of Projectiles. 47

Side os the Squares Z>, be fix’d atg, andthe other End at f> any where belowthe Rule AB, this being done, if youSlide the Side of the Square CD alongthe lower Side of the Rule A B, and, atthe fame Time, keep the Thread con-stantly tight by means of the Pin E,with its Part £g close to the Side of theSquare g D, the Curve EVG, which thePin E describes by its Motion, will bea Parabola. Note , When the Side Dgtouches the Point f, E will be at theVertex, and then the Square must beturn’d to the other Side.

THEOREM IX.

| If from any Point G {Fig. 9th.) inj the Curve of a Parabola, a Right-Line

Gf be drawn to the Focus Points andG R parallel to the Axis V C, and if the! Angle fGR. is bisected, the bisecting| Line Tt will be a Tangent to the Curve

i in the Point G.

D