beneath the col of the pass, and emerges close to the village of Airolo,
on the banks of the Ticino. The length of this tunnel is rather more
than nine and a quarter miles, so that it is about one and a half miles
longer than the Mont Cenis Tunnel. Its northern end is 3,638 feet above
the sea level; its southern end is higher, namely, 3,756 feet; but there
is an intermediate point in the tunnel higher than either-–3,786
feet—and from this there is a uniform incline in each direction. The
tunnel is 300 yards beneath the lowest part of the valley of Andermatt,
and the summits of the mountains it traverses are at least a mile above
it. The motive power by which the rock-drilling machines used in driving
the tunnel were actuated was, as in the case of the Mont Cenis Tunnel,
compressed air; and the power used for compressing the air was, in this
case also, a head of water,—but this was not applied in the same way.
The waters of the Reuss at the northern side, and those of the Tremola
and of the Ticino at the southern side, were taken at a considerable
height in very large cast-iron pipes, and were made to act upon powerful
turbines that gave motion to the compressing machines. These were
capable of compressing the air so that its volume was reduced to
one-twentieth, and the pressure it then exercised would, of course, be
equal to that of twenty atmospheres, or about 300 lbs. on the square
inch,—or more than three times as much as was made use of in the Mont
Cenis Tunnel. The compressed air, carried through pipes to the head of
the workings in the rock, was there allowed to exert its force on the
pistons of the perforators in the manner already described. There was,
in fact, a continual repetition of exactly the same cycle of operations
of boring, charging, firing, etc., that are mentioned on page 355. A
large quantity of the compressed air was always allowed to rush into the
work immediately after each blasting, in order that the smoke and other
products might be driven out and the atmosphere rendered fit for
respiration. In attacking the mountain simultaneously from each side it
was, of course, essential that the tunnels should be driven in precisely
the same direction, and therefore the positions of the points of
departure had to be determined by very careful surveys. At Gœschenen,
the gorge of the Reuss did not naturally admit of a sufficient distance
of vision to fix the direction with the required accuracy, and it became
necessary to pierce a thick mass of rock with a special tunnel for the
purpose of taking a sight sufficiently far back. At Airolo, again, the
tunnel had to enter the valley by curving towards the village; and here
a provisional gallery had to be driven in the straight line.
[Illustration:
PLATE XVI.
THE NORTH MOUTH OF THE GREAT TUNNEL, ST. GOTHARD RAILWAY.
]
Several contractors competed for the work of constructing this great
tunnel, and it was at first supposed that an Italian company, which was
managed by some of the principal engineers engaged on the Mont Cenis,
would be almost certain to obtain the contract. The promoters, however,
intrusted the work to a private individual, M. Louis Favre, of Geneva.
This gentleman undertook to complete the tunnel in eight years, at the
price of 2,800 francs per mètre for the work of excavation merely,
exclusive of masonry, etc. This cost would be not far from £101 per
English yard. The contract was signed on August 7th, 1872, and on
September 12th of the same year M. Favre commenced operations at the
southern end, and the work at the northern end was begun on October 9th
following. The operations were carried on with great energy, and even
during the period of the Company’s financial difficulties there was no
stoppage of the works between Gœschenen and Airolo. It has been
suggested that it was largely due to the regular and successful progress
of this great piece of rock boring that the Company were enabled to
re-establish themselves on a basis that ensured the completion of the
whole undertaking. The contractor, on his part, did not fail to
encounter many physical difficulties. At the southern end much trouble
was caused by torrents of water gushing from the soil, many of these
being of great volume and force; in fact, the work was here carried on
for nearly a whole year in the midst of water—for the ground for the
first mile consisted of glacial and other deposits, which were
intersected by subterranean water-courses. Reaching the solid rock was
here a relief. But at Gœschenen little of loose formation was met with;
but the rock encountered was of extreme hardness—consisting, indeed, of
almost pure quartz, which had the effect of quickly blunting the points
of even the best tempered tools. But another kind of difficulty had to
be overcome when the workings got beneath the vale of Urseren. Here, at
several places, layers of argillaceous matter were found between the
masses of hard rock. These layers were easy enough to pierce through,
but on account of the pressure of the rocks in which they were
interspersed, they were squeezed out and gradually protruded within the
tunnel, which would soon have become entirely obstructed. At first a
very massive lining of timber was tried, but it was soon found that this
must be replaced by a solid vaulting of stone. The first vault failed to
sustain the pressure, and so did the second, although the thickness of
the material was more than a yard. In some places these operations had
to be several times repeated, and from this cause the cost of parts of
the tunnel has been nearly £1,000 per yard.
