§ 327. The Mandan believe that thunder is produced by the wings of a gigantic bird. When the bird flies softly, as is usually the case, he is not heard; but when he flaps his wings violently, he occasions a roaring noise. This bird is said to have two toes on each foot, one behind and one before. It dwells on the mountains, and builds nests there as large as one of the forts. It preys upon deer and other large animals, the horns of which are heaped up around the nest. The glance of its eyes produces lightning. It breaks through the clouds and makes way for the rain. The isolated and peculiarly loud claps of thunder are produced by a large tortoise which dwells in the clouds. ASTRONOMICAL LORE. § 328. The stars are deceased men. When a child is born a star descends and appears on earth in human form; after death it reascends and appears again as a star in the heavens. The rainbow is a spirit which accompanies the sun. Many affirm that the northern lights are occasioned by a large assembly of medicine men and distinguished warriors of several northern nations, who boil their prisoners and slain enemies in huge cauldrons.[238] MYSTERY OBJECTS AND PLACES OF THE MANDAN AND HIDATSA. § 329. The mystery rock of the Mandan and Hidatsa is thus described by Lewis and Clarke:[239] This medicine stone is the great oracle of the Mandans, and whatever it announces is believed with implicit confidence. Every spring and, on some occasions during the summer, a deputation visits the sacred spot, where there is a thick, porous stone 20 feet in circumference, with a smooth surface. Having reached the place, the ceremony of smoking to it is performed by the deputies, who alternately take a whiff themselves, and then present the pipe to the stone; after which they retire to an adjoining wood for the night, during which it may be safely presumed that all the embassy do not sleep. In the morning they read the destinies of the nation in the white marks on the stone, which those who made them are at no loss to decipher. The same stone, as worshiped by the Hidatsa, is thus described by James:[240]