Fig. 4Fig. 4
The "plates," as the electrodes in a secondary battery are termed, are generally large metal plates. Experiment has shown that lead is the best for this purpose. It has also been found that it can be improved by making it porous, since the inner surfaces of the pores are so much added surface through which current can pass into the electrolyte. There are various ways of producing this porosity, which need not trouble us here, however. It will suffice for our purpose to understand that an ordinary secondary cell consists of two lead plates, with the largest possible surface, immersed in a liquid, generally a dilute solution of sulphuric acid in water.
To charge the battery, current is sent to one plate, through the liquid to the other plate, and so away. A thin film of hydrogen is thus formed upon the outgoing plate, while oxygen is formed at the incoming one. Since the hydrogen is spread over such a large area, it does not accumulatesufficiently for much of it to rise to the surface. Most of it remains adhering to the plate. The oxygen combines with the lead of its plate and so is safely stored up there in the form of oxide of lead. This storage of hydrogen upon the one plate and oxygen on the other cannot go on indefinitely, and so as soon as the limit is reached the cell is fully charged. Passage of further current is then simply waste.
The dynamo or primary batteries which are used for charging having been disconnected, the two plates can be connected together through lamps, motors, or in any other desired way, and the current will then flow out again, the opposite way to that in which it entered, just as a stone thrown up in the air returns the opposite way. The current which comes out is, in fact, a sort of reflex action arising from that which went in, the mechanism by which it is produced being the reabsorption of the oxygen and hydrogen into the electrolyte.
Whether a cell is fully charged or not is ascertained by weighing the electrolyte, an operation which at first sight seems to have nothing whatever to do with the matter. It arises from the difference in weight between water and sulphuric acid, the latter being the heavier. We have seen that while a little acid must be added to water before it can be electrolysed, it is the water which is ultimately resolved into its constituent gases. Hence the result of electrolysis is to increase not the amount, but the proportion of acid. Therefore it increases the weight of the electrolyte. This weight is ascertained by means of a "hydrometer," a glass tube, stopped, and loaded with some small shot at its lower end. On the upper part is engraved a graduated scale, so that the exact depth to which it sinks can be easily read. This depth will, of course, vary with the specific gravity of the liquid, and so the depth recorded by the scale will be an indication of the proportion of acid, and that in turn will show how far the process of charging has progressed.
Accumulators are, or have been hitherto at any rate, very troublesome things. They are apt to lose their power. Ifnot properly charged they are easily damaged. Too rapid charging or too rapid discharging, standing for a while only partly charged—all these things have a bad effect, in extreme cases even destroying them altogether. Because of the use of lead they are terribly heavy too, so much so that for traction purposes they are of very little use, for a large amount of the energy stored in the accumulators is then used up in hauling them about.
Yet what a field there is for the successful accumulator! Take the one instance of the electrification of a railway. If good light and efficient accumulators were to be had, no alteration at all would be necessary to the permanent way. The engines or motor carriages would need to go periodically to a depot to be re-charged, but that could easily be arranged. Such things as conductor rails, overhead conductors and so on would be needless.
The world has therefore been interested for years in the rumour that T. A. Edison was engaged upon this problem, and at last he has produced his accumulator, by which he has removed many of the difficulties, if not all. Instead of a case of glass or celluloid, as is usual with the older cells, his cells are enclosed in strong boxes of nickel steel. The positive plate consists of nickel tubes filled with alternate layers of nickel hydroxide, while the negative plate is formed of prepared oxide of iron in a nickel framework. The electrolyte is a solution of potassium hydroxide. The chemical action and the electrical reaction is, of course, on the same principle precisely as in the older cells, but it is claimed that the Edison cells are "fool-proof"—that is to say, they cannot be damaged by careless handling, and they appear to be a little lighter. Thus the problem is partly solved, and with that as a fresh starting-point someone may sooner or later give us a secondary battery which is light as well as strong.
If any would-be scientific inventor reads these words there is a suggestion for a promising line of investigation.
