Part 14
Philip Colomb perhaps is deserving of the credit of initiating modern signaling by flashing a code. He began work on such a system in 1858 and as an officer in the British Navy worked hard to introduce it. Finally, in 1867, the British Navy adopted the flashing-system, in which a light-source is exposed and eclipsed in such a manner as to represent dots and dashes analogous to the Morse code. At first the rate of transmission of words was from seven to ten per minute. Recently much more sensitive apparatus is available, and with such devices the rate is limited only by the sluggishness of the visual process. This initial system was very successful in the British Navy and it was soon found that a fleet could be handled with ease and safety in darkness or in fog. Inasmuch as the "dot-and-dash" system requires only two elements, it may be transmitted by various means. A lantern may be swung in short and long arcs or dipped accordingly.
The blinker or pulsating light-signal consists of a single light-source mechanically occulted. It is controlled by means of a telegraph-key and the code may be rapidly transmitted. The search-light affords a means for signaling great distances, even in the daytime. The light is usually mechanically occulted by a quick-acting shutter, but recently another system has been devised. In the latter the light itself is controlled by means of an electrical shunt across the arc. In this manner the light is dimmed by shunting most of the current, thereby producing the same effect as actually eclipsing the light with a mechanical shutter. By means of the search-light signals are usually visible as far as the limitations of the earth's curvature will permit. By directing the beam against a cloud, signals have been observed at a distance of one hundred miles from the search-light despite intervening elevated land or the curvature of the ocean's surface. By means of small search-lights it is easy to send signals ten miles.
This kind of apparatus has the advantage of being selective; that is, the signals are not visible to persons a few degrees from the direction of the beam. One of the most recent developments has been a special tungsten filament in a gas-filled bulb placed at the focus of a small parabolic mirror. The beam is directed by means of sights and the flashes are obtained by interrupting the current by means of a trigger-switch. The filament is so sensitive that signals may be sent faster than the physiological process of vision will record. With the advent of wireless telegraphy light-signaling for long distances was temporarily eclipsed, but during the recent war it was revived and much development work was prosecuted.
The Ardois system consists of four lamps mounted in a vertical line as high as possible. Each lamp is double, containing a red and a white light, and these lights are controlled from a keyboard. A red light indicates a dot in the Morse code and a white light indicates a dash. The keys are numbered and lettered, so that the system may be operated by any one. Various other systems employing colored lights have been used, but they are necessarily short-range signals. Another example is the semaphore. When used at night, tungsten lamps in reflectors indicate the positions of the arms. The advantage of these signals over the flashing-system is that each signal is complete and easy to follow. The flashing-system is progressive and must be carefully followed in order to obtain the meaning of the dots and dashes.
Smaller signal-lamps using acetylene have been employed in the forestry service and in other activities where a portable device is necessary. In one type, a mixture-tank containing calcium carbide and water is of sufficient capacity for three hours of signaling. A small pilot-light is permitted to burn constantly and the flashes are obtained by operating a key which increases the gas-pressure. The light flares as long as the key is depressed. The range of this apparatus is from ten to twenty miles. An electric lamp supplied from a storage battery has been designed for geodetic operations in mountainous districts where it is desired to send signals as far as one hundred miles. Tests show that this device is a hundred and fifty times more powerful than the ordinary acetylene signal-lamp, and it is thought that with this new electric lamp haze and smoke will seldom prevent observations.
Certain fixed lights are required by law on a vessel at night. When it is under way there must be a white light at the masthead, a starboard green light, a port red light, a white range-light, and a white light at the stern. The masthead light is designed to emit light through a horizontal arc of twenty points of the compass, ten on each side of dead ahead. This light must be visible at a distance of five miles. The port and starboard lights operate through a horizontal arc of twenty points of the compass, the middle of which is dead ahead. They are screened so as not to be visible across the bow and they must be intense enough to be visible two miles ahead. The masthead light is carried on the foremast and the range-light on the mainmast, at an elevation fifteen feet higher than the former. The range-light emits light toward all points of the compass and must be intense enough to be seen at a distance of three miles. The stern light is similar to the masthead, but its light must not be visible forward of the beam. When a vessel is towing another it must display two or three lights in a vertical line with the masthead light and similar to it. The lights are spaced about six feet apart, and two extra ones indicate a short tow and three a long one. A vessel over a hundred and fifty feet long when at anchor is required to display a white light forward and aft, each visible around the entire horizon. These and many other specifications indicate how artificial light informs the mariner and makes for order in shipping. Without artificial light the waterways would be trackless and chaos would reign.
