A geometric construction done with a movable compass alone. All constructions possible with a compass and straightedge are possible with a movable compass alone, as was proved by Mascheroni (1797).
Mascheroni's results are now known to have been anticipated largely by Mohr
(1672).
An example of a Mascheroni construction of the midpoint
of a line segment specified by two points
and
illustrated above (Steinhaus 1999, Wells 1991). Without loss of generality, take
.
1. Construct circles centered at
and
passing through
and .
These are unit circles centered at (0, 0) and (1, 0).
2. Locate ,
the indicated intersection of circles
and ,
and draw a circle centered on
passing through points
and .
This circle has center (1/2, )
and radius 1.
3. Locate ,
the indicated intersection of circles
and ,
and draw a circle centered on
passing through points
and .
This circle has center (3/2, )
and radius 1.
4. Locate ,
the indicated intersection of circles
and ,
and draw a circle centered on
passing through point .
This circle has center (2, 0) and radius .
5. Locate
and ,
the intersections of circles
and .
These points are located at positions (5/4, ).
6. Locate ,
the intersection of circles
and .
This point has position (1/2, 0), and is therefore the desired midpoint of .
Pedoe (1995, pp. xviii-xix) also gives a Mascheroni solution.
Wolfram
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