One of the most remarkable adaptations of scientific knowledge is the "manufacture of cold." At first that phrase seems strange, but it is really quite legitimate. There are machines at work at this moment which are turning out cold as if it were any other manufactured article. It is not that they manufacture cold water or cold air, it is the cold itself which they produce.
Of course, cold has no real existence, since it is simply a negative quantity, an absence of heat, yet its effects are so real that we are in the habit of talking of it as if it were a reality, and in that sense we can regard it as a product of manufacture.
Moreover, we see in this a conspicuous instance of the interdependence of invention and science, for scientific principles were first adapted to produce cold, and then artificial cold was employed in scientific investigations, whereby the rare gases of the atmosphere have been discovered, as we shall see presently.
InMechanical Inventions of To-dayI have dealt with the uses which can be made of heat as a motive power. Here we have in some sense a reversal of the process. In the heat-engine the expenditure of heat produces motion. In the refrigerating machine motion produces heat, on the face of it a strange way of producing cold. Yet it is by the production of heat in the first instance that we are ultimately able to obtain the cold.
One way to make a thing cold is to place it in contact with ice. But that process suffers from severe limitations. In the first place, we may not be able to procure ice when wewant it. And in the second place, we may want to produce a temperature much lower than that of ice.
Now a machine can produce any degree of coldness, almost down to the "absolute zero," the point at which a body is absolutely devoid of any heat whatever, the condition in which its molecules are absolutely still. That point is 274° C.belowfreezing-point. Freezing-point on that scale is "zero," and so thisabsolutezero isminus274°. Or, to put it another way, freezing-point is 274°absolutetemperature. The absolute zero has never been reached, and there is reason to believe that it never can be quite reached, but by methods about to be described a temperature within a few degrees of it has been attained. And all of this can be done without any cooling agent colder than water at an ordinary temperature.
There are several systems, but the one which illustrates the principle most simply is that in which carbonic acid gas is the "working fluid." This is a very compressible gas, and so is well fitted for the purpose. First of all a pump or compressor compresses it. That has the effect of heating it. Such we might expect from the fact that heat is molecular activity: when by compressing the gas we force the molecules closer together, they naturally hit each other and the sides of the containing vessel harder than they did before, and the increased activity is manifested as increased heat. So the first effect, as was remarked just now, is to produce, apparently, increased heat.
But then the hot compressed gas, by being passed through a coil of pipe surrounded by cold water, can be robbed of that heat. According to the speed at which it traverses the coil it will be more or less cooled: by causing it to travel slowly it can be brought down almost to the temperature of the water. So we start with the gas at atmospheric pressure and at somewhere about atmospheric temperature too. This we convert into compressed gas at a high temperature. After cooling it we have compressed gas at a moderate temperature.
Then, to complete the process, we let the gas expand again. Now just as compressing a gas heats it, letting it expand cools it. If we compressed it and then expanded it again we should be just as we were to commence with. But since, in between the two operations we extract a quantity of heat by means of the cooling water, we get at the end a very much lower temperature than that with which we started.
We cannot cool the gas without compressing it, because heat will only flow from one body into another when the second is cooler than the first. But by making the gas hot temporarily by compression we enable the water to draw some heat from it, and then, allowing it to sink back to its original state, we get practically the old temperature, less what the water has extracted. The principle is really absurdly simple when one once gets to understand it. The application is not so simple as far as the designer of the machine is concerned, for he has to adjust the various parts to exactly the right shape and dimensions, so that they may work well with one another and produce the desired result with the minimum expenditure of power.
To the observer, however, and to the user too, the finished machine is wonderful in its simplicity. The principle is illustrated diagrammatically in Fig. 5.
In the centre is the compressor. Its action forces the gas along the pipe to the right and down into the condenser. As it flows downwards through the coil there cold water enters at the bottom of the tank, flows upward past the coil and escapes again at the top. Thus the coil is kept in contact withcoldwater.
Passing then through the bottom of the tank the gas travels from right to left through the "regulating valve" and into an arrangement almost exactly similar to the condenser but called the evaporator. Here the gas expands and suffers a great fall in temperature. This cold is communicated to liquid circulating in the tank which forms a part of the evaporator, and this liquid can be circulatedthrough pipes into any rooms to be cooled or around vessels of water which it is desired to freeze. This liquid, which acts as the carrier of the cold, is called "brine," and is water to which is added calcium chloride to keep it from freezing.
