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plane of their axes is inclined to the VP at an angle of 30. The position the prisms will now assume will be tantamount to swinging the two together as shown in No. 1 (Fig. 173), on the corner figured 2', of the left-hand one, through a horizontal angle of 30. If then the plan of the prisms obtained in No. 2 be drawn in with the line a b, making that angle with the IL, as shown in No. 3, then the elevation of the prisms in this position may be obtained by direct projection from No. 3 and No. 1, care being taken that the projectors used in doing so are drawn from corresponding points in each. If due consideration be given to the relative position of the edges of the prisms to each other, no difficulty should be experienced in obtaining a correct projection of them, as shown in No. 4. The lines of penetration in elevation do not, of course, in this case show as being at right angles to each other, although they are virtually in that position. Nos. 5 and 6 in Fig. 173 show the application of the same method of procedure to the case of the penetration of triangular prisms as to those of square section ; which require no further explanation than that afforded by the projectors shown in the diagram. One side of the penetrating prism is shown as coinciding with and passing through the axis of the penetrated one. In each case the cross-section of the prism is an equilateral triangle, the size of the penetrating one being shown by the end views given at A and B. As a multiplicity of similar examples to the foregoing would only show the application of the same principles in determining the lines of penetration of prisms by prisms, a few problems will now be taken in which the intersections of pyramids are involved. CHAPTER XVIII