Book I.
VIII.
“ A plain angle is the inclination of two lines to one another in a plane, “ which meet together, but are not in the ſame direction.”
IX.
A plain rectilinear angle is the inclination of two ſtraight lines to one another, which meet together, but are not in the fame ſtraight line.
‘ N. B. When ſeveral angles are at one point B, any 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 die ſſtraight lines that contain the angle meet one anodicr, is put between the other two letters, and one of thefe two is ſomewherc upon one of thclc ſtraight lines, and the other upon the other line, thus the angle which is contained by the ſſtraight 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 expreſſed by a letter placed at that point; as the angle at E.’
X.
When a ſtraight line (landing on another ſtraight line makes the adjacent angles equal to one another, each of tilde angles is called a right angle ; and the ſtraight line which (lands cm the other is called a perpendicular to it.
XI.
An obtufe angle is that which is greater than a right angle.
