cut for magnetic determination in the laboratory. The measurements, which were made by Professor Herroun, were obtained by placing the cube near a freely suspended magnetized needle, in a partially neutralized field. The instrument was standardized by means of bars of known moment and by the time of swing. Susceptibility was measured by surrounding the cube with a coil. Specimens from Leicestershire (Charnwood Rocks) gave low values, but a measureable degree of permanent magnetization and

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... tibility was found for the Cleveland Dyke and for the dyke at Lornty in Perthshire. A method is developed for calculating the magnetic profile from the geological model assuming uniformity of magnetization. Calculated and observed curves for the Lornty profiles show a fairly good agreement; they indicate that the north side of the dyke is south-polar, so that the magnetization appears to be in the opposite sense to th at recorded for certain dykes in Germany, and does not coincide with that which would be produced by cooling in the earth's present field. In transparent bodies under plane stress the difference and directions of the two principal stresses are obtained by observations with polarized light. To separate the stresses either additional measurements can be used, for instance changes of thicknessf ; or we may resort to any method of graphical integration. Although several different methods exist, J all are based upon the equations of equilibrium and involve considerable time. A new method, which has now been worked out by the writer, based entirely upon elastic equations, enables * In charge of the photo-elastic investigation in the Mech. Techn. Labor., Technical University, Munich under the direction of Professor L. F6ppl. t Coker and Filon, " A Treatise on Photo-Elasticity," Camb., 1931, § § 2-47-2-49. % Coker and Filon, loc. cit., § 2.30; Filon, ' Brit.