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III. The extremities of a line are points.
VIII. " A plane angle is the inclination of two lines to one another See N.
“ in a plane, wbich meet together, but are not in the same
to one another, which meet together, but are not in the
•N. B. When several angles are at one point B, any one of them is expressed by three letters, of which the letter that
is at the vertex of the angle, that is, at the point in which 'the straight lines that contain the angle meet one another, is • put between the other two letters, and one of these two is
fomewhere upon one of those straight lines, and the other upon the other line: Thus the angle which is contained by the straight lines AB, CB is named the angle ABC, or CBA;
that which is contained by AB, DB is named the angle • ABD, or DBA ; and that which is contained by DB, CB is • called the angle DBC, or CBD; but, if there be only one angle • at a point, it may be expreffed by a letter placed at that point; ( as the Angle at E.'
ther straight line makes the adjacene
xv. A circle is a plane figure contained by one line, which is cal
led the circumference, and is such that all straight lines drawn from a certain point within the figure to the circumference, are equal to one another :
XVII. A diameter of a circle is a straight line drawn through the Sce N. centre, and terminated both ways by the circumference.
XVIII. A femicircle is the figure contained by a diameter and the part of the circumference cut off by the diameter.
XIX. “ A segment of a circle is the figure contained by a straight “ line, and the circumference it cuts off.”
XXIII. Multilateral figures, or polygons, by more than four straight lines.
equal, and all its angles right angles.
are not right angles.
XXXIII. See N. A rhomboid, is that which has its opposite fides equal to one
another, but all its fides are not equal, nor its angles right angles.
which, being produced ever so far both ways, do not meet.
POSTULAT E S.
distance from that centre.
as all its :
A X I OM S.
es equal to 4 sangles ning