[Illustration:
FIG. 187_b._—_The Uppermost Bridge over the Maïenreuss._
]
The instances above mentioned may be taken as mere specimens of the
physical difficulties attending a work of this kind. There are often
others arising from the unusual circumstances under which the workmen
are placed, and others again from accidental causes alone. M. Favre
experienced some of these, as, for example, when one year a fire
destroyed the greater part of the village of Airolo; another year there
was a strike on the part of the workmen. The high temperature in the
workings was, especially towards the end, a source of great trouble. The
cause of the heat is no doubt the same as that which is held to support
the theory of the earth’s central heat. Numberless observations have
established the fact that the temperature of the earth’s crust increases
as we go deeper. The increase appears not to be uniform in different
places—at least there is much discrepancy in the estimates that have
been made. But as a sufficient approximation to a general statement, it
may be taken as proved that for every seventy feet or so that you go
below the surface of the ground, there is an increase of the temperature
of the strata equal to 1° Fahrenheit. Now, the workmen in the two
sections of the tunnel had, at last, to carry on their labour in a
temperature of more than 100° Fahrenheit. This, perhaps, might have been
one cause of some unprecedented kinds of malady that appeared amongst
the tunnel labourers. M. Favre himself was not destined to witness the
completion of his great undertaking, for, on July 19th, 1879, as he was
returning from an inspection of the tunnel, he fell into the arms of his
companions, struck down by a fatal attack of apoplexy. On February 29th,
1880, the last fuse required to blast down the rock separating the two
tunnels was fired by one of the few workmen who had been engaged in the
operations during the whole period from their commencement. It was found
that the two tunnels met exactly and coincided in direction.
[Illustration:
FIG. 187_c_.—_The Bridges over the Maïenreuss near Wasen._
]
The construction of such a line of railway as the St. Gothard tries the
skill of the engineer, and taxes all the resources of his art. The
problems presented by the nature of the route, and the requirements of
the iron road, have in this case been successfully solved by bold
expedients—by new and ingenious devices. The reader will readily
understand that the ordinary cart road may wander about, so to speak, of
its own will; it is not confined to the limited gradient of the line; or
obliged to make its turns and curves of at least a certain radius. Now,
there are portions of the valley where the general slope is too steep
for the railway to follow, and where it was necessary to form it in
zig-zags, so that certain sections of the gorge or valley may be several
times traversed by the line returning upon itself. Fig. 187_d_ is a view
showing an incident of this kind, and one of the most interesting spots
on the route. The dark line on the spectator’s right is the track of the
railway; the white trace, which in the lower part of the view is seen on
the other side of the Reuss, is the ordinary road. If this last be
followed up the valley, it will be seen to cross first the Reuss, and
then a tributary stream (the _Maïenreuss_) descending through a gorge on
the right, after which it zig-zags up a hill to the village of _Wasen_
(the church of which village is seen crowning the eminence in the centre
of our view), and then it continues its course up the valley, passing
through a small village, and disappearing over the shoulder of a hill on
the right bank of the river. Let us now carefully follow the railway
from where the train at the bottom of the picture is seen ascending the
gradient. The line presently passes under a bridge, and then enters the
tunnel, near to the entrance of which a small building will be noticed.
