Marvels of Scientific Invention Part 7

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 gla.s.s 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 gla.s.s 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 gla.s.s panes, and above all a dome-shaped roof.

Having reached the platform we proceed to examine the gla.s.s, 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 gla.s.s 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 sh.o.r.e, 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.

[Ill.u.s.tration: _By permission of Messrs. Chance Bros. and Co., Ltd., Birmingham_

Da.s.sEN 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 compa.s.s, but that is worthy of a chapter to itself.

CHAPTER VII

THE GYRO-COMPa.s.s

The magnetic compa.s.s has been for ages the mariner's guide over the trackless waters. In cloudy weather it has been his only means of knowing the direction in which his craft was heading. Indeed, it is not too much to say that the maritime commerce of the world was based upon the behaviour of that little piece of magnetised steel.

It has always, however, been subject to certain faults. To commence with, it points, not to the geographical north, but to the "magnetic pole," a point some distance from the geographical pole, and one, moreover, which is not quite permanent. The fact that the magnetic pole varies its position is impressively shown by the fact that a special department at Greenwich Observatory is continually employed, by the aid of delicate self-recording instruments, watching and setting down its fluctuations. And the premier observatory of the world, it should be remembered, exists primarily, not in the interests of pure science, but as a department of the British Admiralty in order to study matters of interest to navigation. Thus we have testimony to the importance of these little vagaries on the part of the magnetic compa.s.s.

But in addition to these inherent faults there is a new source of error in the magnetic compa.s.s which man has introduced himself by making his ships of iron instead of wood. Every ship of the present day is a huge magnet. A piece of iron left in the same position for a length of time becomes polarised, which is to say that it acquires the properties of a magnet; and two magnets always exert an influence upon each other.

Consequently the ship, after lying for perhaps a year in one position, during the period of building, becomes itself magnetic and interferes with its own compa.s.s.

Then, again, our methods of ship construction aggravate this trouble. It is believed that every molecule of iron is itself a minute magnet with a north and south pole of its own. These lying in confusion in the ma.s.s of unmagnetised iron neutralise each other, so that the ma.s.s, taken as a whole, does not exhibit any magnetic power. But if by some means the whole of the millions of millions of molecules can be set the same way--with all their north poles in one direction, and their south poles in the opposite direction--then they will all act together. Instead of neutralising each other they will then help each other, and under those conditions the ma.s.s of iron will possess that peculiar power which is distinctive of a magnet. So long as a piece of iron is left in the same position the magnetism of the earth is thus acting upon the molecules.

Just as it tends to place the compa.s.s needle north and south, so it does with every molecule in the iron ma.s.s. And if, while lying still, the iron be hammered, the shaking of the molecules due to the hammering loosens them as it were and a.s.sists the earth's power in pulling them into position.

One has only, then, to watch the riveting up of a ship, and to see the vigorous way in which the riveters wield their hammers, to realise that when the thousands or even millions of rivets have all been finished the material of that ship will have had the very best possible chance of becoming magnetic.

To make matters worse still, ships are often loaded with great weights of iron among their cargo. That, too, may affect the compa.s.s. On warships there are the heavy guns, each weighing, with its turret, hundreds of tons, and they move, so that their effect upon the compa.s.s is not always the same, but may vary from time to time. And finally one may mention the electrical machinery in a modern ship consisting largely of powerful magnets.

Altogether, then, it is not surprising that the old magnetic compa.s.s is somewhat unreliable. It has to be coaxed into doing its duty. Pieces of iron and magnets have to be disposed about it to counteract these disturbing influences with which it is surrounded. Before a voyage experts have to come on board to adjust the compa.s.ses, and even then there is reason to believe that the instrument sometimes plays the ship false.

It is not to be wondered at, then, that the naval authorities in particular throughout the world have welcomed the advent of a new compa.s.s which appears to possess none of these drawbacks. It points to the geographical north, to the actual pivot, if one may so speak, upon which the earth turns. It is non-magnetic, so that the presence of iron or magnets even in its immediate neighbourhood has little or no effect upon it. On the other hand, it has to be driven by a current of electricity, and it seems just possible that in some great crisis it might fail, although every provision is made for alternative sources of supply in case of one failing, and there is always the possibility of falling back upon the old magnetic compa.s.s should the new one go wrong.

