§ 10. I believe conditions of form like those in Fig. 82 (turn the book with its outer edge down) may not unfrequently be thus, owing to stratification, when they occur in the nearer sky. This line of cloud is far off at the horizon, drifting towards the left (the points of course forward), and is, I suppose, a series of nearly circular eddies seen in perspective.
Which question of perspective we must examine a little before going a step farther. In order to simplify it, let us assume that the under surfaces of clouds are flat, and lie in a horizontal extended field. This is in great measure the fact, and notable perspective phenomena depend on the approximation of clouds to such a condition.
§ 11. Referring the reader to my Elements of Perspective for statements of law which would be in this place tiresome, I can only ask him to take my word for it that the three figures in Plate 64 represent limiting lines of sky perspective, as they would appear over a large space of the sky. Supposing that the breadth included was one-fourth of the horizon, the shaded portions in the central figure represent square fields of cloud,[2] and those in the uppermost figure narrow triangles, with their shortest side next us, but sloping a little away from us.
In each figure, the shaded portions show the perspective limits of cloud-masses, which, in reality, are arranged in perfectly straight lines, are all similar, and are equidistant from each other. Their exact relative positions are marked by the lines connecting them, and may be determined by the reader if he knows perspective. If he does not, he may be surprised at first to be told that the stubborn and blunt little triangle, b, Fig. 1, Plate 64, represents a cloud precisely similar, and similarly situated, to that represented by the thin triangle, a; and, in like manner, the stout diamond, a, Fig. 2, represents precisely the same form and size of cloud as the thin strip at b. He may perhaps think it still more curious that the retiring perspective which causes stoutness in the triangle, causes leanness in the diamond.[3]
| 64. Cloud Perspective. (Rectilinear.) |
| 65. Cloud Perspective. (Curvilinear.) |
§ 12. Still greater confusion in aspect is induced by the apparent change caused by perspective in the direction of the wind. If Fig. 3 be supposed to include a quarter of the horizon, the spaces, into which its straight lines divide it, represent squares of sky. The curved lines, which cross these spaces from corner to corner, are precisely parallel throughout; and, therefore, two clouds moving, one on the curved line from a to b, and the other on the other side, from c to d, would, in reality, be moving with the same wind, in parallel lines. In Plate 66, which is a sketch of an actual sunset behind Beauvais cathedral (the point of the roof of the apse, a little to the left of the centre, shows it to be a summer sunset), the white cirri in the high light are all moving eastward, away from the sun, in perfectly parallel lines, curving a little round to the south. Underneath, are two straight ranks of rainy cirri, crossing each other; one directed south-east; the other, north-west. The meeting perspective of these, in extreme distance, determines the shape of the angular light which opens above the cathedral. Underneath all, fragments of true rain-cloud are floating between us and the sun, governed by curves of their own. They are, nevertheless, connected with the straight cirri, by the dark semi-cumulus in the middle of the shade above the cathedral.
| Fig. 83. |
§ 13. Sky perspective, however, remains perfectly simple, so long as it can be reduced to any rectilinear arrangement; but when nearly the whole system is curved, which nine times out of ten is the case, it becomes embarrassing. The central figure in Plate 65 represents the simplest possible combination of perspective of straight lines with that of curves, a group of concentric circles of small clouds being supposed to cast shadows from the sun near the horizon. Such shadows are often cast in misty air; the aspect of rays about the sun being, in fact, only caused by spaces between them. They are carried out formally and far in the Plate, to show how curiously they may modify the arrangement of light in a sky. The woodcut, Fig. 83, gives roughly the arrangement of the clouds in Turner’s Pools of Solomon, in which he has employed a concentric system of circles of this kind, and thus lighted. In the perspective figure the clouds are represented as small square masses, for the sake of greater simplicity, and are so beaded or strung as it were on the curves in which they move, as to keep their distances precisely equal, and their sides parallel. This is the usual condition of cloud: for though arranged in curved ranks, each cloud has its face to the front, or, at all events, acts in some parallel line—generally another curve—with those next to it: being rarely, except in the form of fine radiating striæ, arranged on the curves as at a, Fig. 84; but as at b, or c. It would make the diagram too complex if I gave one of intersecting curves; but the lowest figure in Plate 65 represents, in perspective, two groups of ellipses arranged in equidistant straight and parallel lines, and following each other on two circular curves. Their exact relative position is shown in Fig. 2, Plate 56. While the uppermost figure in Plate 65 represents, in parallel perspective, a series of ellipses arranged in radiation on a circle, their exact relative size and position are shown in Fig. 3, Plate 56, and the lines of such a sky as would be produced by them, roughly, in Fig. 90, facing page 128.[4]
| Fig. 84. |