Journal of the Meteorological Society of Japan. Ser. II

Frequency curve as shown in fig. 1 p. 3. is constracted using the data from 444 ascents of pilot balloon. The authors conclude that (1) prevailing directions of upper winds differs from a stratum to another and this seems to be caused by some disturbing action coherent to each statum; (2) the direction of maximum frequency in the layer of lower handreds maters is determined mainly by the orographic conditions. At Kobe the prevailing direction of wind at that layer is from the east. (3) The direction of gradient wind becomes prevailing at and above the height of about 2000m.

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The forms of cracks caused respetively by simple tension, bending, contraction and shearing are differ each other. The author has investigated them. By a simple tension exerted on a plate in a defiuite direction in the plane of the plate one or many cracks are produced perpendicular to the tension. When the material of the plate is quite uniform, the cracks are parallel and equidistant according to the principle of equality⁽¹⁾. By cylindrical bending of a plate the tension is exerted along convex side of the plate and compression along the concave side. Hence when the material is properly soft, parallel cracks are liable to be formed on the convex side and creases of similar form on the other. By conical bending the cracks on the outer surface are along generating lines. The distances between each cracks are nearly equal when the curvature of the surface is uniform. By ring form bending cracks are concentric circles. If a plate is bent by a normal force applied at a point on it, the conical and ring form bendings are resulted on which radial cracks are produced on the convex side and lateral circular cracks on the concave side. (Fig. 1.) We can see that these cracks are symmetrical when the plate-material is homogeneous. When the material is brittle enough⁽¹⁾ as is a glass plate the cracks on one side reach to the other side so that we see usually radial as well as circular cracks, but when the plate is tenacious inside as enamelled iron plate, then we can see each kind of cracks on the respective side.
Cracks produced by uniform and isotropic contraction are quite different in the form from those mentioned above. Circular crack is most symmetrical by itself, but it is impossible to divide a surface with only a system of circles; hence circles deform to poligons and we see so often the lattice cracks composed of poligons. Small net work seen in fig. 2 is an example which is produced on the surface of wall by thermal expansion of the under lying concrete bed. We can see here that crack has tendency to meet normally with another existing one, sometimes by curving itself purposely. This is also the tendency toward symmetry. When three cracks start from a point they have tendency to run equiangularly i.e. separate from each other by 120°. This is also explained in virtue of the principle of equality.
By ordinary shearing oblique parallel cracks are formed as already well known. If there are two or more strata lying one over the other and if the bottom layer will slip along a line of discontinuity, then cracks will be formed also on the surface layer if it is brittle, in such a form shown in fig. 2. (oblique big cracks) The reason for it is very clear, because at the surface layer there are two tendencies for crack-making that one to run along the slip line of the bottom layer and the other to pass obliquely against that line. The result is the formation of a series of cracks run along the slip-line but a little oblique to it. The sense of obliquity-to the right or left-determines the direction of the slip. Prof Omori⁽¹⁾ and. Mr. T. Tokuda⁽²⁾ have shown actual examples of such cracks. An experiment was carried out with a mass of putty and it was shown that, when the material is plastic, the line of disconstinuity which appears on the surface as the result of the bottom slip will exhibit a sinuosity, an asymmetric sinuosity, and at last a chain of S-shaped links, according to the progress of the bottom slip. The formation of S-shape is benefited by existing of curvature of the slip line, heterogeneity of the material and inequality of the thickness of the plate. A characteristic series of creases are formed starting obliquely from the main line of discontinuity and diverging gradually.

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