Читать книгу Lightships and Lighthouses - Frederick Arthur Ambrose Talbot - Страница 5

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While it is the tower that probably creates the deepest impression upon the popular mind, owing to the round of difficulties overcome associated with its erection, yet, after all, it is the light which is the vital thing to the navigator. To him symmetry of outline in the tower, the searching problems that had to be solved before it was planted in a forbidding spot, the risks that were incurred in its erection—these are minor details. His one concern is the light thrown from the topmost height, warning him to keep off a dangerous spot and by its characteristic enabling him to determine his position.

I have described the earliest type of light, the open wood or coal fire blazing on an eminence. In due course the brazier gave way to tallow candles. This was an advance, certainly, but the range of the naked light was extremely limited. Consequently efforts were made to intensify it and to throw it in the desired direction. The first step was made with a reflector placed behind the illuminant, similar to that used with the cheap wall-lamp so common in village workshops. This, in its improved form, is known as the “catoptric system,” the reflector being of parabolic shape, with the light so disposed that all its rays (both horizontal and vertical) are reflected in one direction by the aid of a highly polished surface. While the catoptric system is still used on some light-vessels, its application to important lighthouses has fallen into desuetude, as it has been superseded by vastly improved methods. But the reflector, made either of silvered glass set in a plaster-of-Paris mould or of brightly polished metallic surfaces, held the field until the great invention of Augustin Fresnel, which completely revolutionized the science of lighthouse optics.

Fig. 2.—Fixed Apparatus of 360 Degrees.

Shows one ray throughout the complete circle.

(By permission of Messrs. Chance Bros. and Co., Ltd.)

Fresnel was appointed a member of the French Lighthouse Commission in 1811, and he realized the shortcomings of the existing catoptric method only too well. Everyone knows that when a lamp is lighted the luminous rays are diffused on every side, horizontally as well as vertically. In lighthouse operations the beam has to be thrown in a horizontal line only, while the light which is shed towards the top and bottom must be diverted, so that the proportion of waste luminosity may be reduced to the minimum. While the parabolic reflector achieved this end partially, it was far from being satisfactory, and Fresnel set to work to condense the whole of the rays into a horizontal beam. Buffon, a contemporary investigator, as well as Sir David Brewster, had suggested that the end might be met by building up a lens in separate concentric rings, but neither reduced his theories to practice.

Fresnel invented a very simple system. He took a central piece of glass, which may be described as a bull’s-eye, and around this disposed a number of concentric rings of glass. But these rings projected beyond one another. Each constituted the edge of a lens which, while its radius differed from that of its neighbour, owing to its position, yet was of the same focus in regard to the source of illumination. The parts were shaped with extreme care and were united in position by the aid of fish glue, the whole being mounted in a metal frame. The advantage of the system was apparent in the first demonstrations. The lenses being comparatively thin, only one-tenth of the light passing through was absorbed, whereas in the old parabolic reflectors one-half of the light was lost.

Fig. 3.—Single Flashing Apparatus (One Panel and Mirror).

(By permission of Messrs. Chance Bros. and Co., Ltd.)

This revolutionary development was perfected in 1822, and in the following year it was submitted to its first practical application on the tower of Cordouan in the Gironde. Several modifications were made by the inventor for the purpose of adapting his system to varying conditions. One of the most important was the disposition of lenses and mirrors above the optical apparatus for the purpose of collecting and driving back the rays which were sent out vertically from the illuminant, so that they might be mingled with the horizontal beam, thereby reinforcing it. At a later date similar equiangular prisms were placed below the horizontal beam so as to catch the light thrown downwards from the luminous source, the result being that finally none, or very little, of the light emitted by the illuminant was lost, except by absorption in the process of bending the rays into the desired direction.

Fig. 4.—A Twenty-Four Panel Light, which was introduced into Certain French Lighthouses.

