The fourth book deals with the circle in its relations to inscribed and circumscribed triangles, **quadrilaterals** and regular polygons.

Owing to the small size of the experimental wire, the method is very quick and sensitive, and the apparatus can be set up in a few minutes when once the experimental **quadrilaterals** have been made.

37 The mensuration of earthwork involves consideration of **quadrilaterals** whose dimensions are given by special data, and of prismoids whose sections are D such **quadrilaterals**.

When dealing with maps not drawn on an equal area projection we substitute **quadrilaterals** bounded by meridians and parallels, the areas for which are given in the " Smithsonian Geographical Tables " (1894), in Professor H.