When it is necessary to revolve apparatus of this weight, the whole mechanism is floated upon mercury contained in a cast-iron vessel of suitable size, and by an ingenious arrangement only a small portion of mercury is required.

The characteristics of navigation lights are varied considerably in order to enable the mariner to distinguish them and thereby to learn exactly where he is. The fixed light is liable to be confused with others, so it has now become a minor light. Flashes of short duration followed by longer periods of darkness are extensively used. The mariner by timing the intervals is able to recognize the light. This method is extended to groups of short flashes followed by longer intervals of darkness. In fact, short flashes have been employed to indicate a certain number so that a mariner could recognize the light by a number rather than by means of his watch. However, a time element is generally used. A combination of fixed light upon which is superposed a flash or a group of flashes of white or of colored light has been used, but it is in disrepute as being unreliable. A type known as "occulating lights"

consists of a fixed light which is momentarily eclipsed, but the duration of the eclipse is usually less than that of the light.

Obviously, groups of eclipses may be used. Sometimes lights of different colors are alternated without any dark intervals. The colored ones used are generally red and green, but these are short-range lights at best.

Colored sectors are sometimes used over portions of the field, in order to indicate dangers, and white light shows in the fairway. These are usually fixed lights for marking the channel.

The distance at which a light may be seen at sea depends upon its luminous intensity, upon its color or spectral composition, upon its height and that of the observer's eyes above the sea-level, and upon the atmospheric conditions. a.s.suming a perfectly clear atmosphere, the visibility of a light-source apparently depends directly upon its candle-power. The atmosphere ordinarily absorbs the red, orange, and yellow rays less than the green, blue, and violet rays. This is demonstrated by the setting sun, which as it approaches closer to the horizon changes from yellow to orange and finally to red as the amount of atmosphere between it and the eye increases. For this reason a red light would have a greater range than a blue light of the same luminous intensity.

Under ordinary atmospheric conditions the range of the more powerful light-sources used in lighthouses is greater than the range as limited by the curvature of the earth. For the uncolored illuminants the range in nautical miles appears to be at least equal to the square root of the candle-power. A real practical limitation which still exists is the curvature of the earth, and the distance an object may be seen by the eye at sea-level depends upon the height of the object. The relation is approximately expressed thus,--

Range in nautical miles = 8/7 square root of Height of object in feet. For example, the top of a tower 100 feet high is visible to an eye at sea-level a distance of 8/7 square root of 100 = 80/7 = 11.43 miles. Now if the eye is 49 feet above sea-level, a similar computation will show how far away it may be seen by the original eye at sea-level.

This is 8/7 square root of 49 = 8 miles. Hence an eye 49 feet above sea-level will be able to see the top of the 100-foot tower at a distance of 11.43 + 8 or 19.43 nautical miles. Under these conditions an imaginary line drawn from the top of the tower to the eye will be just tangent to the spherical surface of the sea at a distance of 8 miles from the eye and 11.43 miles from the tower.

The luminous intensity of a light-source or of the beam of light is directly responsible for the range. The luminous intensity of the early beacon-fires and oil-lamps was equivalent to a few candles. The improvements in light-sources and also in reflecting and refracting optical systems have steadily increased the candle-power of the beams, until to-day the beams from gas-lamps have intensities as high as several hundred thousand candle-power. The beams sent forth by modern lighthouses equipped with electric lamps and enormous light-gathering devices are rated in millions of candle-power. In fact, Navesink Light at the entrance of New York Bay is rated as high as 60,000,000 candle-power.

Of course, light-production has increased enormously in efficiency in the past century, but without optical devices for gathering the light, the enormous beam intensity would not be obtained. For example, consider a small source of light possessing a luminous intensity of one candle in all directions. If all this light which is emitted in all directions is gathered and sent forth in a beam of small angle, say one thousandth of the total angle surrounding a point, the intensity of this beam would be 1000 candles. It is in this manner that the enormous beam intensities are built up.

There is an interesting point pertaining to short flashes of light. To the dark-adapted eye a brief flash is registered as of considerably higher intensity than if the light remained constant. In other words, the lookout on a vessel is adapted to darkness and a flash from a beam of light is much brighter than if the same beam were shining steadily.

This is a physiological phenomenon which operates in favor of the flashing light.