The distress signals of a vessel are rockets, but any burning flame also serves if rockets are unavailable. Fireworks were known many centuries ago and doubtless the possibilities of signaling by means of rockets have long been recognized. An early instance of scientific interest in rockets and their usefulness is that of Benjamin Robins in 1749. While he was witnessing a display of fireworks in London it occurred to him that it would be of interest to measure the height to which the rockets ascended and to determine the ranges at which they were visible. His measurements indicated that the rockets ascended usually to a height of 440 yards, but some of them attained altitudes as high as 615 yards. He then had some special ones made and despatched letters to friends in three different localities, at distances as great as 50 miles, asking them to observe at a certain time, when the rockets were to be sent up in the outskirts of London. Some of these rockets rose to altitudes as great as 600 yards and were distinctly seen by observers 38 miles away. Later he made rockets which ascended as high as 1200 yards and concluded that this was a practical means of signaling. Since that time and especially during the recent war, rockets have served well in signaling messages.
The self-propelled rockets have not been altered in essential features since the remote centuries when the Chinese first used them in celebrations. A cylindrical shell is mounted on a wooden stick and when the powder in the shell burns the hot gases are ejected so violently downward that the reaction drives the shell upward. At a certain point in the air, various signals burst forth, which vary in character and color. One of the advantages of the rocket is that it contains within itself the force of propulsion; that is, no gun is necessary to project it. The illuminating compounds and various details are similar to those of the illuminating shells described in another chapter.
At present the rocket is not scientifically designed to obtain the greatest efficiency of propulsion, but its simplicity in this respect is one of its chief advantages. If the self-propelled rocket becomes the projectile of the future, as some have ventured to predict, much consideration must be given to the design of the orifice through which the gases violently escape in order that the best efficiency of propulsion may be attained. There are other details in which improvements may be made. The combustion products of the black powder which are not gaseous equal about one third the weight of the powder. This represents inefficient propulsion. Furthermore, during recent years much information has been gained pertaining to the air-resistance which can be applied to advantage in designing the form of rockets.
Besides the various rockets, signal-lights have been constructed to be fired from guns and pistols. During the recent war the airman in the dark heights used the pistol signal-light effectively for communication. These devices emitted stars either singly or in succession, and the color of these stars as well as their number and sequence gave significance to the signal. Some of these light-signals were provided with parachutes and were long-burning; that is, light was emitted for a minute or two. There are many variations possible and a great many different kinds of light-signals of this character were used. In the front-line trenches and in advances they were used when telephone service was unavailable. The airman directed artillery fire by means of his pistol-light. Rockets brought aid to the foundered ship or to the life-boats. The signal-tube which burned red, green, or white was held in the hand or laid on the ground and it often told its story. For many years such a device dropped from the rear of the railroad train has kept the following train at a safe distance. A device was tried out in the trenches, during the war, which emitted a flame. This could be varied in color to serve as a signal and the apparatus had sufficient capacity for thirty hours' burning. This could also be used as a weapon, or when reduced in intensity it served as a flash-light.
For many years experiments have been made upon the use of the invisible rays which accompany visible rays. The practicability of signaling with invisible rays depends upon producing them efficiently in sufficient quantity and upon separating them from the visible rays which accompany them. Some successful results were obtained with a 6-volt electric lamp possessing a coiled filament at the focus of a lens three inches in diameter and twelve inches in focal length. This gave a very narrow beam visible only in the neighborhood of the observation post to which the signals were directed. The beam was directed by telescopic sights. During the day a deep red filter was placed over the lamp and the light was invisible to an observer unless he was equipped with a similar red screen to eliminate the daylight. It is said that signals were distinguished at a distance of six miles. By night a screen was used which transmitted only the ultraviolet rays, and the observer's telescope was provided with a fluorescent screen in its focal plane. The ultraviolet rays falling upon this screen were transformed into visible rays by the phenomenon of fluorescence. The range of this device was about six miles. For naval convoys lamps are required to radiate toward all points of the compass. For this purpose a quartz mercury-arc which is rich in ultraviolet rays was surrounded with a chimney which transmitted the ultraviolet rays efficiently and absorbed all visible rays excepting violet light. The lamp appeared a deep violet color at close range, but the faintly visible light which it transmitted was not seen at a distance. A distant observer picks up the invisible ultraviolet "light" by means of a special optical device having a fluorescent screen of barium-platino-cyanide. This device had a range of about four miles.