Fig. 5.—This diagram shows the working of the Refrigerating Machine. The pump compresses the gas and drives it through the coil in the condenser, where it is cooled by water. It passes thence through the coil in the evaporator, where it expands and cools the surrounding brine.Fig. 5.—This diagram shows the working of the Refrigerating Machine. The pump compresses the gas and drives it through the coil in the condenser, where it is cooled by water. It passes thence through the coil in the evaporator, where it expands and cools the surrounding brine.
Now the observant reader may have noticed that there is no apparent reason for the name of the left-hand vessel. It will be quite clear, however, when I explain that although I have spoken of the working fluid all along as gas, I have only done so to avoid bringing in too many explanations at once. It is actually liquid for a good part of its journey. Carbonic acid gas liquefies at a very moderate temperature and pressure, and so while it leaves the compressor as a gas it becomes liquid in the condenser and remains so until it has passed the regulating valve. Then it begins to expand into gas once more, and in that state it passes back to the compressor.
There is a pressure-gauge on the pipe leaving the compressor and another on the one entering it. A comparison of the readings on these two tells how the apparatus isworking. The difference between them indicates how much compression is being given to the gas. Assuming that the compressor is working at a constant speed, this compression can be regulated to a nicety by the valve: close it a little and the compression will increase: open it a little and the compression will decrease. By this means the degree of cold produced can be varied at will.
This is the way in which many ships are enabled to carry cargoes of frozen meat. The chambers in which the meat is stowed are insulated—that is to say, their walls are made as impervious as possible to heat. Then the brine is carried into the chambers in pipes, cooling them much as the hot-water pipes heat an ordinary public building.
Or another method is to carry the pipe which constitutes the evaporator into the chamber to be cooled. A third way is to dispense with brine and to blow air through the coils of the evaporator, whereby the air is made to carry away the cold to wherever it is needed.
Ice can be made easily in moulds of metal or wood around which brine circulates. If made of ordinary water the ice is likely to be cloudy and opaque, which is quite good enough for many purposes. In cases where it is desired that it should be clear, the water is agitated during freezing, or else distilled water is used. To enable the blocks to be got out of the moulds it is sometimes arranged to circulate warm brine for a few moments.
Ice skating rinks are formed by making, first, an insulating layer of sawdust, slag-wool or something of that sort (those by the way, being the materials generally used for insulating cold chambers) underneath the floor. The floor, too, is made waterproof and then upon it is laid as closely as possible a series of iron pipes. Water is flooded on to the floor until the pipes are covered to a depth of several inches, and then brine is pumped through the pipes. In time the water freezes, and so long as the brine circulates it remains so.
But although the "CO2process" described above is the simplest illustration of the principle, there are other systems. In one very popular form ammonia gas is the "working fluid." This is liquefied by pressure and cooling with water, being subsequently expanded just as described above.
By permission of Messrs. J. and E. Hall, Ltd., London and Dartford Machine-made Ice Here we see a huge block of ice being lifted (it may be on a hot summer day) from the mould in which it has been madeBy permission of Messrs. J. and E. Hall, Ltd., London and DartfordMachine-made IceHere we see a huge block of ice being lifted (it may be on a hot summer day) from the mould in which it has been made
Another much-used system is the "ammonia-absorption" process, in which the ammonia is not liquefied, but when under pressure is absorbed by water, returning to gas again when the pressure is released.
But the degree of cold attained in these commercial machines is as nothing to the extremely intense cold generated on the same principles in the liquid-air machine, which is found in every well-equipped physical laboratory.
Briefly, this consists of a coil of many turns of small tube enclosed in a small double vessel, the space between the inner and outer skins of which is packed with insulating material. A compressor pumps air in at the top of the coil at a pressure of from 150 to 200 atmospheres. An "atmosphere," it may be remarked, is a unit often used in scientific matters, meaning the normal pressure of the atmosphere, which is, roughly speaking, 15 lb. per square inch. Hence 200 atmospheres is about 3000 lb. per square inch.