[Ill.u.s.tration: A. A COMPLETED LIGHTHOUSE LENS]

[Ill.u.s.tration: B. TORRO POINT LIGHTHOUSE, PANAMA Ca.n.a.l]

[Ill.u.s.tration: AMERICAN SEARCH-LIGHT POSITION ON WESTERN FRONT IN 1919]

[Ill.u.s.tration: AMERICAN STANDARD FIELD SEARCH-LIGHT AND POWER UNIT]

Doubtless, the reader has noted that reliability, simplicity, and low cost of operation are the primary considerations for light-sources used as aids to navigation. This accounts for the continued use of oil and gas. From an optical standpoint the electric arc-lamps and concentrated-filament lamps are usually superior to the earlier sources of light, but the complexity of a plant for generating electricity is usually a disadvantage in isolated places. The larger light-ships are now using electricity generated by apparatus installed in the vessels.

There seems to be a tendency toward the use of more buoys and fewer lighthouses, but the beam-intensities of the latter are increasing.

In the hundred years since the Boston Light was built the same great changes wrought by the development of artificial light in other activities of civilization have appeared in the beacons of the mariner.

The development of these aids to navigation has been wonderful, but it must go on and on. The surface of the earth comprises 51,886,000 square statute miles of land and 145,054,000 square miles of water. Three fourths of the earth's surface is water and the oceans will always be highways of world commerce. All the dangers cannot be overcome, but human ingenuity is capable of great achievements. Wreckage will appear along the sh.o.r.e-lines despite the lights, but the harvest of the shoals has been much reduced since the time described by Robert Louis Stevenson, when the coast people in the Orkneys looked upon wrecks as a source of gain. He states:

It had become proverbial with some of the inhabitants to observe that "if wrecks were to happen, they might as well be sent to the poor island of Sanday as anywhere else." On this and the neighboring island, the inhabitants have certainly had their share of wrecked goods. On complaining to one of the pilots of the badness of his boat's sails, he replied with some degree of pleasantry, "Had it been His [G.o.d's] will that you come na here wi these lights, we might a' had better sails to our boats and more o' other things."

In the leasing of farms, a location with a greater probability of shipwreck on the sh.o.r.e brought a much higher rent.

XIV

ARTIFICIAL LIGHT IN WARFARE

When the recent war broke out science responded to the call and under the stress of feverish necessity compressed the normal development of a half-century into a few years. The airplane, in 1914 a doubtful plaything of daredevils, emerged from the war a perfected thing of the air. Lighting did not have the glamor of flying or the novelty of chemical warfare, but it progressed greatly in certain directions and served well. While artificial lighting conducted its unheralded offensive by increasing production in the supporting industries and helped to maintain liaison with the front-line trenches by lending eyes to transportation, it was also doing its part at the battle front. Huge search-lights revealed the submarine and the aerial bomber; flares exposed the manoeuvers of the enemy; rockets brought aid to beleaguered vessels and troops; pistol lights fired by the aerial observer directed artillery fire; and many other devices of artificial light were in the fray. Many improvements were made in search-lights and in signaling devices and the elements of the festive fireworks of past ages were improved and developed for the needs of modern warfare.

Night after night along the battle front flares were sent up to reveal patrols and any other enemy activity. On the slightest suspicion great swarms of these brilliant lights would burst forth as though flocks of huge fireflies had been disturbed. They were even used as light barrages, for movements could be executed in comparative safety when a large number of these lights lay before the enemy's trenches sputtering their brilliant light. The airman dropped flares to illuminate his target or his landing field. The torches of past parades aided the soldier in his night operations and rockets sent skyward radiated their messages to headquarters in the rear. The star-sh.e.l.l had the same missions as other flares, but it was projected by a charge of powder from a gun. These and many modifications represent the useful applications of what formerly were mere "fireworks." Those which are primarily signaling devices are discussed in another chapter, but the others will be described sufficiently to indicate the place which artificial light played in certain phases of warfare.

The illuminating compounds used in these devices are not particularly new, consisting essentially of a combustible powder and chemical salts which make the flame luminous and give it color when desired. Among the ingredients are barium nitrate, pota.s.sium perchlorate, powdered aluminum, powdered magnesium, pota.s.sium nitrate, and sulphur. One of the simplest mixtures used by the English is,

Barium nitrate 37 per cent.

Powdered magnesium 34 per cent.

Pota.s.sium nitrate 29 per cent.

