9042An ESSAY
B and C, which will be the two moſt extreme
ones that can be ſeen.
ones that can be ſeen.
To do this another Way.
61.
If the upper Face of the Cylinder or Priſm
11Fig. 26,
27. be otherwiſe requir’d to be found, the ſame Things
being given as in the foregoing Method, we draw
the Line P Q in the perſpective Plane, parallel
to the Baſe Line, whoſe Diſtance therefrom we
make equal to the Height of the Priſm or Cy-
linder, whoſe Perſpective is requir’d. Then we
change its Geometrical Plane, ſo that the Baſe
Line coincides with P Q, and that in this Tran-
ſpoſition a Perpendicular to the Baſe Line coin-
cides with this ſame Perpendicular continued to-
2246. wards P Q. Finally we find the Perſpective of the Baſe of the Priſm or Cylinder, thus changed
in Situation by uſing P Q for a Baſe Line, and
the ſaid Perſpective is the Repreſentation of their
upper Faces.
11Fig. 26,
27. be otherwiſe requir’d to be found, the ſame Things
being given as in the foregoing Method, we draw
the Line P Q in the perſpective Plane, parallel
to the Baſe Line, whoſe Diſtance therefrom we
make equal to the Height of the Priſm or Cy-
linder, whoſe Perſpective is requir’d. Then we
change its Geometrical Plane, ſo that the Baſe
Line coincides with P Q, and that in this Tran-
ſpoſition a Perpendicular to the Baſe Line coin-
cides with this ſame Perpendicular continued to-
2246. wards P Q. Finally we find the Perſpective of the Baſe of the Priſm or Cylinder, thus changed
in Situation by uſing P Q for a Baſe Line, and
the ſaid Perſpective is the Repreſentation of their
upper Faces.
Demonstration.
If we ſuppoſe the Plane of the upper Surface
of the Priſm to be continued, it will meet the
Perſpective Plane in P Q; and the upper Face
in this Plane continued, will have the ſame
Situation in Reſpect to P Q, as the Baſe hath
on the Geometrical Plane with Regard to the
Baſe Line. If then the ſaid continued Plane be
conceived to lye on the perſpective Plane, the
upper Faces of the Priſm or Cylinder, will be
as the Baſes changed in the Manner aforeſaid;
therefore the Appearance of the ſaid Baſes
changed, will be that of the upper Surfaces.
of the Priſm to be continued, it will meet the
Perſpective Plane in P Q; and the upper Face
in this Plane continued, will have the ſame
Situation in Reſpect to P Q, as the Baſe hath
on the Geometrical Plane with Regard to the
Baſe Line. If then the ſaid continued Plane be
conceived to lye on the perſpective Plane, the
upper Faces of the Priſm or Cylinder, will be
as the Baſes changed in the Manner aforeſaid;
therefore the Appearance of the ſaid Baſes
changed, will be that of the upper Surfaces.
Note, By folding the Paper it is eaſy to
tranſpoſe Figures, and when the Height of
tranſpoſe Figures, and when the Height of