## 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

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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