In principle the improved compa.s.s is, like its older brother, simplicity itself. The latter is but a small piece of iron magnetised; the former is nothing more than a spinning-top.

It is rather strange that although the spinning object has been a familiar toy for years, and that, moreover, its behaviour has been the subject of investigation by some very eminent scientific men, it is only of recent years that its principles have been put to practical use.

Everyone is familiar with the fact that a round block of wood will support itself upon a comparatively tall peg so long as it is rapidly rotating. And that is but one of the curious things which a rotating body will do. For example, imagine a wheel mounted upon an axle the ends of which are supported inside a ring, while the ring again is supported on pivots between the two p.r.o.ngs of a fork, the fork being free to swivel round in a socket. The wheel is then free to move in any direction. Technically, it is said to have "three degrees of freedom."

It can spin round, its axle can turn over and over with the pivoted ring inside which it is fixed, while it can also swing round and round as the fork turns in its socket. a.s.suming that the joints are all perfectly free, that the pivots move in their sockets with perfect freedom--which, of course, they do not--then a wheel so mounted could move in any direction under the influence of any force that might act upon it. Now a wheel so mounted if left alone remains in precisely the same position so long as it goes on rotating. If it be turning sufficiently quickly its tendency to remain will be strong enough to overcome the friction of any ordinarily well-made instrument. Consequently a wheel of that description has been used to demonstrate the rotation of the earth, it remaining still (except, of course, for its rotating movement) while the earth has moved under it.

Could we entirely eliminate the effects of friction that might be used as a compa.s.s, for it could be set, say with its axle pointing north and south, at the commencement of the voyage, and it would remain so despite all the evolutions through which the ship might go.

But there is a better scheme even than that, based upon the peculiar behaviour of a revolving wheel when it has only two degrees of freedom.

Suppose that we dispense with the ring employed in the previous arrangement, pivoting the ends of the axle between the p.r.o.ngs of the fork. The wheel is then free to rotate, and its axle can slew round through a complete circle by the turning of the fork in its socket, but there can be no tilting of the axle. Being thus deprived of one of its movements the gyroscope with three degrees becomes a gyroscope with two degrees of freedom, and in that form it supplies the need for an efficient and reliable compa.s.s.

The secret of the whole thing is the curious fact that a gyroscope with two degrees of freedom exhibits a keen desire to place its axis parallel with the axis of the earth. Owing to the shape of the earth, a device such as has been described, with its fork standing up vertically, cannot possibly have its axis really parallel with that of the earth, except on the Equator. Still it gets as nearly parallel as possible. To be scientifically accurate, we ought to say that it places it own axis "in the same plane" as that of the earth.

To understand this we need to realise that all movement is relative. In ordinary language, when we say a thing is still we mean that it is still in relation to the surface of the earth, but since the earth is moving the stillest thing, apparently, is really travelling at enormous speed.

Saint Paul's Cathedral in London, or a tall sky-sc.r.a.per in New York, would usually be regarded as supreme instances of immobility. It would be hard to find better examples of stationariness, as we ordinarily look at things. Each stands, firm and strong, upon a horizontal base. Yet each is really turning a somersault every twenty-four hours. The plateau upon which St Paul's stands, though it seems still and motionless beneath our feet, is continually tilting; its eastern edge is continually going downwards and its western edge upwards, as the earth performs its daily spin. It is only a north and south line which does not share in some degree this continual tilting action. Every plane, large or small, so long as it remains horizontal, is being tilted thus, down at the eastern edge and up at the western. And the plane in which the axle of a gyroscope with "two degrees" is free to move is a horizontal plane. Owing to its being held between the p.r.o.ngs of the fork, while it can swing round to point north, south, east or west, or towards any point between them, it cannot deviate from the horizontal plane. Therefore such axle is always being tilted by the motion of the earth, _except when it happens to be lying exactly north and south_.

