. Smithsonian miscellaneous collections. , and the contractile force which produces cracks atcertain distances in a given mass will exert itself equally in alldirections over a surface uniformly subjected to the cooling forces,and will, at the instant of fracture, act towards centres, whose dis-tance apart is dependent on the rate of cooling. If the mass is per-fectly homogeneous the centres of contraction will be disposed overthe surface with the greatest uniformity possible, that is, they willbe equidistant throughout, and the resultant fractures Avill be in asystem of hexagons. If from any
. Smithsonian miscellaneous collections. , and the contractile force which produces cracks atcertain distances in a given mass will exert itself equally in alldirections over a surface uniformly subjected to the cooling forces,and will, at the instant of fracture, act towards centres, whose dis-tance apart is dependent on the rate of cooling. If the mass is per-fectly homogeneous the centres of contraction will be disposed overthe surface with the greatest uniformity possible, that is, they willbe equidistant throughout, and the resultant fractures Avill be in asystem of hexagons. If from any irregularity in the compositionor petrographic structure of the rock the contractile force acts un-equally in different directions, the form of the polygons will be lessregular. The mutual influence of the forces producing different columns asthey approach each other is readily understood from the the case of two columns, S, S, approaching one another (,) and suppose the progress of the maximum strain to have reached. a b, a b, the forces producing contraction acting through a and awill meet and react on each other before those acting through band b, so that the points of maximum strain at any given time willhave advanced farther along the lines through a and a than throughb and V. The lines of greatest strain will then be c d and c d, andthe cracks normal to them will take the directions e e and c e\This will continue till they become parallel. If there were but two columns forming at equal rates they wouldcurve symmetrically and continue in parallel directions and of width, but if one column progresses more rapidly than the GENERAL MEETING. 23 other they will no longer curve to the same extent, and the slowerone will curve more than the faster one. Now, instead of two single columns, there,are always two groupsapproaching one another, and these prevent the continuation of thecolumns beyond the bend, pinching them out and causing them totaper off
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Keywords: ., bookcentury1800, bookdecade1860, bookidsmiths, booksubjectscience
