Page:The Elements of Euclid, viz. the first sixs books, together with eleventh and twelfh.djvu/12

2 Book I. “ A plain angle is the inclination of two lines to one another in a plane, “ which meet together, but are not in the ame direction.”

A plain rectilinear angle is the inclination of two traight lines to one another, which meet together, but are not in the fame traight line.

‘ N. B. When everal angles are at one point B, any of them is expreed by three letters, of which the letter that is at the Vertex of the angle, that is at the point in which die traight lines that contain the angle meet one anodicr, is put between the other two letters, and one of thefe two is omewherc upon one of thclc traight lines, and the other upon the other line, thus the angle which is contained by the traight 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 expreed by a letter placed at that point; as the angle at E.’ When a traight line (landing on another traight line makes the adjacent angles equal to one another, each of tilde angles is called a right angle ; and the traight line which (lands cm the other is called a perpendicular to it. An obtufe angle is that which is greater than a right angle. Rh