Now for a reason which is too complex to go into here a gyroscope strongly objects to having its axle tilted in this manner. If it be compelled by superior force to submit to tilting, it tries to wrench itself round sideways. Anyone who has a gyroscope top and cares to try the experiment will feel this action quite easily. Hold the spinning-top in your hand and turn it over so as to tilt the axle, when it will, if you are not careful, twist itself out of your grasp.

So a gyroscope of the kind we are considering, when the motion of the earth tilts its axis, turns itself round in its socket until at last it reaches the north and south position, when the tilting, and therefore the twisting, ceases. Hence the axle of the gyroscope if left to itself (the rotation of the wheel being maintained the while) will place itself in a north and south direction. And, moreover, it will keep in that direction. It will take some force to slew it round into any other. And if moved into any other by some extraneous means it will restore itself to the old position again.

Hence a wheel thus arranged has all the attributes which we need for a mariner's compa.s.s. But unfortunately there are mechanical difficulties in the way of using such a simple contrivance for that purpose.

Chief of all these is the fact that it is not what engineers call "dead-beat." That means that it will not go to the proper position and then remain there quite still. Instead, it will first slightly overshoot the mark, which being followed by the reverse action, it will come back and overshoot it just as far in the opposite direction. Instead, therefore, of a steady pointing, always in the same direction precisely, it will oscillate more or less, the exact north and south line being the mean or average position, the centre of the oscillations.

It would of course be possible to damp this, to apply a break as it were, if the apparatus were to remain stationary. For example, if the whole concern were immersed in water the resistance of the liquid would restrain any quick movement of the axle, yet it would not prevent it from slowly finding its true position. Thus the oscillations would be reduced to such a small range as to be for practical purposes negligible. But the drawback to a device of that kind, applied to a gyroscope on board ship, would be that the axle would be carried round to some extent every time the ship turned. As she changed direction it would more or less carry round the water with it; that in turn would carry the gyroscope, and so the direction of the latter would be for a time untrue. It would in course of time regain its accuracy, but in the meantime it would be leading the ship astray.

Consequently the application of this, in itself wonderfully simple, idea, to this extremely important purpose was accompanied with a difficulty which was for a long time insuperable.

But all was overcome at last by the genius of Dr Anschutz, of Hamburg, whose firm were the first to turn out the practicable article. Taking advantage of another movement of the gyroscope when arranged as has been described, and using the revolving wheel itself as a centrifugal fan, he was able to make the wheel blow air "against itself," as it were, when in any position other than north and south. Thus, if it deviates towards the east, this jet of air tends to blow it back; if it turns westwards the jet again comes into operation, tending to bring the erring gyro back to its proper place; and so the tendency to oscillate is checked.

The finished instrument as it is installed on the latest warships is, of course, quite different in detail from the simple contrivance which we have been considering so far, although it is the same precisely in principle. The essential part is a heavy metal wheel combined with which is an electric motor which keeps it rotating at a speed of 20,000 or so times per minute.

The bearings of the wheel are supported upon a metal ring which floats upon the surface of a trough of mercury. Thus friction is brought down almost to the irreducible minimum. The only place where the wheel and its supports touch anything solid is at one delicately made pivot which serves to keep the floating mechanism in the centre of the mercury basin, and to prevent it from rubbing against the side of it. The current which drives the motor reaches it through this pivot and leaves through the mercury. Thus arranged, although the floating part is of considerable weight, a very slight force indeed is enough to move it; while, looking at it the other way, we can see that the ship might turn rapidly to right or to left, carrying round the mercury bowl with it, without turning the floating part at all. Thus the gyroscopic action is very free indeed to exercise its function of keeping the contrivance pointing always in the one way.

The float has mounted upon it a compa.s.s card much like that of the ordinary magnetic instrument, and the sailor reads it in precisely the same way. To outward appearance there is little essential difference; in one case there is a magnet under the card to keep it still, in the other there is the float with the revolving wheel mounted upon it.

It is customary to have one "master compa.s.s" of this kind on a ship, with an electrical repeater in each of the steering positions. As the "master" turns in its casing it sends a rapid series of currents to all the others, causing them to turn in unison with it. The "master" is fitted in some safe part of the ship where it is least likely to be the victim of any accidental damage.