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.

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.

Fig. 60.

60.

109.

113. For Perpendiculars of the ſame Length.

The Figures in the Geometrical Plane being

tranſpoſed in the Manner aforeſaid , aſſume T t,