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TRIGONOMETRY, as the name implies (Tprywvov jetpew), is that science which furnishes rules for the computation of triangles. As Geometry enables us to construct a triangle from three independent data, so Trigonometry enables us, from the same data expressed in numbers, to calculate its sides and angles. Triangles may be drawn either on a plane or spherical surface, and accordingly this branch of Mathematics is divided into Plane and Spherical Trigonometry. As a necessary preliminary to the investigation of trigonometrical rules, we proceed to explain the methods which are used for measuring angles, and expressing them by numbers.