Note Footnote 7 — EXPERIMENTS ON RETAINING WALLS AND PRESSURES ON TUNNELS.

: To attain the greatest accuracy, in constructions like that shown in Fig. 11, the scale should be as large as possible; the arcs of circles, especially, must be drawn with a large radius, and the points, , , etc., determined with care. The angle, , can be computed and laid off by aid of a table of chords. The construction in this figure corresponds to a vertical height of ft., , . The value of the component, , perpendicular to , is now to be found, by drawing lines from and , perpendicular and parallel to , to intersection, and measuring the component to scale. For , it is found that is the plane of rupture. The line, , through the new , representing the thrust, is very small; but it can be easily magnified by laying off the polygon, , to a scale two or three times as large, and thus the thrust can be found as accurately as before.] Return to text