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Measurement of Angles

The angle between two straight lines drawn on a sheet of paper may be roughly measured by means of a protractor, a circle or semi-circle with its rim divided into degrees. Its centre is marked, and can therefore be placed so as to coincide with the point of intersection of the two straight lines; the angle between them can then be read off on the graduations along the rim of the protractor. An analogous method of measuring angles is employed in the case of a compass-needle such as that required for 69.

The more accurate methods of measuring angles depend on optical principles, and their consideration is accordingly deferred until the use of the optical instruments is explained (see 62, 71).



Last Update: 2011-03-15