Now portions of air are often made of different temperature by the sun's rays, by evaporation, as from the surface of the sea, or of lakes, or from marshy districts, and by winds operating under particular local circumstances. I do not want to lengthen out this dry story, but if you have read attentively what I have said, you will see that various positions of masses of air heated to different degrees, is all that is necessary to account for the instances which I have mentioned, in which objects have appeared inverted and out of their true positions; such as Dover Castle, Captain Scoresby's father's ship, the French coast opposite Hastings, and the islands, and horses with their legs upwards, described by Humboldt.
The apparitions of Souter Fell may be accounted for in like manner. The latter one was seen at a time of civil commotion, when there were private troops of horse exercised in all parts of the country, and so the fact of armed horsemen being in the neighbourhood, is rendered very probable. We have only to suppose the image of such a troop to be brought to the side of Souter Fell, perhaps from the opposite side of the mountain, by a complicated refraction, like that which appeared to move Dover Castle out of its place.
But the Spectre of the Brocken, the Fata Morgana, and the image which my friend saw of the side of the hill on which he was, require another sort of explanation, because the object and the image are seen both at once; the latter could therefore have been no other than the reflection of the first.
It seems likely that some vapours are capable of receiving shadows. When I have been bathing in a river with a muddy bottom, I have often seen my shadow on the cloud of muddy particles which I have disturbed from the bottom, in a manner something similar to that in which I should think this may occasionally take place.
There is, however, another theory of it. When rays pass from a thin medium into a denser medium, the whole do not go through, but they are strained, as it were, and a part are kept back and reflected. It is thus that you see a reflection on a transparent pane of glass.
If you breathe very lightly upon it, the reflection will be still more distinct, and the resemblance to the phenomenon we are describing probably greater. There are then two causes of reflection, the change of refracting power, and the presence of the watery particles.
Something of this kind perhaps occurs on the top of the Brocken. A rush of cold air may set up from deep ravines, with water and marshy land at the bottom, on the West side, while the rising sun is genially warming the air on the east side of the mountains. Mind, I do not say that it is so, but it does not seem unlikely that two currents, one of cold air and the other of hot, thus ascend close to each other; and according to what I have told you, there would be a reflecting power in the plane of contact, which might be increased by the watery particles carried upwards.
A kind of aerial screen would thus be formed, which might catch the shadow of a person on the opposite summit, cast upon it by the horizontal rays of the morning sun. Thus you may account for the image, and its being so greatly magnified, requires no further explanation than I have given above; as it only needs the supposition of a mass of heated air, with two colder ones on each side, being between the persons and the reflecting substance.
The doubling of one of the figures was possibly occasioned by a reflecting surface having been formed on a different plane from the first, which might very easily occur.