In this ingenious manner the circle of light is divided into sections, called “panels,” each of which comprises its bull’s-eye and its group of concentric rings and prisms. The extent of this division varies appreciably, as many as sixteen panels being utilized in some instances. In this direction, however, subdivision can be carried too far. Thus, in some of the French lighthouses no less than twenty-four panels were introduced. The disadvantage is obvious. The total volume of light emitted from the luminous source has to be divided into twenty-four parts, one for each panel. But the fewer the panels, the more light is thrown through each, and the correspondingly greater power of the beam. Thus, in a four-panel light each beam will be six times as powerful as that thrown from a twenty-four panel apparatus of the same type.

Fresnel also introduced the system of revolving the optical apparatus, and by the introduction of suitable devices was able to give the light a flashing characteristic, so that it became possible to provide a means of identifying a light from a distance entirely by the peculiarity of its flash. The French authorities were so impressed with the wonderful improvement produced by Fresnel’s epoch-making invention that it was adopted immediately for all French lights. Great Britain followed suit a few years later, while other countries embraced the system subsequently, so that the Fresnel lens eventually came into universal use.

Fig. 5.—A Four-Panel Light.

The ray thrown through each panel is six times as powerful as the beam thrown through a twenty-four panel apparatus.

But the Frenchman’s ingenious invention has been developed out of recognition. To-day only the fundamental basis is retained. Marked improvements were made by Mr. Alan Stevenson, the famous Scottish lighthouse engineer. In fact, he carried the idea to a far greater degree than Fresnel ever contemplated, and in some instances even anticipated the latter’s subsequent modifications and improvements. This was demonstrated more particularly in the holophotal revolving apparatus, the first example of which he designed for the North Ronaldshay lighthouse in 1850, a similar apparatus being devised some years later by Fresnel. In 1862 another great improvement was made by Mr. J. T. Chance, of the well-known lighthouse engineering firm of Birmingham, which proved so successful that it was incorporated for first and third order apparatuses in the New Zealand lights designed by Messrs. Stevenson in the same year.

Fig. 6.—Single Apparatus in Four Panels.

(By permission of Messrs. Chance Bros. and Co., Ltd.)

The French and British investigators, however, were not having things entirely their own way. The United States played a part in these developments, although they did not enter very successfully into the problem. The first lighthouse at Boston Harbour carried candles until superseded by an ordinary lamp, which was hung in the lantern in much the same way as it might have been suspended behind the window of a private dwelling. An inventor, Mr. Winslow Lewis, who confessed that he knew nothing about lighthouse optics, patented what he called a “magnifying and reflecting lantern” for lighthouse work, which he claimed was a lamp, a reflector, and a magnifier, all in one. It was as crude a device as has ever emanated from an inventive brain, but the designer succeeded in impressing the Government so effectively that they gave him £4,000, or $20,000, for his invention. The reflector was wrought of thin copper with a silvered surface, while the magnifier, the essence of the invention, was what he called a “lens,” but which in reality comprised only a circular transparent mass, 9 inches in diameter, and varying from 2½ to 4 inches in thickness, made of bottle-green glass. The Government considered that it had acquired a valuable invention, and was somewhat dismayed by the blunt opinion of one of its inspectors who held contrary views concerning the magnifier, inasmuch as he reported cynically that its only merit was that it made “a bad light worse.”

Fig. 7.—Double Flashing Apparatus: Two Panels and Mirror.

(By permission of Messrs. Chance Bros. and Co., Ltd.)

Fig. 8.—Double Flashing Apparatus: Two Groups each of Two Panels.

(By permission of Messrs. Chance Bros. and Co., Ltd.)

The inventor did not manifest any antagonism to this criticism, but immediately pointed out the great economy in the consumption of oil that was arising from the use of his idea. Indeed, he prosecuted his claims so successfully that he clinched a profitable bargain to himself with the Government. His apparatus had been fitted to thirty-four lights, and he contracted to maintain them on the basis of receiving one-half of the oil previously consumed by the lamps which his invention superseded. This arrangement was in vogue for five years, when it was renewed, with the difference that on this occasion the Government, concluding that the inventor was making too much out of the transaction, reduced the allowance to one-third. Subsequently the invention received higher commendation from the officials than that advanced by the critical inspector, although it must be pointed out that meanwhile the magnifying bull’s-eye had been abandoned, and a new type of reflector introduced, so that the sole remaining feature of the wonderful invention was the lamp. Even that had been modified. When the Lighthouse Board was established in 1852 it abolished the much-discussed invention, and introduced the Fresnel system, bringing the United States into line with the rest of the world.

