Transactions . Fig. 12.) The stress in each of the two directionsx and y, reckoned as water head, is H. Each of these creates astress along z of H j = H (for glass). The resultant com-pressive stress along z due to these two is therefore H (for glass).The tensile stress produced along z by the water pressure is H, so THE LAWS OF JOINTING. 109 that the net horizontal tension along z is ( — ) H = H (appro*..for glass). The minimum depth necessary to produce jointing under theseconditions may be directly read from the second column of Table I. The imperfect or partly
Transactions . Fig. 12.) The stress in each of the two directionsx and y, reckoned as water head, is H. Each of these creates astress along z of H j = H (for glass). The resultant com-pressive stress along z due to these two is therefore H (for glass).The tensile stress produced along z by the water pressure is H, so THE LAWS OF JOINTING. 109 that the net horizontal tension along z is ( — ) H = H (appro*..for glass). The minimum depth necessary to produce jointing under theseconditions may be directly read from the second column of Table I. The imperfect or partly formed nature of many secondary jointingsystems is thus explained by the fact that considerably more depth isrequired for their development than for the primary joints. Effect of Earthquakes.—Prof. W. 0. Crosby has speculated on theinfluence of earthquakes in producing Earthquakes of the largest known violence have within the solidground an amplitude of vibration (a) of about 2 in. and a complete. Fig. 12.—Stresses Producing Secondary Joints (Maximum).period (t) of sec. The velocity (v) is about 4,000 ft. per following formula gives the resultant stress : h = — —, where h is the maximum pressure produced in rock head. Space is not takenhere to show how this formula is obtained because it is easily derivedfrom those given in books on earthquakes. The greatest difference of stress from the normal, produced by anearthquake, is thus seen to be equivalent to 261 ft. of rock is a small amount relatively to the joint-forming stresses andtherefore earthquakes cannot be considered as a factor in jointing. 18 Proceedings of the Boston Society of Natural History, vol. xxii., p. 72 (1882-83). 110 the laws of jointing. Discussion. F. L. GRAMMER,Leesburg, Va. (communication to the Secretary*):—Mr. Stevens refers to an explanation of parallel jointing by W. }, which I noted when reading one of his excellent text books. It seemed to me ear
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