Light-signals are essential for the operation of railways at night and they have been in use for many years. In this field the significance of light-signals is based almost universally on color. The setting of a switch is indicated by the color of the light that it shows. With the introduction of the semaphore system, in which during the day the position of the arm is significant, colored glasses were placed on the opposite end of the arm in such a manner that a certain colored glass would appear before the light-source for a certain position of the arm. A kerosene flame behind a glass lens was the lamp used, and, for example, red meant "Stop," green counseled "Caution," and clear or white indicated "All clear." For many years the kerosene lamp has been used, but recently the electric filament lamp is being installed to some extent for this purpose. In fact, on one railroad at least, tungsten lamps are used for light-signals by day as well as by night. Three signals--red, green, and white--are placed in a vertical line and behind each lens are two lamps, one operating at high efficiency and one at low efficiency to insure against the failure of the signal. The normal daylight range is about three thousand feet and under the worst conditions when opposed to direct sunlight, the range is not less than two thousand feet. It is said that these lights are seen more easily than semaphore arms under all circumstances and that they show two or three times as far as the latter during a snow-storm.
The standard colors for light-signals as adopted by the Railway Signal Association are red, yellow, green, blue, purple, and lunar white. These are specified as to the amount of the various spectral colors which they transmit when the light-source is the kerosene flame. Obviously, the colors generally appear different when another illuminant is used. The blue and purple are short-range signals, but the effective range of the best railway signal employing a kerosene flame is only about four miles.
It has been shown that the visibility of point sources of white light in clear atmosphere, for distances up to a mile at least, is proportional to their candle-power and inversely proportional to the square of the distance. Apparently the luminous intensities of signal-lamps required in clear weather in order that they may be visible must be 0.43 candles for one nautical mile, 1.75 candles for two nautical miles, and 11 candles for five nautical miles. From the data available it appears that a red or a white signal-light will be easily visible at a distance in nautical miles equal to the square root of its candle-power in that direction. The range in nautical miles of a green light apparently is proportional to the cube root of the candle-power. Whether or not these relations between the range in miles and the luminous intensity in candles hold for greater distances than those ordinarily encountered has not been determined, but it is interesting to note that the square root of the luminous intensity of the Navesink Light at the entrance to New York Harbor is about 7000. Could this light be seen at a distance of seven thousand miles through ordinary atmosphere?
The most distinctive colored lights are red, yellow, green, and blue. To these white (clear) and purple have been added for signaling-purposes. Yellow is intense, but it may be confused with "white" or clear. Blue and purple as obtained from the present practicable light-sources are of low intensity. This leaves red, green, and clear as the most generally satisfactory signal-lights.
There are numerous other applications, especially indoors. Some of these have been devised for special needs, but there are many others which are general, such as for elevators, telephones, various call systems, and traffic signals. Light has the advantages of being silent and controllable as to position and direction, and of being a visible signal at night. Thus, in another field artificial light has responded to the demands of civilization.
XVI
THE COST OF LIGHT
Artificial light is so superior to natural light in many respects that mankind has acquired the habit of retiring many hours after darkness has fallen, a result of which has brought forth the issue known as "daylight saving." Doubtless, daylight should be used whenever possible, but there are two sides to the question. In the first place, it costs something to bring daylight indoors. The architectural construction of windows and skylights increases the cost of daylight. Light-courts, by sacrificing valuable floor-area, add to the expense. The maintenance of windows and sky lights is an appreciable item. Considering these and other factors, it can be seen that daylight indoors is expensive; and as it is also undependable, a supplementary system of artificial lighting is generally necessary. In fact, it is easy to show in some cases that artificial lighting is cheaper than natural lighting.
