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196 up hill. To meet this difficulty, Laplace resorted to an ingenious hypothesis. Let us suppose that the rigid solid ring is not of uniform density or thickness. Let us, for example, imagine that a portion of it is made of lead, while the rest is lighter than cork, and its gravitation insignificant in comparison; it is plain that such a system would approximate to that of a simple satellite, and so far be stable. The lead would move as a satellite, and the light portion, by supposition, is not sufficiently powerful to alter the conditions. If we attach a sovereign to a light ring of paper and throw both into the air, the curve described will be that due to the sovereign and not that due to the paper; simply because the gold so far preponderates, that the effect of the paper may be regarded as nil. This is, however, a wholly gratuitous supposition of Laplace. Observation does not confirm any notable difference in thickness or density. No doubt, when the edge of the ring is presented to us, some bright points remain when the rest disappears; but this appearance, first ascribed to lofty mountains, is not due to any fixed features of the ring; for the bright points are stationary, while the ring rotates. These points of light are believed to be due to the reflection of the inner edges of the rings presented to the sun. They do not appear to move, for the same reason that the sun's image does not move on with the flowing river from which it is reflected.