Fig. 9.—Triple Flashing Apparatus: Three Panels and Mirror.

(By permission of Messrs. Chance Bros. and Co., Ltd.)

One feature of the subject cannot fail to arrest attention. This is the possibility of producing a variety of combinations by the aid of the lenses to fulfil different requirements. The Fresnel, Stevenson, and Chance developments in the science of lighthouse optics facilitated this work very significantly. Accordingly, to-day a variety of lights, evolved from the variations in the mounting of the lenses, is in vogue. For purposes of identification they have been divided into a number of classifications, and, for the convenience of the navigator, are described as lights of the first order, second order, and so on. Broadly speaking, there are seven main groups, or orders, the rating only applying to dioptric or catadioptric lights, indicating the bending of the luminous rays in the desired direction, either by refraction and reflection through the medium of prisms, or a combination of both. Actually there is a distinction between these two, the true dioptric system referring only to refraction, where the ray is bent in the desired direction by a glass agent, known as a “refracting prism.” In the catadioptric system, on the other hand, both methods are employed, since the prism performs the dual purpose of reflecting and refracting the rays. However, in modern lighthouse parlance both are grouped under the one distinction “dioptric.”

The rating or classification of the lights varies according to the inside radius or focal distance of the lens—in other words, the distance from the centre of the light to the inner surface of the lens. The main groups are as follows:

Hyperradial,	1,330	millimetres	(52·3 inches)	focal distance.
1st order,	920	”	(36·2 ”)	”
2nd ”	700	”	(27·6 ”)	”
3rd ”	500	”	(19·7 ”)	”
3½ ”	375	”	(14·7 ”)	”
4th ”	250	”	( 9·8 ”)	”
5th ”	187·5	”	( 7·4 ”)	”
6th ”	150	”	( 5·9 ”)	”

The most powerful apparatus used to-day, however, is that known as the “hyperradiant,” and it is the largest which has yet been devised. For this, lighthouse engineering is indebted to Messrs. Stevenson, the engineers to the Commissioners of Northern Lighthouses. It was first suggested as far back as 1869, and experiments were carried out which emphasized the fact that such an apparatus was required, since it was found that when large gas-burners were used much of the light in revolving apparatuses was out of focus and escaped condensation. The Scottish engineers thereupon suggested that an apparatus should be used having a focal distance of 1,330 millimetres, or 52·3 inches. In fact, they went farther and suggested even larger apparatuses, but this idea has not matured. But it was not until 1885 that Messrs. Stevenson had such a system manufactured, and then it was tested at the South Foreland beside the powerful lenses which had just been built for the new Eddystone and the Mew Island lighthouses. The merits of the theories advanced by Messrs. Stevenson were then completely proved, for it was found that with a ten-ring gas-burner the hyperradiant apparatus threw a light nearly twice as powerful as that given by the rival lenses with the same burner.

Fig. 10.—Quadruple Flashing Apparatus: Four Panels.

(By permission of Messrs. Chance Bros. and Co., Ltd.)

At the present moment the hyperradiant is regarded as the ultima thule of lighthouse optical engineering, and Messrs. Chance Brothers and Co., of Birmingham, have built some very magnificent apparatuses of this order. At present there are not more than a dozen such powerful lights in operation. Three are on the English coast, at Bishop Rock, Spurn Point, and Round Island, respectively; two in Scotland, at Fair Isle and Sule Skerry; two in Ireland, at Bull Rock and Tory Island; one in France, at Cap d’Antifer; one in China, at Pei Yu-shan; one in India, at Manora Point, Karachi; and the Cape Race light in Newfoundland. The hyperradiant apparatus is a massive cage of glass, standing some 12 feet in height, and, as may be supposed, is extremely expensive.

