182. _Birth and Decay of Sun-Spots._--The formation of a spot is sometimes gradual, requiring days or even weeks for its full development; and sometimes a single day suffices. Generally, for some time before its appearance, there is an evident disturbance of the solar surface, indicated especially by the presence of many brilliant faculæ, among which _pores_, or minute black dots, are scattered. These enlarge, and between them appear grayish patches, in which the photospheric structure is unusually evident, as if they were caused by a dark mass lying below a thin veil of luminous filaments. This veil seems to grow gradually thinner, and finally breaks open, giving us at last the complete spot with its penumbra. Some of the pores coalesce with the principal spot, some disappear, and others form the attendant train before described (179). The spot when once formed usually assumes a circular form, and remains without striking change until it disappears. As its end approaches, the surrounding photosphere seems to crowd in, and overwhelm the penumbra. Bridges of light (Fig. 199), often much brighter than the average of the solar surface, push across the umbra; the arrangement of the penumbra filaments becomes confused; and, as Secchi expresses it, the luminous matter of the photosphere seems to tumble pell-mell into the chasm, which disappears, and leaves a disturbed surface marked with faculæ, which, in their turn, gradually subside. [Illustration: Fig. 200.] 183. _Motion of Sun-Spots._--The spots have a regular motion across the disk of the sun from east to west, occupying about twelve days in the transit. A spot generally appears first on or near the east limb, and, after twelve or fourteen days, disappears at the west limb. At the end of another fourteen days, or more, it re-appears at the east limb, unless, in the mean time, it has vanished from sight entirely. This motion of the spots is indicated by the arrow in Fig. 200. The interval between two successive appearances of the same spot on the eastern edge of the sun is about twenty-seven days. [Illustration: Fig. 201.]