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THERE are several natural phenomena which may be seen from mountains, but most of them not from low ground, at least not in this country. They are so rarely seen that most people do not know the names of them, or only have a dim recollection of them in scientific books, along with the water-spout and mirage. Names are constantly given to them which do not belong to them. The word halo is the one that has been most misused. A halo is a large circle round the sun or moon. Coronae are smaller circles round the sun or moon. These halos and coronae are not peculiar to mountains, so I take no further notice of them. Rainbows, solar and lunar, are also not peculiar to mountains. All the bows that I am going to describe are opposite the sun or moon, not round them.
In May 1887, a paper of mine was read before the Royal Meteorological Society on Brocken spectres and the bows that often accompany them. I am not a member of that Society. In order to write that paper I searched in a great many places, and succeeded in collecting thirty descriptions. I took no notice of general descriptions in scientific books, because I did not know what they were based upon. I send a copy of my paper to the Club Library, so that any member who wishes may look further into the subject.
Brocken spectre is the name given to a shadow on cloud or mist which appears to be gigantic, or at any rate larger than life. The Brocken is a mountain in Germany, from which the first descriptions came. The earliest explanation attempted is what I call the candle theory. The fog is like a wall on which the shadows grow larger the further the object that produces them is from the wall; the sun takes the place of the candle. This theory will not stand examination. The sun is not a point; it is 850,000 miles in diameter; its distance from us is 92,700,000, that is 109 times its diameter; consequently, the further the shadow is from the man, the smaller it is.
I have found by experiment with my own shadow cast by the sun, that at sixty yards it could not be distinguished as the Shadow of a man, that at thirty-five yards the arms and legs were barely visible, that the furthest distance at which a good shadow could be made was seventeen yards. The subject has been much complicated by writers of scientific books, who have given pictures of what no one ever saw, and who have made statements without having enough descriptions to work upon.
I consider that the apparently gigantic size is caused by a mistake in judging the distance. The observer thinks that his shadow is a long way off and gigantic, when it really is closer to him and life size. When two people together have seen a Brocken spectre, they have usually differed about the size. Another fact pointing in the same direction is this. When two people stand close together they can see each other's Brocken spectres; when they are a few yards apart sideways, each can see only his own. My explanation is this. The shadow is not cast on a wall, but on a number of fog particles stretching a long way from the man. Looking into this from near the man you see a great many yards into these darkened particles and can distinguish the shadow. Looking across it you only look through a foot or two and do not notice it. If the shadow was as far off as it appears to be, moving a few yards to the side would not make it disappear.
My explanation is confirmed by experiments made by Mr A. W. Clayden, F. R. Met. Soc., with shadows cast from a lantern on to London fog, the distance of which he was able to calculate. He called in several people to guess the distance, and they all over-estimated it. I have found the same difficulty in judging distance when coming across animals in a fog on a mountain. A sheep may look bigger than a cow. The same occurs in sand-storms in a desert. Nordenskiold mentions some extraordinary mistakes in fogs in the Arctic regions.
If there are any people who believe that these shadows are really gigantic, let them test them in any way that they can. If two are together, first let each guess the height and distance, and write it down before comparing. Let one go back a little towards the sun and see if his friend's shadow still looks gigantic and the same size as his own. Then let them go a little apart sideways and see what effect that has on the shadows.
The usual state of the weather when Brocken spectres are seen is, the observer in bright sunshine, the shadow on fog blown up from below, wind strong. They are seldom seen unless there is a sharp drop in the ground in front. When the observer is in the fog, the shadow does not appear to be gigantic. The phenomenon often only lasts a few seconds. The longest time recorded is a quarter of an hour or a little more. I use the words fog and mist as meaning the same thing. Some more correct people make a difference.
Sometimes the shadows of the arms cannot be seen when they are still, but can be seen when they are moving. An explanation is wanted. Sometimes dark rays are seen stretching away from the arms or head of the shadow. These are upon fog nearer than the rest of the shadow. The explanation is too difficult to give here.
The Brocken spectre that I saw was surrounded by a bow. In other descriptions of Brocken spectres I continually came across this bow. Of the thirty descriptions in my paper nineteen had the bow. It also occurred seven times with shadows not said to be gigantic. It has never been recorded without a shadow. It had no name of its own in scientific books, and observers usually called it by a name belonging to some other phenomenon - rainbow, fog-bow, cloud-bow, halo, prismatic halo, prismatic circle, fog circle, mirage. I was obliged to invent a name, and thought Brocken bow the most suitable. I give here a woodcut of a Brocken bow, and also one of a Glory, to which I shall refer later on. They are not supposed to be exact, but are intended to impress upon the eye the difference between the two phenomena.
A Brocken bow is like a small Rainbow. Its colours are the same as those of the Rainbow, in the same order, with the red outside. When faint, the colours on the inner side are hardly visible. This is also sometimes the case with a Rainbow. The breadth of the colours bears something like the same proportion to the space inside them in a Brocken bow as it does in a Rainbow. A Glory, on the contrary, touches the head of the shadow. A Brocken bow has sometimes a secondary bow outside, colours reversed, red inside and fainter, as is also the case with the secondary Rainbow. Rough estimates of the angular measurement have been given, varying from 8º to 20º. The Rainbow is 84º 44'. Measurements are much wanted. It may turn out that the size is always the same.
Brocken bow and glory
Some observers have described the bow as not circular, but oval, ovoid, elliptic, or horseshoe. I was at first inclined to doubt the possibility of this, but so many are described in this way that I cannot pass them by as wrong. Some people think that the eccentricity is due to false perspective. The observer thinks that the bow is in one plane when it really is in another.