There is another point in lighthouse optics which demands explanation. This is the term “divergence,” which plays an important part in the duration of the flash. In speaking about focus, the engineer follows somewhat in Euclid’s footsteps in regard to the definition of a point; in a way it is equally imaginary. The focal point does not mean the whole of the flame, but the centre of the luminous source, and, as is obvious, it is impossible to secure a flame without dimensions. It may be an attenuated, round, oval, or fan-shaped light—the result is the same. The focal point is the theoretical centre of the luminous source, and the rays, coming from the top, sides, and bottom of the flame cannot come from the true focus. If they did, all the light from one panel would be emitted in absolutely parallel lines, and therefore in a revolving apparatus the beam would pass any given point on the horizon in an infinitely short period of time—to be precise, instantaneously. But the ex-focal rays of the flame, in passing through the lens, emerge at an angle to those coming from the absolute centre, so that the whole beam becomes “diverged,” and throws a cone of light from the lens. Consequently the beam occupies an appreciable period of time in passing a given point on the horizon.

As may be supposed, the intricate character of the lenses constituting the optical apparatus of the modern lighthouse demands the highest skill and infinite care in their preparation, while the composition of the glass itself is a closely guarded secret. There are less than half a dozen firms in the world engaged in this delicate and highly specialized work, of which France claims three, Germany one, and Great Britain one. All the lighthouse authorities of the various nations have to secure their requirements from one or other of these organizations. The industry commenced in France, and for many years the French reigned supreme. Then it contrived to make its entrance into England, and was taken up by the family of Chance in Birmingham, who soon proved themselves equal to their French leaders.

Fig. 11.—Red and White Flashing Apparatus.

(By permission of Messrs. Chance Bros. and Co., Ltd.)

The British firm has established a unique reputation, as it has been responsible for the majority of the great lights of the world, some of which are not only of huge dimensions and weight, but also of novel form. The hyperradial apparatuses which have been placed recently in the towers of Manora Point and Cape Race probably rank as the most powerful and the finest in existence. These are used in conjunction with the petroleum vapour incandescent burner. The Cape Race light, for instance, comprises a revolving optic of four panels, subtending a horizontal angle of 90 degrees, with a vertical angle of 121½ degrees. Each lens comprises the central disc, or bull’s-eye, around which are placed nine rings of glass, giving a total refracting angle of 57 degrees. In order to bend the vertical rays into a horizontal path twenty-two catadioptric reflecting prisms are disposed above the lens, while below are thirteen similar prisms. The total amount of glass worked into the four panels is about 6,720 pounds, and the prisms are mounted in gun-metal frames, which weigh approximately 4,800 pounds, so that the total weight of the glass portion and its mounting alone, standing some 12 feet in height, is over 11,500 pounds. The installation completed for the equipment of the Manora Point lighthouse, Karachi, is very similar.

In some cases the demand for a powerful light has been met with a system differing from the “hyperradiant.” The lenses and respective groups of refractors are superimposed, each tier having its individual burner and flues for carrying off the products of combustion. In this way we have the biform, comprising two such panels arranged one above the other, as in the Fastnet and Eddystone lights; and the quadriform, wherein four tiers are built one above the other, as installed at the Mew Island light in Ireland. The advantage of this arrangement is that a beam of great intensity is secured with a lantern of comparatively small diameter.

The French authorities adopted a modification of this system. Instead of placing two lenses and refractors one above the other, they ranged them side by side, the effect being analogous to a couple of squinting eyes, the panels being parallel and therefore throwing out parallel beams. But these adaptations have not come into extensive use, as they have been superseded by more simple means of achieving similar requirements with an even more powerful ray. The hyperradiant stands as the finest type of apparatus yet devised, and therefore is employed when an extremely powerful light is required.

