CIRCLE allows you to select an arc or circle for a
perimeter calculation.

Move the cursor near to a desired arc or circle on the
screen and press the space bar or cursor button.

The projected (Proj len) and actual (Actual len) perimeter
lengths of the selected entity are displayed at the bottom
of the screen.  The projected perimeter differs from the
actual perimeter only if the arc or circle is not in the
current display plane.

You can Press <Enter> to continue selecting desired 
entities.
