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    EARLY HISTORY OF THE SOLAR SYSTEM.—An important contribution to the mathematical investigation of the evolution of the solar system has been made by Dr. Harold Jeffreys in a communication to the Royal Astronomical Society (Monthly Notices, vol. lxxviii., p. 424). It is first shown to be improbable that the planets were formed by the gradual condensation of a gaseous mass, and it would seem that they were strongly condensed from the beginning, and were formed catastrophically. The tidal theory is therefore adopted, according to which a star of mass several times, greater than that of the sun approached it so closely that the tidal action resulted in the extrusion of one or two streams of matter having a considerable velocity. These streams would break up almost at once into a series of fluid masses, and the gaseous matter set free in the initial disruption would form a resisting medium, the effect of which would be to reduce the eccentricities of the original orbits. From the rate of change of eccentricity it is provisionally estimated that the age of the solar system is 3 × 109 years, which is in general agreement with the age derived by radio-active methods. Among other results of interest it is shown that all the bodies having diameters less than 1000 km., if assumed to be composed of silica, must have been liquid or solid from the beginning, as smaller masses could not have been held together by their own gravitation when in the gaseous state. Dr. Jeffreys considers that the asteroids were probably formed from a primitive planet which approached Jupiter so closely as to be broken up by tidal action.

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