SCIENTIFIC INVENTIONS AT SEA
The safety of our fellow-creatures has always been a strong stimulus to
our inventive faculties. The occurrence of a bad railway accident, and,
roughly, its nature, can be inferred from the files of the Patent
Office, for such an event brings men's thoughts to devising ways and
means of preventing a recurrence, and an avalanche of such inventions
descends upon the patent department in consequence. In like manner a
particularly distressing accident to a lifeboat some years ago brought
out many inventions for the improvement of those romantic craft. Many of
the inventions which arise under these conditions are, of course,
utterly worthless, but some of them "come to stay."
It is not surprising, therefore, when we think of the almost innumerable
wrecks which happen, even with modern shipping, that human ingenuity has
been extremely busy in devising ways for bringing more of safety and
less of risk into the lives of those who go down to the sea in ships. Of
these perhaps none is more fascinating than the modern lighthouse, with
its tall tower, its brightly flashing light, standing undisturbed in the
wildest storm, quietly and persistently sending forth its guiding rays,
no matter how the elements may be buffeting it. There is something
specially attractive in this perfect embodiment of quiet strength and
devotion to duty.
Of course, its origin is very ancient. One of the earliest inventions,
no doubt, was the bright thought of a very primitive man who lit a fire
on a hill to serve as a guide to some belated friends out in their
fishing canoes. From some such beginning the modern lighthouse, a
magnificent product of the science of civil engineering and the science
of optics, has arisen.
Of the difficulties encountered in the construction of lighthouse towers
on outlying rocks much has been written. The historic Eddystone, for
example, has quite a voluminous literature of its own. Of the light
itself, however, much less is known.
It will be interesting first to note the different purposes for which a
light may be required, and then see how the apparatus of the lighthouse
is made to serve these purposes.
There is the "making" light, perched, if possible, upon some high
eminence, deriving its name from the fact that the sailor sights it as
he is "making" the land. Vessels approaching England from the south-west
by night first see the light at the Lizard. The transatlantic vessels
know they are approaching land by catching sight of the Fastnet Rock
light off the coast of Ireland. Cape Race light serves in the same way
for those about to enter the St Lawrence and Navesink for the entrance
to New York harbour. All such as these have to be of the greatest power
practicable, so that they may be visible not only at the longest
possible distance, but also under unfavourable conditions, such as haze
and slight fog. No light, of course, can penetrate thick fog, but in
light fog and haze a powerful light can be seen at considerable
distances. For the same reason these lights must be high up, or the
curvature of the ocean's surface will limit their range. A light
elevated 100 feet above the sea-level will be visible nearly 16 miles
away, but if only 50 feet up it will be invisible at 13 miles. To be
seen 40 miles away it must be as high as 1000 feet.
But then again height is in some cases a disadvantage, for sometimes fog
hovers a little distance above the sea, while below it the air is clear,
and the higher a light may be the more likely is it to have its lantern
immersed in a floating cloud of fog. Many readers familiar with the
south coast of Britain will remember that the light which used to show
on the summit of Beachy Head is there no more, but has been replaced by
a tower at the foot of the cliffs, the reason being that it may be below
the clouds of fog which are prevalent at that point.
But the mention of Beachy Head introduces us to another class of lights,
known as "coasting" lights, since they are intended to lead the mariner
on from point to point along a coast. It will be seen at once that in
many cases they do not need to be visible at such great distances as the
making lights. When the mariner has sighted the Lizard, for example, he
knows where he is. In order that he may learn that important fact as
soon as possible it is desirable that that light should have the
greatest possible range, but having thus located himself, when he begins
to feel his way along the English Channel he is guided by the coasting
lights, and so long as they are of such range that he will never be out
of sight of one or two of them that will be sufficient. Thus the Beachy
Head light, in its present low position, has a sufficient range for its
purpose, with the added advantage of more freedom from obscuration by
fog. Thus we see how the local conditions and the purpose of each
particular light have to be taken into consideration in determining its
position and power.
The Eddystone, again, is an example of a further class. It simply serves
to denote the position of a group of dangerous rocks. Its function is
not so much guidance, although no doubt it often serves for that, but
for warning. The Lizard light beckons the on-coming ship to the safety
of the English Channel; the Eddystone warns it away from danger. The
latter, therefore, and similar lights are "warning" lights.
[Illustration: _By permission of Messrs. J. and E. Hall, Ltd._
A COLD STORE
Interior of a cold store, in which meat and poultry are kept good and
fresh by the use of machine-made cold.--_See_ p. 67]
Right at the entrance to the English Channel, that greatest of all
highways for shipping, there lie the Scilly Isles. This group comprises
some few islands of fair size from which we draw those plentiful
supplies of beautiful spring flowers, but it also includes a large
number of rocky islets which have sent many a strong ship to its doom.