While the design and arrangement of the optical apparatus is certainly a most vital and delicate task, the mounting thereof upon a substantial support in such a way that it may perform its work with the highest efficiency is equally imperative, since the finest apparatus might be very adversely affected by being improperly mounted.

Fig. 12.—Apparatus showing a Double Flash, followed by a Single Flash.

(By permission of Messrs. Chance Bros. and Co., Ltd.)

Obviously, owing to the great weight of the glass, the support must be heavy and substantial. A massive cast-iron pedestal is employed for this purpose. When the light is of the revolving character, means have to be incorporated to secure the requisite rotation. In the early days the turntable upon which the lens is mounted ran upon rollers, but now a very much better system is universally employed. This has been brought to a high standard of perfection by Messrs. Chance of Birmingham, who have carried out unceasing experiments in this field. The objection to rollers was the enormous friction that was set up, and the great effort that was required, not only to set the lenses revolving, but to keep them rotating at a steady pace. In the modern apparatus the rollers are superseded by an iron trough filled with mercury, upon which floats the turntable carrying the lenses. When the apparatus is properly built and balanced, the friction is so slight that the turntable can be set in motion by the little finger, notwithstanding that several tons have to be moved. Although the optical part of the apparatus floats upon the bed of quicksilver in the same way as a cork lifebelt floats upon water, it is provided with rollers which serve to hold the whole apparatus steady and to overcome any oscillation.

In the case of an immense apparatus such as a hyperradiant lens, which, together with the turntable, may have a total weight of 17,000 pounds, an enormous quantity of mercury is required. The trough of the Cape Race hyperradiant light carries 950 pounds of quicksilver, upon which the lantern is floated. In such an instance, also, the pedestal is a weighty part of the apparatus, representing in this case about 26,800 pounds, so that the complete apparatus utilized to throw the 1,100,000 candle-power beam from the guardian of the Newfoundland coast aggregates, when in working order, some 44,000 pounds, or approximately 20 tons.

Within the base of the pedestal is mounted the mechanism for rotating the optical apparatus. This is of the clockwork type driven by a weight. The latter moves up and down a tube which extends vertically to a certain depth through the centre of the tower. The weight of the driving force and the depth of its fall naturally vary according to the character of the light. In the Cape Race light the weight is of 900 pounds, and it falls 14½ feet per hour. Similarly, the length of time which the clock will run on one winding fluctuates. As a rule it requires to be rewound once every sixty or ninety minutes. A longer run is not recommended, as it would demand a longer weight-tube, while many authorities prefer the frequent winding, as the man on duty is kept on the alert thereby. As the weight approaches the bottom of its tube it sets an electric bell or gong in action, which serves to warn the light-keeper that the mechanism demands rewinding.

Fig. 13.—The Classification of Lights, showing the Respective Radius or Focal Distance of Lens from 150 to 1,330 Millimetres.

(By permission of Messrs. Chance Bros. and Co., Ltd.)

The weight and clockwork mechanism perfected by Messrs. Chance is regarded as one of the best in service. The rotation is perfect and even, owing to the governing system incorporated, while the steel wire carrying the weight is preferable to the chain, which is subject to wear and is noisy in action. In the Chance clockwork gear the weight is just sufficient to start the apparatus from a state of rest, the advantage of such a method being that, should the apparatus be stopped in its revolution from any untoward incident, it is able to restart itself.

Of course, the clockwork mechanism is required only in those cases where the lenticular apparatus has to be revolved. This introduces the question of avoiding confusion between lights. When beacons were first brought into service, the lights were of the fixed type, and the navigator, although warned by the glare to keep away from the spot so marked, was given no information as to his position. Accordingly, lighthouse engineers sought to assist him in this direction during the blackness of the night by providing a ready visual means of identification. Owing to the ingenuity which has been displayed, it has been rendered possible to ring the changes upon a light very extensively.

