generally for small angles, though the departure from the law with increase of angle would appear to be less in the case of the square plane than for planes of elongate form, whether in pterygoid or apteroid aspect. This is rendered evident by reference to Figs. 95, 96, 99.
Fig. 105. If the above form of expression held good each curve would be represented by a straight line passing through the origin. The actual curves are, in the vicinity of the origin, sensibly straight, but the departure from the straight is more marked in the case of planes of extreme proportion, and the applicability of the straight line law in this case is consequently more restricted, or subject to greater error.
Let us write the general expression for small angles in the form where is a constant. Then the value of which determines the slope of the line when plotted, depends upon the shape and aspect of the plane, or, in the case of a rectangular plane, its aspect and aspect ratio.
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