On one of the islets, therefore, the Bishop's Rock, there now stands a
very powerful light which exemplifies many whose purpose is the
double one of welcoming the mariner as he approaches our shores and at
the same time warning him of a local danger. Such are both making and
warning lights.
Of no less importance, though less impressive, are the guiding lights,
which guide the ships into and out of harbours and through narrow
channels. These are generally arranged in pairs, one of the pair being a
little way behind and above the other. Thus when the sailor sees them
both, one exactly over the other, he knows he is on the right course.
Sometimes lighthouses have subsidiary lights as well as the main light,
to mark a passage between two dangers, or to give warning of some
danger. The subsidiary lights are often coloured, and they are generally
"sectors" showing not all round a complete circle, or even a
considerable portion of one, but just in one certain direction. They are
generally shown from a window in the tower lower down below the main
light.
Finally, it is important to remember that every light must be
distinguishable from its neighbours. Hence every one in any given
locality has a different "character" from all the others. This character
is given to it by means of flashes. Instead of showing, as the primitive
lights did, a steady light, the modern lighthouse exhibits a series of
flashes, the duration of which, together with the intervals between,
give it its distinctive character. This flashing arrangement has a
further advantage over the steady light. Each flash can be made more
powerful than a steady light could be. But of that more later.
The actual source of light varies with circumstances. The electric arc
is, as we all know, a very powerful light, in fact it can be made the
most powerful of all, but its light is decidedly bluish. Now the time
when a light is most of all needed is when the weather is thick. Fogs
varying from a slight haze to a thick pall of darkness are of very
common occurrence, and the lighthouse light must be able as far as
possible to penetrate them.
As a matter of fact clean fog, such as one gets at sea, is not by any
means opaque. The black fogs of the great cities are another matter, but
they are not the sort which afflict the mariner. On a foggy day in the
open country or by the sea it is often particularly light; indeed the
light is of a peculiarly diffuse nature which gives a nice even
illumination to everything. Thus we see that fog is really transparent,
but it diffuses the light. It does not stop the light rays, but simply
bends them about and scatters them in all directions. Thus we can see
nothing through the fog, yet a flood of light reaches us through it. In
its effect it is like that "crinkled" glass which is often used for
partitions between rooms, which lets light through, but which cannot be
_seen_ through.
We see, then, that the effect which a fog produces is mainly to refract
the light rays. Each little drop of water (for it must be remembered
that fog is myriads of tiny drops of liquid; it is not vapour) acts like
a minute lens, and bends the rays which pass through it. And the more
blue a ray is the more it is bent. On the contrary, the more red it is
the less is it bent. When a beam of light is analysed in the
spectroscope the red rays are bent least and the blue rays most, so that
the red rays fall at one end of the spectrum and the blue at the other.
Now we only _see_ a thing when light rays proceeding from every part of
it fall straight (or nearly so) upon our eyes. Consequently, since red
rays are bent and scattered by the fog less than blue rays are, a red
light will be more easily seen through a fog than a blue one. It might
seem from this that a red glass put in front of a light would make it
better for this purpose, but that is not the case, for the simple reason
that filtering the light through red glass does not really make it any
redder than it was before: it simply makes it look redder by extracting
from the original light all except the red. But a source of light which
is _naturally_ reddish is so because it is more plentifully endowed with
red rays, while a bluish light like the electric arc is naturally
deficient in red rays. Consequently we should be inclined to expect from
theory that the electric arc would not be a good light for a lighthouse,
since it would lack penetrating power in foggy weather. Some readers may
have noticed themselves, in towns where electric lights and gas lamps
are in use near each other, that the latter, though relatively feebler
under normal conditions, seem to give more light in fog. And experiments
show that this is really the case. So although there are some
lighthouses with electric arc lights, that which is now believed to be
the best is an oil lamp of special design, using a mantle of the
Welsbach type.
The oil is stored in strong steel reservoirs into which air is pumped by
means of a pump not unlike those used to inflate bicycle tyres. By this
means a pressure is maintained upon the oil of about 65 lb. per square
inch. This forces the oil up a pipe and drives it in a jet into a
vaporiser, a tube heated from the outside so that in it the oil is
turned into gas. This gas then rises to the burner and heats the mantle,
just as the gas does in the ordinary incandescent gas light. Indeed in
the case of lights on the mainland near a town the gas from the town
main is often utilised. But this simple arrangement for using vaporised
oil, as will readily be seen, can be employed anywhere. A little of the
gas produced is led through a branch pipe and burnt to heat the
vaporiser. To start the apparatus the vaporiser is heated with a little
methylated spirit. Thus everything is quite self-contained and so simple
that there is little to get out of order. The largest size of lamp will
give 2400 candle-power, with an expenditure of 2-1/4 pints of oil per
hour, just common oil, too, of the kind used with ordinary wick lamps.
