a
,
b
,
c
, etc.) of course are drawn to the vanishing-point of B F (the other side of the house), if F B V represents a right angle; if not, their own vanishing-point must be found separately for these returns. But see [Practice on Problem XI].
Fig. 55.
Interior angles, such as E B C, [Fig. 55.] (suppose the corner of a room), are to be treated in the same way, each side of the room having its measurements separately carried to it from the measuring-line. It may sometimes happen in such cases that we have to carry the measurement up from the corner B, and that the sight-magnitudes are given us from the length of the line A B. For instance, suppose the room is eighteen feet high, and therefore A B is eighteen feet; and we have to lay off lengths of six feet on the top of the room wall, B C. Find D, the dividing-point of B C. Draw a [p78] ]measuring-line, B F, from B; and another,
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