51�Թ�

Spending long nights in antiaircraft positions watching the skies, he imagined spherical triangles drawn across the sky and worked out their geometry.

Containing or having two right angles; as, a birectangular spherical triangle.

Thus we might have a spherical triangle that is concave, with its base 359°, and its other two sides each 90°, the sum of the sides being 539°.

The sum of the interior angles of a spherical triangle is always greater than two right angles, and increases with the area.

The clerestory windows are spherical triangles enclosing three circles with quatrefoil cusps; the form of these windows is somewhat rare, but similar windows are to be seen at Westminster, Hereford, Carlisle, and York.