The course of the tunnel is shown by the _curve_ marked in dots, for
this tunnel makes a round within the rock, and the railway emerges to
day again at a point lower down in the course of the valley than at the
entrance to the tunnel, but at a higher level. It is seen in the figure
appearing from behind the rocks in the right-hand lower corner, passing
under a short tunnel and continuing along the mountain side. The curved
tunnel resembles in direction part of the turn of a corkscrew; it is one
of a series of _helicoidal_ tunnels of which there are several examples
on the line. The entrance to this tunnel is 2,539 feet, the exit 2,654
feet, above the sea level. It is known as _Pfaffensprung_ (Monk’s Leap)
Tunnel. The line again enters a short tunnel, and immediately crosses
the deep gorge of the Maïenreuss, to plunge again into another tunnel at
the base of this hill on which Wasen stands. Higher up it crosses the
Reuss and enters the helicoidal tunnel of _Wattingen_ (dotted line). On
emerging from this, the line re-crosses the Reuss, and may now be traced
_down_ the valley, but higher up on the mountain side, coming in the
reverse direction, and after passing _Wasen_ on the other side,
re-crossing the Maïenreuss gorge by a second bridge. Then turning back
again through another helicoidal tunnel (_Leggistein_) the line crosses
the Maïenreuss for the third time, and continues its course up the
valley. Fig. 187_c_ gives us a near view of _Wasen_, and a glimpse up
the gorge of the Maïenreuss from its junction with the Reuss. The bridge
with the large single arch is that which carries the ordinary road, and
higher up we see the three iron bridges that carry the railway backwards
and forwards in its doublings. We can well imagine the perplexity of
anyone ascending the valley in the train for the first time, and
ignorant of the peculiarities of this extraordinary railway. In crossing
the first, or lowest bridge, over the Maïenreuss, he would catch a
glimpse of the church of Wasen, perched on its hill, high above him, and
on his right. After being carried through more tunnels, and over more
bridges, he would some minutes afterwards be disposed to think that his
eyes were deceiving him, for there, still on his right, he would see the
same church, but now on about the same level as the train. Again, after
more tunnels and bridges, the church would once more appear, transferred
to the left of the line, and sunk very far down. These several
apparitions of the same building in different positions, after the train
has seemed to have been pursuing its onward course the while,—which
course would not be judged by any impressions the traveller would
usually receive to be other than rectilinear,—are indeed a regular
bewilderment to the inexperienced traveller. He is then obliged finally
to resign himself passively to be carried he knows not whither or how,
for his sense of direction is completely at fault;—the train comes out
of tunnels which seem turned the wrong way; the river, which he expected
to find on the left hand, he sees on the right; and the Reuss appears to
have reversed the direction of its flow.
[Illustration:
FIG. 187_d_.—_Windings of the Line near Wasen._
]
It is understood that the St. Gothard line has been a great commercial
success, for the number of passengers entering and leaving Italy by that
route has been enormous, and still shows a large annual increase.
Indeed, the prosperity of the line has been so great that the project
has been revived of carrying another railway over the Alps to connect
Italy and Switzerland by way of the Simplon. If this scheme should be
carried out, the mountains will be pierced by a tunnel of a length
double that of the St. Gothard.
[Illustration:
FIG. 188.
]
LIGHT.
The foregoing pages have been devoted to the description of inventions
or operations in which mechanical actions are the most obvious features.
Some of the contrivances described have for their end and object the
communication of motion to certain bodies, others the arrangement of
materials in some definite form, and all are essentially associated with
the idea of what is called _matter_. But we are now about to enter on
another region—a region of marvels where all is enchanted ground—a
region in which we seem to leave far behind us our grosser conceptions
of matter, and to attain to a sphere of more refined and subtile
existence. For we are about to show some results of those beautiful
investigations in which modern science has penetrated the secrets of
Nature by unfolding the laws of light—
“Light
Ethereal, first of things, quintessence pure.”
The diversity and magnificence of the spectacles which, by day as well
as by night, are revealed to us by the agency of light, have been the
theme of the poet in every age and in every country. It cannot fail to
arrest the attention to find Science declaring that all the loveliness
of the landscape, the fresh green tints of early summer and the golden
glow of autumn, the brilliant dyes of flowers, of insects, of birds, the
soft blue of the cloudless sky, the rosy hues of sunset and of dawn, the
chromatic splendour of rubies, emeralds, and other gems, the beauties of
the million-coloured rainbow,—are all due to light—to light alone, and
are not qualities of the bodies themselves, which merely _seem_ to
possess the colours. The following quaint stanzas, in which a poet of
the seventeenth century addresses “Light” have a literal correspondence
with scientific truth:
“All the world’s bravery, that delights our eyes,
Is but thy several liveries;
Thou the rich dye on them bestowest,
Thy nimble pencil paints this landscape as thou goest.
