At intervals of from a few years to nine or ten years the crater has been periodically drained, at which times the moving platform of frozen lava has sunk more or less rapidly to levels far below the black ledge and from 900 to 1700 feet below the crater rim. Following this descent a slow progressive rise is inaugurated, which has sometimes gone on at a rate of more than a hundred feet per year, though it is usually much slower than this. When the platform has reached a height varying from 700 to 350 feet below the crater rim, another sudden settlement occurs which again carries the pit floor downward a distance of from 300 to 700 feet. [Illustration: FIG. 103.—View of the open lava lake of Halemaumau within the crater of Kilauea, the molten lava shown cascading over the raised lava walls on to the floor of the pit (after Pavlow).] =The draining of the lava caldrons.=—The changes which go on within the crater of Mokuaweoweo, though less studied than those of Kilauea, appear to be in some respects different. Here every eruption seems to be preceded by a more or less rapid influx of melted lava to the pit of the crater, this phenomenon being observed from a distance as a brilliant light above the crater—the reflection of the glow from overhanging vapor clouds. The uprising of the lava has often been accompanied by the formation of high lava fountains upon the surface, and the molten lava sometimes appears in fissures near the crater rim at levels well above the lava surface within the pit. Although in many cases the lava which has thus flooded the crater has suddenly drained away without again becoming visible, it is probable that in such cases an outlet has been found to some submarine exit, since under-ocean discharge effects have been observed in connection with eruptions of each of the volcanoes. [Illustration: FIG. 104.—Map showing the manner of outflow of lava from Kilauea during the eruption of 1840. The outflowing lava made its appearance successively at the points _A_, _B_, _C_, _m_, _n_, and finally at a point below _n_, from whence it issued in volume and flowed down to the sea at Nanawale (after J. D. Dana).]