Of course air so highly compressed as that is hot, but after it has passed down the coil and has escaped from the valve which liberates it at the bottom it is much cooler. But that is only the beginning of the operation. The expanded, and therefore cooled, air finds its way upward through the turns of the coil down which the following air is coming. That, expanding in its turn, is colder still, because of the cooling action of the first air, and so the process goes on.
This is perhaps easier to understand if we imagine that the air comes through the coil in gusts and we notice what happens to each succeeding gust. The first comes down, expands, cools and ascends, thereby cooling the second gust as it comes down. The second then, after expansion, will be cooler than the first was. That in its turn will cool the third, and so the third after expansion will be cooler thanthe second. And that will go on, each succeeding gust being cooler than the one before. And although the flow of air is continuous, and not in gusts, the result is just the same: it goes on getting cooler and cooler until at last the air comes out in its liquid form. This liquid collects in a little chamber formed at the bottom of the vessel which contains the coil and can be drawn off when desired.
Air in its liquid state looks very much like water. In fact it is difficult to get chance observers to believe that it is not water. It boils at a temperature far below the freezing-point of water, so that liquid air if placed in a cup made of ice will boil furiously. Ice is so much the hotter that it behaves towards liquid air as a very hot fire does to water.
The feature of the above machine, it will be noticed, is that no cooling water is required, as in the refrigerating machine, although the principle of the two is the same. The coil is the "condenser" and the vessel in which it is enclosed is the "evaporator," and so the cold air produced by the process in the evaporator cools the coil of the condenser. Thus it is "self-intensive," as the makers call it.
Hydrogen can be liquefied in a similar machine, except that it needs a little preliminary cooling with liquid air. Liquid hydrogen is the coolest thing known approaching the region of absolute zero.
And now we can turn to the wonderful discoveries which have followed upon the manufacture of liquid air.
To make the story complete we need to go back to the time of Priestly and Cavendish, early in last century. They investigated the atmosphere and showed that it consisted of oxygen and nitrogen in certain invariable proportions, with under certain conditions a small proportion of carbonic acid. These facts were so well authenticated, and they seemed to explain everything so satisfactorily, that it was quite thought almost up to the end of the nineteenth century that there was nothing more to learn about the atmosphere.
Nevertheless there was an idea in the minds of some scientists that there must be another group of elementssomewhere, the existence of which was then undiscovered, but it was never dreamed that these were in the air.
Soon after the weights of the atoms had been found a medical student named Prout in an anonymous essay called attention to the fact that there were curious numerical relationships between them. Speculation on the subject went on for many years, until in 1865 the great Russian chemist Mendeléeff published his conclusions. He had arranged the elements in the form of a tablein the order of their atomic weights. The table consisted of twelve rows of names forming eight vertical columns, and the remarkable thing was that all those elements which fell into any particular column, although their atomic weights were very widely different, had similar properties. This enabled him topredictthe discovery of certain new elements, for the table contained a number of blank spaces. Three elementshave been foundsince, and their atomic weights and properties are just such as to fill three of the blank spaces. One blank space, it is thought, may be filled some day by the gas coronium, which like helium has been discovered in the sun, but unlike it has not yet been detected here. When it is, there is the place in the table which it may fill. The table then commenced with what is still called Group 1, but for reasons too complicated to explain here it appeared as if there must be a group before that, a group the chief characteristic of which would be the inactivity of the elements included in it. These were expected to be of various atomic weights, but these weights, it was anticipated, would so occur in the intervals between the others that they would all fall into a new column to the left of "Group 1."
In the year 1892 Lord Rayleigh was investigating the question of the density of a number of different gases, including, so it happened, nitrogen. Now there are several ways of procuring nitrogen. One is to get it from the atmosphere by ridding it of the oxygen with which it is normally mixed. Another way is to split up some compound, such as ammonia, of which it forms a part, in such away as to catch the nitrogen and leave the other elements with which it was combined elsewhere.