“A crimson garment in the rose thou wearest:;
A crown of studded gold thou bearest;
The virgin lilies, in their white,
Are clad but with the lawn of almost naked light.
“The violet, Spring’s little infant, stands
Girt in thy purple swaddling-bands;
On the fair tulip thou dost dote;
Thou clothest it in a gay and parti-coloured coat.”
All these beauties are indeed derived from the imponderable and
_invisible_ agent, light; and the variety and changefulness of the
effects we may constantly observe show that light possesses the power of
impressing our visual organs in a thousand different ways, modified by
the surrounding circumstances, as witness that ever-shifting
transformation scene—the sky. In the skies of such a climate as that of
England there are ceaseless changes and ever-beautiful effects,
producing everywhere more perfect and diversified pictures than the
richest galleries can show. In the night how changed is the spectacle,
when the sun’s more powerful rays are succeeded by the soft light of the
moon, sailing through the azure star-bestudded vault! What limitless
scope for the artist is afforded by these innumerable modifications of a
single subtile agent, in light and shade, brightness and obscurity, in
the contrasts and harmonies of colours, and in the countless hues
resulting from their mixtures and blendings!
It will be necessary, before attempting to explain the discoveries and
inventions which prove how successfully science, aided by the powerful
mathematical analysis of modern times, has acquired a knowledge of the
ways of light, to discuss such of the ordinary phenomena as have a
direct bearing upon the subjects to be considered.
[Illustration:
FIG. 189.—_Rays._
]
_SOME PHENOMENA OF LIGHT._
It may be considered as a matter of common experience that light is able
to pass through certain bodies, such as air and gases, pure water,
glass, and a number of other liquids and solids, which, by virtue of
this passage of light, we term _transparent_, in opposition to another
class of bodies, called _opaque_, through which light does not pass.
That light traverses a vacuum may be held as proved by the light of the
sun and stars reaching us across the interplanetary spaces; but it may
also be made the subject of direct experiment by an apparatus described
below, Fig. 190. Another fact, very obvious from common observation, is
that light usually travels in straight lines. Some familiar experiences
may be appealed to for establishing this fact. For example, every one
has observed that the beams of sunlight which penetrate an apartment
through any small opening pursue their course in perfectly straight
lines across the atmosphere, in which their path is rendered visible by
the floating particles of dust. It is by reason of the straightness with
which rays of light pursue their course that the joiner, by looking
along the edge of a plank, can judge of its truth, and that the engineer
or surveyor is able by his theodolite and staff to set out the work for
rectilinear roads or railways. On a grander scale than in the sunbeam
traversing a room, we witness the same fact in the effect represented in
Fig. 189, where the sun, concealed from direct observation, is seen to
send through openings in the clouds, beams that reveal their paths by
lighting up the particles of haze or mist contained in the atmosphere.
It is not the air itself which is rendered visible; but whenever a beam
of sunlight, or of any other brilliant light, is allowed to pass through
an apartment which is otherwise kept dark, the track of the beam is
always distinctly visible, and, especially if the light be concentrated
by a lens or concave mirror, the fact is revealed that the air, which
under ordinary circumstances appears so pure and transparent, is in
reality loaded with floating particles, requiring only to be properly
lighted up to show themselves.
[Illustration:
FIG. 190.
]
Professor Tyndall, in the course of some remarkable researches on the
decomposition of vapours by light, wished to have such a glass tube as
that represented in Fig. 190, filled with air perfectly free from these
floating particles. When the beam of the electric lamp passed through
the exhausted tube, no trace of the existence of anything within the
tube was revealed, for it appeared merely like a black gap cut out of
the visible rays that traversed the air; thus proving that light,
although the agent which makes all things become visible, _is itself
invisible_—that, in fact, we see not light, but only illuminated
substances. When, however, air was admitted to the tube, even after
passing through sulphuric acid, the beam of the light became clearly
revealed within the tube, and it was only by allowing the air to stream
very slowly into the exhausted glass tube through platinum pipes, packed
with platinum gauze and intensely heated, that Professor Tyndall
succeeded in obtaining air “optically empty,” that is, air in which no
floating particles revealed the track of the beams. The destruction of
the floating matter by the incandescent metal proves the particles to be
organic; but a more convenient method of obtaining air free from all
suspended matter was found by Professor Tyndall to be the passing of the
air through a _filter of cotton wool_. It must not be supposed that it
is only occasionally, or in dusty rooms, laboratories, or lecture-halls,
that the air is charged with organic and other particles—
“As thick as motes in the sunbeams.”
