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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