Full text: Gravesande, Willem Jacob: An essay on perspective

An ESSAY until C D be equal to twice the Length of the
given Perpendicular. This being done, move
the Needle along the Horizontal Line, ſuppoſe
to R, until the Point of the Thread R S paſſes
through the Point T; then keeping the Thread
tight in this Manner, if the Ruler M N be
mov’d, till its Edge alſo paſſes through the
Point T, and P is the Point wherein the Edge of
the ſaid Ruler croſſes the Part of the Thread
R D, the Line T P will be the Repreſentation
ſought.

The Demonſtration of this is evident from
what is ſaid in n. 59.

178. Method II.

112. When all the Perpendiculars have the ſame
Length.

Let F G be parallel to the Baſe Line, and F O
equal to the Height of the Eye; aſſume F f, e-
qual to the Length of either of the given Per-
pendiculars, and faſten the Thread fixed in F, in
the Point f. Then raiſe R S perpendicular to
the Baſe Line, which make equal to F f, and
draw S Q parallel to the Baſe Line. This being
done, tranſpoſe the Figures in the Geometri- cal Plane, in ſuch Manner, that the Point R co-
incides with S, and R H with S Q. Then if S Q
be taken for a Baſe Line, and the Appearances
of the Feet of the Perpendiculars be found, the Repreſentations of their Extremities will be
had.

178.1.

Fig. 60.
60.
109.

179. Method III.

113. For Perpendiculars of the ſame Length.

The Figures in the Geometrical Plane being
tranſpoſed in the Manner aforeſaid , aſſume T t,

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