, we raise a perpendicular and make D F equal to E V, a line C F, drawn from any point C on the measuring-line to F, will mark the distance A B on the inclined line, A B being the portion of the given inclined line which forms the diagonal of the vertical rectangle of which A C is the base.
[Footnote 27: ] The demonstration is in Appendix II., [p. 104].] [Return to text]
[p57]
][PROBLEM XVIII.]
TO FIND THE SIGHT-LINE OF AN INCLINED PLANE IN WHICH TWO LINES ARE GIVEN IN POSITION.[Footnote 28] ]
As in order to fix the position of a line two points in it must be given, so in order to fix the position of a plane, two lines in it must be given.
Fig. 48.
Let the two lines be A B and C D, [Fig. 48.]
[p58]
]As they are given in position, the relative horizontals A E and C F must be given.
Then by [Problem XVI.] the vanishing-point of A B is V, and of C D, V′.