Having got a source of powerful light, the next thing is to collect that
light and throw it in the direction required. For the light proceeds
from the lamp in all directions (practically), and much of it would be
entirely wasted could it not be collected and guided in the required
direction.
The earliest attempt at this was to use a reflector of bright polished
metal. In the most improved form these were made to that peculiar curve
known as a parabola. This is a curve obtained by cutting a cone in a
certain way, wherefore it is one of the "conic sections," and its
particular appropriateness for this work resides in the fact that if a
light be placed at a certain point known as the "focus" all the
diverging rays which fall upon the reflector will be reflected in the
same direction, parallel to each other. An ordinary spherical mirror
would reflect them either back to the lamp or in diverging directions.
At any distance the beam from the parabolic reflector will be more
intense than that from the spherical one, since the rays will be closer
together. But even with the parabolic one there is some diffusion, for
the simple reason that whereas the focus is a mathematical point
(position without magnitude) the most concentrated form of light known
has a considerable magnitude. Hence the rays proceeding from the centre
of the mantle are reflected as per the theory, but those from the
outlying parts of it are somewhat diffused. This difficulty cannot
possibly be overcome, and hence even in the finest examples of
lighthouse architecture the flashes are not quite sharp and clear-cut.
There is a central moment, so to speak wherein the flash is almost
blinding in its intensity, but it is preceded by a period of growing
brightness and succeeded by one of decreasing light.
In the modern apparatus, however, metallic mirrors are entirely
dispensed with, their place being taken by reflecting prisms of glass.
The metallic ones had to be continually rubbed to keep them clean, and
this soon dulled their brightness, while the glass prisms need only to
be wiped carefully, which operation has little effect upon their
surface.
It may come as a surprise to some that reflecting prisms are possible.
The idea of refraction through a prism is quite familiar. Such forms the
essential principle of the spectroscope. Refraction is explained to
every school child in order to account for the rainbow. But _reflection_
by a piece of the clearest glass seems a contradiction in terms almost.
Yet it is only a question of shape. In some prisms the light is simply
bent as it passes through. In others it is bent twice, so that it leaves
the prism just as if it had been reflected off a mirror. Both devices
are used in the lighthouse. Let us see how they are combined so as to
perform the work to be done.
Take first of all the case of a light upon an isolated rock where the
warning is needed equally all round. All that is necessary here is to
pick up those rays which, if left to themselves, would fall upon the
water near the foot of the tower, and those which would waste themselves
skywards, and then to gather all the rays into several bundles or beams.
We will suppose a simple case in which the light is supposed to give
flashes at regular intervals.
We are in the topmost room of the lighthouse, the lantern, as it is
called. In the centre there stands the murette or pedestal. In this
several columns support a circular platform on the top of which there
moves what we might call a turntable, which in turn bears a frame of
gun-metal into which are fitted a maze of glass bars triangular in
section and curved to form concentric circles. The whole structure,
possibly, is of great size. From the floor to the platform is as high as
an ordinary man. Indeed around the turntable there is a gallery which
forms a roof over our heads, so that it is only after mounting some iron
steps on to this gallery that we are able to examine the glass part.
As we ascend we notice that the walls of the chamber as far up as the
gallery are formed of iron plates, while above that there is a metal
framework filled in with glass panes, and above all a dome-shaped roof.
Having reached the platform we proceed to examine the glass, and we find
that the metal framework forms a cage with four sides, each
approximately flat, but really slightly spherical. Each of these sides
is called a "panel." In the centre of each is a lens. Peeping through
the interstices between the prisms, we perceive that the lamp is inside
this structure, exactly in the centre, so that its light shines
directly through the central lens or bull's eye. Around this bull's-eye
are many circles of glass bar, forming refracting prisms. Around this
again are more bars in the form of segments, which together form
circles, some being refracting prisms and others reflecting prisms. All
the light rays from the lamp which fall on any one prism are deflected,
so that they proceed approximately in the same direction. Those prisms
in the upper part lay hold of the rays which would otherwise go up into
the sky. Those at the bottom collect those which would fall near the
foot of the tower. So scarcely any are lost. But for the fact that the
lamp itself is comparatively large and not a theoretical point, as
already explained, the beam from this panel would be perfectly straight,
parallel, and of uniform density everywhere. As it is, it widens
slightly as it proceeds, but, practically speaking, we might call it a
solid beam of light.
Each of the panels sends forth such a beam, so that they strike out in
four directions from the central lamp much as four spokes from the hub
of a wheel.