Lord Rayleigh tried both ways, and he found that the nitrogen from the atmosphere was denser than that derived from ammonia. Sir William Ramsey then carried the matter a step further. He heated atmospheric nitrogen in the presence of magnesium, under which conditions some of the nitrogen combines with the latter element to form nitride of magnesium. That, it was found, made the remaining nitrogen denser still. The explanation then seemed obvious. Suppose we imagine a mixture of sawdust and iron filings: it will be heavier than an equal quantity of pure sawdust. And if we contrive to take away some of the sawdust from the mixture we shall find that what is left is heavier still, when compared with an equal bulk of pure sawdust. For it is clear that as we take away sawdust we thereby increase the proportion of the heavier iron filings and so we make the mixture heavier.
Applying a similar process of reasoning to these discoveries, the conviction grew that the nitrogen of the air was not pure, but that it had mixed with it a small proportion of some other gas of greater density. They soon succeeded in isolating this denser gas, to which they gave the name of argon. Its atomic weight was found, and, wonderful to relate, it was such that argon fell into a new column to the left of Group 1, as had been anticipated.
The discovery of argon was announced in 1894. The next year Sir William Ramsey, investigating a gas which had been discovered locked up in the interstices of a mineral called clevite, was able to state that it was helium, the element which had been previously noticed by the spectroscope in the sun. Like argon, it was found to be extremely inactive, and its atomic weight turned out to be such that it too fell into the "Zero Group."
In 1898 Professors Ramsey and Travers found two more gases in the air, krypton and neon, and a little later still, there was found mixed with the krypton a further new gas,xenon. All of these had their atomic weights found, and fell into that new column in the periodic table.
But what has all this got to do with liquid air? The two subjects are closely related, for it is by liquid-air machines that these rare gases are now obtained, and it was from liquid air that the last three were first discovered.
For air, as we well know, is a mixture of gases, and when extreme cold and pressure are applied these gases liquefy, each behaving according to its own nature. They do not all liquefy at the same time, nor on being relieved from the pressure and heated do all evaporate again at the same temperature. Although they emerge from the liquid-air machine in the form of a single liquid, it is really a mixture of liquids, each with its own boiling-point.
In an earlier chapter we saw how petroleum can be separated into its various constituents, such as petrol, by fractional distillation, advantage being taken of the difference in the "boiling-point" of the various "fractions." The boiling-point of a liquid is, of course, the temperature at which it turns freely into vapour, and just as petroleum when heated gives off first cymogene, next rhigolene, then petrol, benzine, kerosene and so on, in the order named, so liquid air, when it is evaporated, gives off its different constituents in order. Nitrogen, oxygen, argon, helium, krypton, neon and xenon can all be separated each from the others in this way, by "fractional distillation." The heat from the surrounding objects is allowed to get at the liquid, and the gases are then given off in the order of their boiling-points.
And thus we see how the mechanical production of cold has assisted in the pursuit of pure science. The newly-found gases are not of any great use at present. They are so inactive that possibly they never will be, with one exception, and that is neon. If an electric discharge be made to pass through a tube filled with this gas, a beautiful glow is the result, and it is just possible that neon tubes may become the electric light of the future. That is only a prediction, however, and a hesitating one at that.
The inactive elements may become of value in explosives. We have seen how important nitrogen is in these dangerous substances, the chief feature of which is their instability—their readiness, that is, to change into something else—which instability is due to the reluctance with which nitrogen enters into them. Now nitrogen, though inactive, is much less so than these others, and if a way should ever be found of inducing them to enter into a compound, that compound will probably be an extremely powerful explosive.
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. Fromsome 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 showon 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.
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. 67By permission of Messrs. J. and E. Hall, Ltd.A Cold StoreInterior of a cold store, in which meat and poultry are kept good and fresh by the use of machine-made cold.—See p. 67
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.
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 verypowerful 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 beseenthrough.
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 onlyseea 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 isnaturallyreddish is so because it is more plentifully endowed with red rays, while a bluish light like the electric arc is naturallydeficient 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 21⁄4pints 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 brightpolished 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. Butreflectionby a piece of the clearest glass seems a contradiction in termsalmost. 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 shinesdirectly 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." Thenagain 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 topof 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 71⁄2seconds. The optical apparatus weighs seven tons.