In practical astronomy, metallic circles ac curately divided are used to measure angles. The general principle is to have a telescope moving in a given plane, while a divided circle. situated in a parallel plane, measures the angle through which the telescope moves. Verniers or microscopes are used to read the graduated circle with great precision.
The term *meridian circle' is applied to an instrument in which the telescope revolves in the plane of the meridian, and a divided circle measures angles in this plane.
In logic, a circle is the fault of an argument that assumes the principle it should prove, and afterward proves the principle by the thing which it seemed to have proved. The same fault takes place in definitions when an idea is defined by others which suppose the knowl edge of the first. Arguing in a circle is a fault into which men are very liable to fall, particu larly in theological discussions.
In astronomy, the heavens being considered as a spherical surface drawn round the earth as centre, an imaginary line drawn round the heavens so as to lie in one plane is a drcle of the sphere. It is a *great circle' if the plane of it passes through the centre; thus the celes tial equator and the ecliptic are great circles; if the plane of the circle does not pass through the centre it is called a *small circle' ; all paral lels of declination are small circles.
See also •GEOMETRY, various sub-headings. Consult McClelland, 'Geometry of the Circle' (London 1891); Smith, D. E., 'The History and Transcendance of ) (in Young, 'Mono Topics in Modern Mathematics' New York 1911). CIRCLE, Magic, a space in which sor cerers were wont to protect themselves from the fury of the evil spirits they had raised. This circle was usually formed on a piece of ground about nine feet square (in the East seven feet appears to have been considered sufficient), in the midst of some dark forest, churchyard, vault or other lonely and dismal spot. It was described at midnight in certain conditions of the moon and weather. Inside the outer circle was another somewhat less, in the centre of which the sorcerer had his seat. The spaces between the circles, as well as between the paral lel lines which enclosed the larger one, were filled with all the holy names of God, and a variety of other characters supposed to be potent against the powers of evil. Without the pro tection of this circle, the magician, it was be lieved, would have been carried off by spirits Another figure which, described upon the ground, could bar the passage of a demon, was the pentagram.