Then descending once more to the floor from which we started, we see
that among the columns there is a large clockwork arrangement, the
purpose of which is to drive round the turntable and all that it
carries--in the language of the lighthouse engineer the "optical
apparatus" or, more briefly, "the apparatus." And as this turns the
radiating beams of light sweep round the horizon and in succession
strike into the eyes of any mariner who may be within range. Each time a
beam strikes him he sees a flash. If the apparatus revolve once a minute
he will see four flashes every minute, one from each panel.
Let us consider, then, the advantages of this wonderful mechanism, with
its cunning arrangement of prisms. It is these latter, of course, which
are the important thing. The rest, the mechanical portion, is simply for
the purpose of holding them and turning them at the proper speed. In the
first place, the contrivance gives us flashes instead of a steady light;
it gives the lighthouse its "character." Then again it enhances the
brightness of the light. Instead of shining all round, the light is
concentrated in four special directions, and the light which would be
wasted upwards or downwards is saved and brought into use.
But suppose that the lighthouse we are considering be near the shore, so
that there is no need for it to throw any light in one--the
landward--direction. Then we should see inside the revolving framework
with its prisms a fixed frame with reflecting prisms which would catch
any rays going from the lamp in the direction of the land and simply
hurl them, as it were, back into the flame. Thus the intensity of the
flame becomes increased by those rays thrown back which would else have
been wasted.
Or suppose that the character of the light is such that the flashes have
to be at irregular intervals. Then the framework, instead of being
symmetrically four-sided, would be of an irregular shape.
And that brings us to a beautiful feature of the mechanism of the
apparatus. We have been discussing a four-panel arrangement. Suppose
that we were to reduce it to three. Then, since all the light would be
concentrated into three beams instead of four, each beam would be more
intense. We should thereby have increased the range of our apparatus
without any increase in the cost of oil--for nothing, as it were. But to
get the same number of flashes per minute we should have to drive it
round so much the faster. But increased speed means increased burden on
the keepers who have to wind up the heavy weights which operate the
clockwork. So there is a limit to the speed which can be attained.
But if friction can be almost eliminated the apparatus can revolve at a
high speed without throwing undue burden upon the men. But how can
friction thus be got rid of? Messrs Chance Bros., the great lighthouse
constructors, of Birmingham, have done it, almost entirely, by floating
the apparatus on mercury. The turntable has on its under side a large
ring which nearly fits a cast-iron trough on the top of the pedestal.
In this trough there is mercury, so that upon the liquid metal the
apparatus floats as if upon a circular raft. The table with its lenses,
prisms and other fittings may weigh six or seven tons, yet it can be
pushed round by one finger.
The various sizes of optical apparatus are known as "orders." One of the
"first order" has a focal distance of 920 millimetres. This means that
there is that distance between the centre of the lamp and the
bull's-eye. They descend by successive stages down to the sixth order,
with a focal distance of 150 millimetres, while the most important
lights are of an order superior even to the so-called "first," termed
the "hyper-radial," the focal distance of which is 1330 millimetres.
A recent example of a hyper-radial light is at the well-known Cape Race
in Newfoundland. It revolves once every 30 seconds, giving a flash of 3
seconds every 7-1/2 seconds. The optical apparatus weighs seven tons.
[Illustration:
_By permission of Messrs. Chance Bros. and Co., Ltd., Birmingham_
DASSEN ISLAND LIGHTHOUSE, CAPE OF GOOD HOPE
This lighthouse, 80 feet high, is built of cast-iron plates,
bolted together]
Most lighthouses are fitted with fog signals of some kind which have a
distinctive character the same as the lights. Some are horns blown at
intervals by compressed air often obtained from a special air-pump
driven by an oil-engine. Another thing is to let off detonators at
stated intervals. But perhaps the most interesting of all is the
submarine telephone. The trouble with audible signals is that they are
apt to vary as the conditions of the atmosphere change. For, strange
though it may appear, the air which is the natural medium by which
sounds are carried to our ears is really a very bad substance for the
purpose. Water is much superior. A swimmer who cares to try the
experiment of lying upon the water with his ears immersed while a friend
beats a gong under the water some distance off will be astounded at the
result. So many modern ships are fitted with under-water ears,
waterproof telephone receivers, really. One is fixed each side of the
vessel, the wires from them being led to telephone receivers near the
bridge. Many lighthouses and lightships in like manner are fitted
with under-water bells which can be rung at intervals. The sounds so
conveyed through the water are always the same. Atmospheric or similar
changes have no effect upon them. And, moreover, the officer can tell
which side of his ship the bell is. If it be on his port-side it sounds
louder in his port telephone, and vice versa. By turning his ship until
he hears them equally he knows that he is pointing directly to or from
the bell. Thus if the bell belong to a warning light he can steer
confidently right away from the danger even in the thickest fog.
But science has not only provided the mariner with lights of marvellous
power and of strange distinctive characters, and reliable sound-signals
for foggy weather, it has also found him a reliable compass, but that is
worthy of a chapter to itself.