These may be subdivided broadly as follows:

Type of Light.	Symbol.	Characteristics.
Fixed	F.	A steady continuous light.
Flashing	Fl.	A revolving light showing a single flash at regular intervals, or a fixed light with total eclipses.
Fixed and flashing	F.Fl.	A fixed light varied at regular intervals by a single flash of greater brilliancy.
Group flashing	Gp.Fl.	Various combinations of flashes shown at regular intervals.
Occulting	Occ.	A steady light suddenly and totally eclipsed at regular intervals.

In the foregoing classifications only a white light is used. But it may so happen that the lighthouse, owing to its position and the dangerous character of the spot which it marks, carries a light which changes colour from white to red or green, which are shown alternately in various combinations. These characteristics are indicated as follows:

Type of Light.	Symbol.	Characteristics.
Alternating	Alt.	White and colour alternating.
Alternating flashing	Alt.Fl.	Flashing alternations by revolving mechanism.
Alternating fixed and flashing	Alt.F.Fl.	Fixed and flashing alternating.
Alternating group flashing	Alt.Gp.Fl.	Group flashing alternating.

In timing a revolving or flashing light, the cycle is taken from the beginning of one flash to the beginning of the next. In these readings the flash is always shorter than the duration of the eclipse, while an occultation is shorter than, or equal to, the length of the light interval. Since flashing and occulting may be carried out with a fixed light suddenly extinguished or eclipsed, the characterization is determined solely according to the relative duration of light and darkness, irrespective of the type of apparatus employed or the relative brilliancy. There is one peculiarity of the flashing light which may be remarked. At short distances and in clear weather a faint continuous light may be shown.

Hand in hand with the development of the optical apparatus has been the wonderful improvement in regard to the illuminants and the methods of producing a brilliant clear flame. The fuel first used upon the introduction of the oil lamp was sperm or colza oil, the former being obtained from the whale, and the latter from seeds and a wild-cabbage. Both were very expensive, so that the maintenance of a light was costly—so much so that the United States authorities devoted their efforts to the perfection of a high-class lard-oil. This proved highly satisfactory, possessing only one drawback. In winter it congealed so much under the low temperature that it had to be heated before it could be placed in the lamp; but once the light was set going, the heat radiated from the burner served to keep the oil sufficiently fluid to enable it to mount the wick to the point of combustion under capillary action.

So far as the American authorities were concerned, the advantages of lard-oil sufficed to bring a cheaper medium than colza-oil into vogue. A company, which had been induced by the Government to install an elaborate and expensive plant for the production of colza-oil, after prolonged experiment and efforts to reduce the cost of production, announced that it could not compete with the lard-oil, and suggested that the latter should be employed in preference to the colza. The Government agreed, but, to compensate the company for its trouble, purchased the plant which the latter had laid down.

The advances in the processes for refining petroleum, and the exploitation of the extensive resources of the latter, led to “earth-oil,” in some form or other, being employed for lighthouse purposes. The attempt was facilitated by the invention and improvement of the Argand burner, whereby a brilliant white annular sheet of flame is produced. Various lighthouse engineers devoted their attention to the improvement of this burner in conjunction with paraffin. Their results were completely successful, and at last paraffin became universally utilized as the cheapest and most efficient illuminant known.

The general method of feeding the lamps was to pump the oil from a low level to the burner, thereby producing practically a pressure-feed system in preference to the capillary action which is used in the ordinary household lamp. By increasing the number of rings the intensity of the flame was increased, until at last it was thought that with this development perfection had been attained so far as lamps were concerned.

Then came another radical revolution. The invention of the incandescent gas mantle by Dr. von Auer, and the complete change that it wrought in connection with gas lighting, induced lighthouse engineers to experiment in this field. As they could not use coal-gas, they devoted their investigations to the perfection of a gas from petroleum, which should be capable of combustion with the incandescent burner. Many years were devoted to these experiments, and many petroleum vapour systems were devised. One of the best known, most successful, and most scientifically perfect, is the Chance incandescent light. This burner is used in many of the most powerful lights of the world and has given complete satisfaction. The mantle varies in size with the size and type of the light, ranging from 35 to 85 millimetres in diameter, the latter, in conjunction with a hyperradial apparatus, producing a light exceeding 1,000,000 candle-power.

