If the circles in Fig. 38., instead of being inscribed in the squares
_bc_ and _fg_, be inscribed in the sides of the solid figure _be_ and
_df_, those sides being made square, and the line _bd_ of any given
length, the resulting figure will be, according to the constructions
employed, a cone, polygonal pyramid, cylinder, or polygonal pillar,
drawn in position about a horizontal axis parallel to _bd_.
Similarly, if the circles are drawn in the sides _gd_ and _ec_, the
resulting figures will be described about a horizontal axis parallel
to _ab_.
[24] Or if the length of its side, _AC_, is given instead, take
_ae_, Fig. 37., equal to half the excess of _AB_ over _CD_;
from the point _e_ raise the perpendicular _ce_. With center
_a_, and distance _AC_, describe a circle cutting _ce_ in _c_.
Then _ce_ is the vertical height of the portion of cone
required, or _CE_.
[25] The direction of the side of the square will of course be
regulated by convenience.