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The use of arrays of seismometers for seismic refraction measurements of crustal structure using explosions is reviewed, and the general factors to be considered in the design of equipment for the acquisition and processing of such data are discussed. The design and construction of a particular system using digital magnetic tape recording are described in detail. The three main modes of operation are the recording of ten channels of seismic information in the field, the replay in the laboratory of these ten channels to a multi-channel galvanometer oscillograph, and the transfer of the digital information to punched paper tape for input to a general purpose digital computer. The programmes that have been developed for handling this data on an Elliott 803 computer are described. The system was used during a crustal experiment using depth charges in September, 1%$. The performance during this test is evaluated. II - Stress systems in an inhomogeneous crust. The plane strain stress in an elastic half-space in which there are discrete variations in density over rectangular areas of the cross section is calculated by the double integration of .the result for a point force acting within an homogeneous elastic half-space. The resulting stress system is shown to be a valid solution of the equations of elasticity, and analytical results are derived for several special cases. The application of the theory of elasticity to crustal processes is reviewed, and numerical results are used to discuss the stress systems associated with isostatic compensation and with a crust of varying density.