By courtesy of Messrs. Chance Bros. & Co., Ltd.

THE HYPERRADIAL APPARATUS FOR THE MANORA POINT LIGHT, KARACHI, INDIA.

Of 1,330 millimetres focus, this is the most powerful and largest lighthouse apparatus made.

Not only was a far more powerful light obtained in this manner with the assistance of the petroleum vapour burner and incandescent mantle, but the cost of maintaining the light was reduced, owing to the great economy in oil consumption that was effected thereby, the largest mantle and burner—85 millimetres—burning only 2½ pints of oil per hour. The light thus obtained, while being vastly superior to that derived from a six-wick oil-burner, enables a saving of nearly £48, or $240, per annum to be recorded, taking the cost of the petroleum at 1s., or 25 cents, per gallon delivered to the lighthouse.

While petroleum is generally used, some countries have adopted other oil fuels for small permanent lights. Thus, in Germany compressed oil-gas, water-gas associated with benzine vapour, and Blau liquid gas, are utilized. The last-named is coming very extensively into vogue, also, in Holland, Denmark, and Austria. Blau gas has the advantage that it can be transported in small steel tanks under extremely high pressure—up to 100 atmospheres, or approximately 1,400 pounds per square inch. It is an extract of oil-gas produced at a low pressure in the gas retorts, and then compressed so severely that it liquefies. The fuel, as it is drawn from the cylinder in which it is stored, has the pressure reduced by means of a valve, so that it reaches the burner in a gaseous form at a pressure equivalent to that of the coal-gas used in private houses, and is burned in the same way with an incandescent mantle. The advantage of this method lies in the facility with which large volumes of gas may be transported, a steel cylinder containing 7,500 cubic feet weighing only 132 pounds. It is also inexpensive, a bottle of the foregoing capacity costing only 12s. 6d., or $3. In some cases the incandescent mantles, the average life of which is about a fortnight, are of large diameter, running up to 100 millimetres, or about 4 inches.

Recently Mr. Gustaf Dalén, of the Gas Accumulator Company of Stockholm, the inventor of the Dalén flasher and sun-valve, which are described elsewhere, has introduced a new illuminant, which is coming into vogue, especially on the Continent. This is called “Daléngas,” and is a mixture of 9 per cent. dissolved acetylene and 91 per cent. atmospheric air. Here the dissolved acetylene gas is conducted from a storage reservoir or high-pressure gas cylinder, of special construction, to a governor, where the pressure is reduced, and then to the mixing apparatus, where the acetylene gas is associated with the air in the above proportions. The idea of this combination and method is to enable an acetylene gas mixture to be used with the ordinary incandescent mantles.

By courtesy of Messrs. Chance Bros. & Co., Ltd.

FIRST ORDER TRIPLE FLASHING LIGHT OF 920 MILLIMETRES FOCAL DISTANCE FOR CHILANG LIGHTHOUSE, CHINA.

The advantage of the Daléngas, according to present experience, is the increased candle-power that is obtainable as compared with other systems, the superiority being about 75 per cent. under ordinary conditions. With the largest Fresnel lenses a lighting power of 200,000 Hefner candle-power is secured, while with revolving lenses of the latest type a beam of 3,000,000 candle-power can be obtained. The flame is small, and thus becomes concentrated more in the focus of the lens, so that the divergence of the light may be diminished if desired. When a light of a certain range is to be installed, the optical apparatus can be made smaller for Daléngas than for other illuminants, and the cost is reduced correspondingly. Similarly, if the system is introduced into an existing light, the latter can be made appreciably more powerful, without changing the optical apparatus or affecting the divergence.

In this system the gas is conducted into the lens apparatus from above, and the lighting arrangement is quite independent of, and does not interfere in any way with, the revolving apparatus, while the time spent in changing the mantle is less than half a minute.

