If you look through a magnifying glass at distant objects, they will appear upside down. You may learn why this takes place from books on optics. The same effect is produced by rays passing through a medium which becomes gradually denser, instead of suddenly passing from one state to another. Thus, if you take a square glass bottle and put some clear syrup into it, and then carefully pour water on the top of that, anything, such as a written or printed line, seen through the space where the liquids are mixing, will appear inverted. Again, if you take a tin tube full of water, stopped with a piece of plain glass at each end, warm the middle of the tube, and then look at one end, you will see an object at the other end, if held at a proper distance, magnified, and distant objects turned upside down, just as they would be by a convex lens. If, on the contrary, you cool the middle of the tube, by applying ice to it, the same ensues as by using a concave lens. If a space of cold air be between two spaces of hot air, or the contrary, a space of hot air be between two spaces of cold air, the effects would be the same, only they could not be produced in so small a compass as they could with water. The space where the two different temperatures were gradually mixing, would influence the rays of light in the same manner as a lens on a very large scale. 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.