The average middle-class home is now lighted artificially for about $15.00 to $25.00 per year, with convenient light-sources which are available at all times. There is no item in the household budget which returns as much satisfaction, comfort, and happiness in proportion to its cost as artificial light. It is an artistic medium of great potentiality, and light in a narrow utilitarian sense is always a by-product of artistic lighting. The insignificant cost of modern lighting may be emphasized in many ways. The interest on the investment in a picture or a vase which cost $25.00 will usually cover the cost of operating any decorative lamp in the home. A great proportion of the investment in personal property in a home is chargeable to an attempt to beautify the surroundings. The interest on only a small portion of this investment will pay for artistic and utilitarian artificial lighting in the home. The cost of washing the windows of the average house may be as great as the cost of artificial lighting and is usually at least a large fraction of the latter. It would become monotonous to cite the various examples of the insignificant cost of artificial light and its high return to the user. The example of the home has been chosen because the reader may easily carry the analysis further. The industries where costs are analyzed are now looking upon adequate and proper lighting as an asset which brings in profits by increasing production, by decreasing spoilage, and by decreasing the liability of accidents.
Inasmuch as daylight saving became an issue during the recent war and is likely to remain a matter of concern, its history is interesting. One of the outstanding differences between primitive and civilized beings is their hours of activities. The former automatically adjusted themselves to daylight, but as civilization advanced, the span of activities began to extend more and more beyond the coming of darkness. Finally in many activities the work-day was extended to twenty-four hours. There can be no insurmountable objection to working at night with a proper arrangement of the periods of work; in fact, the cost of living would be greatly increased if the overhead charges represented by such items as machinery and buildings were allowed to be carried by the decreased products of a shortened period of production. There cannot be any basic objection to artificial lighting, because most factories, for example, may be better illuminated by artificial than by natural light.
Of course, the lag of comfortable temperature behind daylight is responsible to some extent for a natural shifting of the ordinary working-day somewhat behind the sun. The chill of dawn tends to keep mankind in bed and the cheer of artificial light and the period of recreation in the evening tends to keep the civilized races out of bed. There are powerful influences always at work and despite the desirable features of daylight-saving, mankind will always tend to lag. As years go by, doubtless it will be necessary to make the shift again and again. It seems certain that throughout the centuries thoughtful persons have seen the difficulty of rousing man from his warm bed in the early morning and have recognized a simple solution in turning the hands of the clock ahead. Among the earliest advocates of daylight saving during modern times, when it became important enough to be considered as an economic issue, was Benjamin Franklin. In 1784 he wrote a masterful serio-comic essay entitled "An Economical Project" which was published in the _Journal_ of Paris. The article, which appeared in the form of a letter, began thus:
MESSIEURS: You often entertain us with accounts of new discoveries. Permit me to communicate to the public through your paper one that has lately been made by myself and which I conceive may be of great utility.
I was the other evening in a grand company where the new lamp of Messrs. Quinquet and Lange was introduced and much admired for its splendor; but a general inquiry was made whether the oil it consumed was not in exact proportion to the light it afforded, in which case there would be no saving in the use of it. No one present could satisfy us on that point, which all agreed ought to be known, it being a very desirable thing to lessen, if possible, the expense of lighting our apartments, when every other article of family expense was so much augmented. I was pleased to see this general concern for economy, for I love economy exceedingly.
I went home, and to bed, three or four hours after midnight, with my head full of the subject. An accidental sudden noise waked me about 6 in the morning, when I was surprised to find my room filled with light, and I imagined at first that a number of those lamps had been brought into it; but, rubbing my eyes, I perceived the light came in at the windows. I got up and looked out to see what might be the occasion of it, when I saw the sun just rising above the horizon, from whence he poured his rays plentifully into my chamber, my domestic having negligently omitted the preceding evening to close the shutters.
I looked at my watch, which goes very well, and found that it was but 6 o'clock; and, still thinking it something extraordinary that the sun should rise so early, I looked into the almanac, where I found it to be the hour given for his rising on that day. I looked forward, too, and found he was to rise still earlier every day till toward the end of June, and that at no time in the year he retarded his rising so long as till 8 o'clock.
Your readers who, with me, have never seen any signs of sunshine before noon, and seldom regard the astronomical part of the almanac, will be as much astonished as I was when they hear of his rising so early, and especially when I assure them that he gives light as soon as he rises. I am convinced of this. I am certain of my fact. One cannot be more certain of any fact. I saw it with my own eyes. And, having repeated this observation the three following mornings, I found always precisely the same result.