All combustible gases, mixed with air in certain proportions, may produce more or less violent detonations when fired. But the quantity of mixed gas in this instance is confined in the length of piping between the burner and the mixing apparatus, and this quantity is so small that an explosion cannot be dangerous. In fact, all such danger has been guarded against completely—is, indeed, impossible in any circumstances.

Electric light has been adopted in one or two cases; but while the foremost authorities agree that it throws the best, most brilliant and most powerful beam of light, the system is generally impracticable on account of its great cost. When tests with this light were made some years ago in comparison with the light thrown from oil burners, it was claimed that the latter, owing to its reddish-yellow tinge, was the most suitable from the all-round point of view, and that it could penetrate to a greater distance in foggy weather. I have been informed by several authorities, who have gone more deeply into this question since, that this is a fallacy, and that the advantage rests completely with electric light. Experience in Germany, which has two magnificent electric lighthouses, and in Scotland, certainly supports this contention, and I have been assured that the sole reason why electric lighting has not been adopted more widely is the heavy cost, both of installation and of maintenance. When electric lighting is rendered cheaper and is brought more to the level of existing lighting arrangements, one may expect another complete change in lighthouse practice. In this direction, as explained in another chapter, the Germans have carried out practical experiments in their characteristic manner, and have brought the cost of maintaining a most powerful electric light to the minimum.

One very great advantage of the electric light is the ease with which the power of the beam may be increased during thick weather, so as to secure penetration to the greatest distance, and decreased to suit easier conditions in clear weather.

This point raises the question, “From how far can a light be seen out at sea?” This factor is influenced by climatic conditions, and also by the curvature of the earth. The higher the light, or the spectator, or both, is elevated above the water, the greater the distance from which the light can be seen. The table on p. 52, prepared by Mr. Alan Stevenson, the eminent Scottish lighthouse engineer, gives the distances at which objects can be seen at sea, according to the respective elevations of the object and the eye of the observer.

For instance, the passenger on a liner the boat-deck of which is 40 feet above the water, approaching the English Channel, will sight the Bishop Rock light from a distance of about 22 miles, because the focal plane—that is, the bull’s-eye of the lens—is 163 feet above the water, which, according to the following table, equals about 14½ miles, to which must be added the height of the boat’s deck, 40 feet representing 7·25 miles. Similarly, the ray of the Belle Ile light will come into view when the vessel is 32½ miles distant—height of focal plane of light, 470 feet = 25 miles, + eye of observer on board the liner, 45 feet = 7·69 miles; while the Navesink light, being 246 feet above the water, may be picked up by the captain of a liner from a distance of 28 miles. The range of many lights, however, owing to the curvature of the earth, is greatly in excess of their geographical range, and with the most powerful lights the glare of the luminous beams sweeping the clouds overhead may be seen for a full hour or more before the ray itself comes into view.

TABLE OF DISTANCES AT WHICH OBJECTS CAN BE SEEN AT SEA, ACCORDING TO THEIR RESPECTIVE ELEVATIONS AND THE ELEVATION OF THE EYE OF THE OBSERVER.

Heights in Feet.	Distances in Statute or English Miles.	Distances in Geographical or Nautical Miles.
5	2·958	2·565
10	4·184	3·628
15	5·123	4·443
20	5·916	5·130
25	6·614	5·736
30	7·245	6·283
35	7·826	6·787
40	8·366	7·255
45	8·874	7·696
50	9·354	8·112
55	9·811	8·509
60	10·246	8·886
65	10·665	9·249
70	11·067	9·598
75	11·456	9·935
80	11·832	10·260
85	12·196	10·570
90	12·549	10·880
95	12·893	11·180
100	13·228	11·470
110	13·874	12·030
120	14·490	12·560
130	15·083	13·080
140	15·652	13·570
150	16·201	14·220
200	18·708	16·220
250	20·916	18·14
300	22·912	19·87
350	24·748	21·46
400	26·457	22·94
450	28·062	24·30
500	29·580	25·65
550	31·024	26·90
600	32·403	28·10
650	33·726	29·25
700	35·000	30·28
800	37·416	32·45
900	39·836	34·54
1,000	41·833	36·28