“The air of our London rooms,” says Tyndall, “is loaded with this
organic dust, nor is the country air free from its pollution. However
ordinary daylight may permit it to disguise itself, a sufficiently
powerful beam causes the air in which the dust is suspended to appear as
a semi-solid, rather than as a gas. Nobody could, in the first instance,
without repugnance, place the mouth at the illuminated focus of the
electric beam and inhale the dust revealed there. Nor is this disgust
abolished by the reflection that, although we do not see the nastiness,
we are drawing it in our lungs every hour and minute of our lives. There
is no respite to this contact with dirt; and the wonder is, not that we
should from time to time suffer from its presence, but that so small a
portion of it would appear to be deadly to man.” The Professor then goes
on to develop a very remarkable theory, which attributes such diseases
as cholera, scarlet fever, small pox, and the like, to the inhalation of
organic _germs_ which may form part of the floating particles. But we
must return to our immediate subject by a few words on the
_VELOCITY OF LIGHT._
[Illustration:
FIG. 191.—_Telescopic appearance of Jupiter and Satellites._
]
It may be stated at once, that this velocity has the amazing magnitude
of 185,000 miles in one second of time, and that the fact of light
requiring time to travel was first discovered, and the speed with which
it does travel was first estimated, about 200 years ago, by a Danish
astronomer, named Roemer, by observations on the eclipses of the
satellites of Jupiter. The satellites of Jupiter are four in number, and
as they revolve nearly in plane of the planet’s orbit, they are subject
to frequent eclipses by entering the shadow cast by the planet; in fact,
the three inner satellites at every revolution. Fig. 191 represents the
telescopic appearance of the planet, from a drawing by Mr. De La Rue,
and in this we see the well-known “belts,” and two of the satellites,
one of which is passing across the face of the planet, on which its
shadow falls, and is distinctly seen as a round black spot, while the
other may be noticed at the lower right-hand corner of the cut. The
satellite next the planet (Io) revolves round its primary in about 42½
hours, and consequently it is eclipsed by plunging into the shadow of
Jupiter at intervals of 42½ hours, an occurrence which must take place
with the greatest regularity as regards the duration of the intervals,
and which can be calculated by known laws when the distance of the
satellite from the planet has been determined. Nevertheless, Roemer
observed that the actual intervals between the successive immersions of
Io in the shadow of Jupiter did not agree with the calculated period of
rotation when the distance between Jupiter and the earth was changing,
in consequence chiefly of the movement of the latter (for Jupiter
requires nearly twelve years to complete his revolution, and may,
therefore, be regarded as stationary as compared for a short time with
the earth). Roemer saw also, that when this distance _was increasing_,
the observed intervals between the successive eclipses were a little
greater, and that when the distance _was decreasing_ they were a little
less, than the calculated period. And he found that, supposing the
earth, being at the point of its orbit nearest to Jupiter, to recede
from that planet, the _sum of all the retardations_ of the eclipses
which occur while the earth is travelling to the farthest point of its
orbit, amounts to 16½ minutes, as does also the _sum of the
deficiencies_ in the period when the earth, approaching Jupiter, is
passing from the farthest to the nearest point of her orbit. While,
however, the earth is near the points in her orbit farthest from, or
nearest to Jupiter, the distance between the two planets is not
materially _changing_ between successive eclipses, and _then_ the
observed intervals of the eclipses coincide, with the period of the
satellite’s rotation. The reader will, after a little reflection, have
no difficulty in perceiving that the 16½ minutes represent the time
which is required by the light to traverse the diameter of the earth’s
orbit; or, if he should have any difficulty, it may be removed by
comparing the case with the following.