The magnesium is coated with hot wax or paraffin, which not only acts as a binder for the mixture when it is pressed into its container but also serves to prevent oxidation of the magnesium when the sh.e.l.ls are stored.

The barium and pota.s.sium nitrates supply the oxygen to the magnesium, which burns with a brilliant white flame. The pota.s.sium nitrate takes fire more readily than the barium nitrate, but it is more expensive than the latter.

Owing to the cost of magnesium, powdered aluminum has been used to some extent as a subst.i.tute. Aluminum does not have the illuminating value of magnesium and it is more difficult to ignite, but it is a good subst.i.tute in case of necessity. An English mixture containing these elements is,

Barium nitrate 58 per cent.

Magnesium 29 per cent.

Aluminum 13 per cent.

Mixtures which are slow to ignite must be supplemented by a primary mixture which is readily ignited. For obtaining colored lights it is only necessary to add chemicals which will give the desired color. The mixtures can be proportioned by means of purely theoretical considerations; that is, just enough oxygen can be present to burn the fuel completely. However, usually more oxygen is supplied than called for by theory.

The illuminating sh.e.l.l is perhaps the most useful of these devices to the soldier. It has been constructed with and without parachutes, the former providing an intense light for a brief period because it falls rapidly. These sh.e.l.ls of the larger calibers are equipped with time-fuses and are generally rather elaborate in construction. The sh.e.l.l is of steel, and has a time-fuse at the tip. This fuse ignites a charge of black powder in the nose of the sh.e.l.l and this explosion ejects the star-sh.e.l.l out of the rear of the steel casing. At the same time the black powder ignites the priming mixture next to it, which in turn ignites the slow-burning illuminating compound. The star-sh.e.l.l has a large parachute of strong material folded in the rear of the casing and the cardboard tube containing the illuminating mixture is attached to it. The time of burning varies, but is ordinarily less than a minute.

Certain structural details must be such as to endure the stresses of a high muzzle velocity. Furthermore, a velocity of perhaps 1000 feet per second still obtains when the star-sh.e.l.l with its parachute is ejected at the desired point in the air.

The non-parachute illuminating sh.e.l.l is designed to give an intense light for a brief interval and is especially applicable to defense against air raids. Such a light aims to reveal the aircraft in order that the gunners may fire at it effectively. These sh.e.l.ls are fitted with time-fuses which fire the charge of black powder at the desired interval after the discharge of the sh.e.l.l from the gun. The contents of the sh.e.l.l are thereby ejected and ignited. The container for the illuminating material is so designed that there is rapid combustion and consequently a brilliant light for about ten seconds. The enemy airman in this short time is unable to obtain any valuable knowledge pertaining to the earth below and furthermore he is likely to be temporarily blinded by the brilliant light if it is near him.

The rifle-light which resembles an ordinary rocket, is fired from a rifle and is designed for short-range use. It consists of a steel cylindrical sh.e.l.l a few inches long fastened to a steel rod. A parachute is attached to the cardboard container in which the illuminating mixture is packed and the whole is stowed away in the steel sh.e.l.l. Sh.o.r.e delay-fuses are used for starting the usual cycle of events after the rifle-light has been fired from the gun. The steel rod is injected into the barrel of a rifle and a blank cartridge is used for ejecting this rocket-like apparatus. Owing to inertia the firing-pin in the sh.e.l.l operates and the short delay-fuse is thus fired automatically an instant after the trigger of the rifle is pulled.

Illuminating "bombs" of the same general principles are used by airmen in search of a landing for himself or for a destructive bomb; in signaling to a gunner, and in many other ways. They are simple in construction because they need not withstand the stresses of being fired from a gun; they are merely dropped from the aircraft. The mechanism of ignition and the cycle of events which follow are similar to those of other illuminating sh.e.l.ls.

The value of such artificial-lighting devices depends both upon luminous intensity and time of burning. Although long-burning is not generally required in warfare, it is obvious that more than a momentary light is usually needed. In general, high candle-power and long-burning are opposed to each other, so that the most intense lights of this character usually are of short duration. Typical performances of two flares of the same composition are as follows:

Flare No. 1 Flare No. 2 Average candle-power 270,000 95,000 Seconds of burning 10 35 Candle-seconds 2,700,000 3,325,000 Cubic inches of compound 6 7 Candle-seconds per cubic inch 450,000 475,000 Candle-hours per cubic inch 125 132

The illuminating compound was the same in these two flares, which differed only in the time allowed for burning. Of course, the measurements of the luminous intensity of such flares is difficult because of the fluctuations, but within the errors of the measurements it is seen that the illuminating power of the compound is about the same regardless of the time of burning. The light-source in the case of burning powders is really a flame, and inasmuch as the burning end hangs downward, more light is emitted in the lower hemisphere than in the upper. The candle-power of the largest flares equals the combined luminous intensities of 200 street arc-lamps or of 10,000 ordinary 40-watt tungsten lamps such as are used in residence lighting.

