of all, being 1311 m. 
To balance the economy of distance and small altitude, we 
have to consider the length of tunnelling necessary, and unfor- 
tunately the length required is in the inverse proportion to the 
conveniences of the route. The first-made tunnel, that of 
Mont Cenis, is 12,849 m. long, the Gothard 14,984 m., 
while that of the Simplon is computed at 19,731 m., so that 
the ratio of the first is to the third as 2: 3. 
The geological conditions have been well studied, and it will 
be interesting to compare the forecast framed from an examina- 
tion of the external appearance of the mountain with the char- 
acter and extent of the rocks actually encountered. On the south 
side, for a distance of about four miles, the principal rocks will 
be clay and mica slate with gneiss. In the central portion, ex- 
tending over something like six miles, the boring tool will have 
to work its way through gneiss, alternating with mica schist, 
and limestone. On the northern side, towards the Rhone, slate 
and beds of gypsum will form the principal constituents ; and 
here, though the rocks may not be so hard as in the centre of 
the mountain, greater difficulty is anticipated owing to the ex- 
treme precaution that will have to be taken in protecting the 
sides of the tunnel. The direction given to the perforation will 
make the axis of the tunnel practically perpendicular to the 
various seams. 
If the extent and hardness of the rock were the only physical 
difficulties with which the engineers had to contend, doubtless 
the boring would have been attempted long since ; but another, 
and a greater obstacle has to be overcome in the temperature 
of the rocks themselves. To excavate under a mountain is to 
some extent comparable with sinking a mine ; and recalling the 
comparatively low altitude at which the tunnel is to be con- 
structed, and the consequent height to which Monte Leone 
will tower above it (more than 2000 m.), it is evident that a 
pretty deep mine is contemplated. We are going to learn 
something of the internal heat of the earth at a considerable 
distance from the surface. One can make a pretty shrewd guess. 
at what these temperatures will be, from previous experience 
in the Cenis and Gothard tunnels; and evidently the advan- 
tage of keeping the tunnel nearer to the sea-level, which will 
facilitate traffic, isaccompanied with the disadvantage of having 
