Plane trigonometry, by S.L. Loney.

CHAPTER IV.

APPLICATION OF ALGEBRAIC SIGNS TO TRIGONOMETRY.
46. Positive and Negative Angles. In Art. 6 in
treating of angles of any size we spoke of the revolving
line as if it always revolved in a direction opposite to that
in which the hands of a watch revolve, when the watch is
held with its face uppermost.
This direction is called counter-clockwise.
When the revolving line turns in this manner it is said
to revolve in the positive direction and to trace out a
positive angle.
When the line OP revolves in the opposite cirection,
i.e. in the same direction as the hands of the watch, it is
said to revolve in the negative direction and to trace out
a negative angle. This negative direction is clockwise.
47. Let the revolving line start from OA and revolve
until it reaches a position OP which
lies between OA' and OB' and which B
bisects the angle A'OB'.
If it has revolved in the positive A' -A
direction it has traced out the positive
angle whose measure is + 225~. B