It is interesting to note the candle-hours obtained per cubic inch of compound and to find that the cost of this light is less than that of candles at the present time and only five or ten times greater than that of modern electric lighting.

Illuminating sh.e.l.ls in use during the recent war were designed for muzzle velocities as high as 2700 feet per second and were gaged to ignite at any distance from a quarter of a mile to several miles. The maximum range of illuminating sh.e.l.ls fired from rifles was about 200 yards; for trench mortars about one mile; and from field and naval guns about four miles.

The search-light has long been a valuable aid in warfare and during the recent conflict considerable attention was given to its development and application. It is used chiefly for detecting and illuminating distant targets, but this covers a wide range of conditions and requirements. In order that a search-light may be effective at a great distance, as much as possible of the light emitted by a source is directed into a beam of light of as nearly parallel rays as can be obtained. Reflectors are usually employed in military search-lights, and in order that the beam may be as nearly parallel (minimum divergence) as possible, the light must be emitted by the smallest source compatible with high intensity.

This source is placed at the proper point in respect to a large parabolic reflecter which renders the rays parallel or nearly so.

Ever since its advent the electric arc has been employed in large search-lights, with which the army and the navy were supplied; however, the greatest improvements have been made under the stress of war. The science of aeronautics advanced so rapidly during the recent war that the necessity for powerful search-lights was greatly augmented and as the conflict progressed the enemy airmen came to look upon the newly developed ones with considerable concern. The rapidly moving aircraft and its high alt.i.tude brought new factors into the design of these lights. It now became necessary to have the most intense beam and to be able to sweep the heavens with it by means of delicate controlling apparatus, for the targets were sometimes minute specks moving at high speed at alt.i.tudes as high as five miles. Furthermore, owing to the shifting battle areas, mobile apparatus was necessary.

The control of light by means of reflectors has been studied for centuries, but until the advent of the electric arc the light-sources were of such large areas that effective control was impossible. Optical devices generally are considered in connection with "point sources," but inasmuch as no light can be obtained from a point, a source of small dimensions and of high brightness is the most effective compromise.

Parabolic mirrors were in use in the eighteenth century and their properties were known long before the first search-light worthy of the name was made in 1825 by Drummond, who used as a source of light a piece of lime heated to incandescence in a blast flame. He finally developed the "lime-light" by directing an oxyhydrogen flame upon a piece of lime and this device was adapted to search-lights and to indoor projection.

It is said that the first search-light to be used in warfare was a Drummond lime-light which played a part in the attack on Fort Wagner at Charleston in 1863.

In 1848 the first electric arc lamp used for general lighting was installed in Paris. It was supplied with current by a large voltaic cell, but the success of the electric arc was obliged to await the development of a more satisfactory source of electricity. A score of years was destined to elapse, after the public was amazed by the first demonstration, before a suitable electric dynamo was invented. With the advent of the dynamo, the electric arc was rapidly developed and thus there became available a concentrated light-source of high intensity and great brilliancy. Gradually the size was increased, until at the present time mirrors as large as seven feet in diameter and electric currents as great as several hundred amperes are employed. The beam intensities of the most powerful search-lights are now as great as several hundred million candles.

The most notable advance in the design of arc search-lights was achieved in recent years by Beck, who developed an intensive flame carbon-arc.

His chief object was to send a much greater current through the arc than had been done previously without increasing the size of the carbons and the unsteadiness of the arc. In the ordinary arc excessive current causes the carbons to disintegrate rapidly unless they are of large diameter. Beck directed a stream of alcohol vapor at the arc and they were kept from oxidizing. He thus achieved a high current-density and much greater beam intensities. He also used cored carbons containing certain metallic salts which added to the luminous intensity, and by rotation of the positive carbon so that the crater was kept in a constant position, greater steadiness and uniformity were obtained.