The weather suitable for Brocken bows is the same as that mentioned with Brocken spectres. The temperature is sometimes above and sometimes below freezing-point.
Descriptions should be written on the spot, and should contain the following particulars and anything else that may seem important:- Time of day; state of atmosphere; 1, towards sun; 2, where observer stood; 3, away from sun; description of ground, wind, temperature, whether freezing or not. Date, place, and height above the sea, may be added afterwards. Before writing the description the bow should be measured, as that is the point upon which information is most wanted.
Of course no one carries instruments. I suggest this method if no better can be thought of: Shut one eye; hold up a stick or pocket-handkerchief in one hand at arm's length; let one end cover one side of the bow, and mark where the other side of the bow comes to; this fixes the three points of a triangle. When next you come across a measure, the assistance of a friend and a little mathematics will enable you to find out the angle at the eye. An observation of this sort should be right within a degree or two. Bows of all sorts should be photographed when there is a chance. I am told that it cannot be done. It ought to be tried.
I have seen all Mr Omond's reports of "Glories, Halos, and Coronae, on Ben Nevis," that have been printed by the Royal Society of Edinburgh. These include his observations to the end of 1887. He has seen a large number of Glories, but not a single Brocken bow. This is peculiar, because from all other sources I have collected about the same number of descriptions of Brocken bows as of Glories. The state of the atmosphere up there, or the shape of the mountain, may have something to do with it.
I believe that bows in the spray of waterfalls and fountains are Rainbows, not Brocken bows. They should be measured.
The state of the atmosphere with Glories is sometimes different from that which is suitable for Brocken bows. In descriptions of Glories the observer sometimes states that he is in the fog, and that there is fog towards the sun. The weather is more settled, and there is less wind. Glories are sometimes seen for hours at a time. Though usually seen from mountains, they are also sometimes seen from low ground, and in the Arctic regions from the sea. The temperature is sometimes above and sometimes below freezing-point.
Glories differ from one another in every particular. The most simple form is a bright circular disc round the shadow of the head. Hence the name. They often consist of a bow, of the colours of the Rainbow, touching the shadow of the head, red outside. Outside this may be a similar bow, red outside.
A Glory never has a secondary bow outside it, with the colours reversed, as the Brocken bow has. The outer bow may touch the inner bow, or there may be a space between. Sometimes there are more bows outside. If the inner one is of the colours of the Rainbow and in that order, the outer ones are of the same colours and in the same order. The largest number of bows reported in a Glory is seven.
A great number of angular measurements of Glories have been taken at Ben Nevis Observatory with instruments made for the purpose. These show that there is no fixed size for the bows, and that there is no fixed proportion between the inner bow and the outer ones. The largest bow measured on Ben Nevis was 12º radius. There were three inside this. The inner bow may be only 1º 30' radius. A Glory may consist of a bright light round the shadow of the head and a bow outside that. I have seen one description at least of a Glory that was not circular.
Glories are not always of the same colours as the Rainbow. Unfortunately, when we come to talk of colours, there are great difficulties. Many men are slightly colour-blind, or from want of practice in judging colours are not able to distinguish slight differences. Some men talk of the Rainbow as composed of three colours, others say seven. Mr Omond does not state in each case whether the glories that he saw were of the colours of the Rainbow. I understand that they nearly always were of those colours. No two Glories which vary from the Rainbow agree with one another in colour. If there are two or more bows, the outer ones usually vary a little from the inner one, but the colours are never reversed. The red is not always outside, but I believe that there is red in every bow. In all Glories seen from Ben Nevis the red was outside in each bow.
Outside Glories are sometimes seen one or two large bows called Fog-bows. They are usually white. Observations show that their sizes vary only within narrow limits. The inner bow has a radius of about 33º 30' or 34º 30'. It is sometimes called Ulloa's bow, from a Spaniard who first described and measured it in 1736. Mr Omond twice reports having seen red on the inner edge of this bow. No one else has reported colour. The outer Fog-bow is more often coloured than the inner one. It has been reported red outside, white inside; and sometimes of all the colours of the Rainbow, red outside. When white, the breadth of the white has been reported 5º to 7º. When of all the colours of the Rainbow, the breadth has been reported 2º.
These measurements require confirmation. The radius of this bow to the middle between the inner and outer edge is reported from 37º 30' to 42º. The Rainbow is 41º to the same place. Neither of these bows can be a secondary to the other, because each is sometimes seen alone. The outer one is the brightest, and most often seen. The bows just described are solar. Mr Omond has also seen two lunar Fog-bows. Radius of inner bow, to the inside 34º to 38º, to the outside 41º, probably very difficult to take; white, except on one occasion, when there was a trace of red on the outside. This inner lunar Fog-bow appears to agree in colour and nearly in measurement with the outer solar Fog-bow, and both may be variations of the Rainbow. The outer lunar Fog-bow was white, and no measurements were taken. It is much to be regretted that Mr Omond's observations since 1887 have not been printed. No one else is likely to see a lunar Fog-bow.
Temperatures reported with Fog-bows have varied from 19º to 56º. It is peculiar that temperature does not appear to make any difference to any of the bows that I have described. This seems to point to the conclusion that water in fog or cloud is in the same state, whether the temperature is above or below freezing-point.
What I have said about fog-bows suggests these questions:- Can there be a white solar Rainbow? What is the size of a white lunar Rainbow? Has a coloured one ever been seen?
Many of my statements will require modification, when more observations have been recorded. I can only generalise from descriptions that I have seen. Two hundred years ago Sir Isaac Newton explained the Rainbow; since then no other bow opposite the sun has been explained. What is now wanted is more observations, particularly of the sizes of Brocken bows and Fog-bows.
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