This page is dedicated to curves that are made from more than one circle.
In Venn diagrams each collection is represented by a circle, as a visual help to
derive rules for the intersections.
A polygon build of circle
arcs is called a curvilinear polygon.
An example with two arcs is the lens.
A Reuleaux
polygon is a curvilinear polygon where the center of each circle arc is
the opposite point of the polygon.
Each Reuleaux polygon is a curve
with constant width.
Most known example is the three arc Reuleaux triangle, a
curvilinear triangle.
Two circles
Two circles we find in the
English bicycle^{1)}, and the French bicyclette.
By two
(equal) arcs a lens or lemon
curve can be formed. This is a curvilinear
polygon.
This is an old symbol, denoting a fish as well as the female vulva.
For Pythagoras it was the symbol of the intersection of the world of the divine
(one circle) with the world of matter (other circle).
Later the sign was adopted by the Christians, with names as:
Extending the arcs in one side results in the Christian ichtus symbol.
An annulus is the form of two concentric circles, the shape of a ring.
A more recent use is a pattern formed in the shape of two circle arcs, the stamp of Truchet. The pattern
consists of two different orientations of the stamp next to each other, in an arbitrary sequence orientation.
These patterns have been used in the construction of games, e.g. the 'black path game' and 'meander'.
The pattern was found by Cyril Stanley Smith, who wrote an article ^{2)}
about Truchet. Smith was a metallurgist and historian of technology at MIT who studied crystals.
The pattern described by Truchet was the Truchet tiling.
Three circles
A circular
triangle is formed by three (equal) arcs.
In the case of three equal arcs beginning in the angles of an equal
triangle, it is called the Reuleaux triangle.
This triangle can be rotated inside a square,
so making it possible to drill a hole with (nearly) the shape of a square.
Archimedes described the arbelos
or shoemaker's knife. The curve
resembles the blade of a knife used by ancient cobblers.
It appears that the arbelos has some remarkable qualities, for instance:
 the arc length along the
enclosing semicircle is the same as along the two smaller semicircles
 draw a perpendicular from the tangent of the two semicircles to the
edge of the large circle. Then the area of the circle with as diameter the
perpendicular is
the same as the area of the arbelos
The two circles inscribed on each half of the perpendicular are called the Archimedes' circles or the twin circles of
Archimedes.
Circles in the arbelos that are congruent (i.e. equal radius) to these circles are called Archimedean
circles.
Now, let's denote the left semicircle as C, with center (c, 0) and the right
semicircle as D, with center (d,0). C and D touch at O, E is the large
semicircle.
Now define semicircles C(nc) and D(nd), with centers (nc,0), (nd,0)
respectively, which touch in O. Then
Schoch's circle S is the circle that touches
E internally and C(2c) and D(2d) externally.
Woo considered the Schoch line w through the
center of the Schoch's circle and parallel to the yaxis.
Now it appears that the circles with center on the Schoch line and touching C(nc)
and D(nd) externally are also Archimedean circles,
and they are called Woo
circles.
Thomas Schoch and Peter Woo studied these circles, H. Okumura and M. Watanabe
made a generalization of the circles.
The Pappus chain is made by constructing
tangent Pappus circles in the arbelos, from right to left in the picture.
The centers of the circles lie on an ellipse.
The Bankoff circle B is the circle through
the cusp of the arbelos and the tangent points of the first
Pappus circle.
Leon Bankoff (19081997) was an American dentist and mathematician. He
became a prominent Los Angeles dentist with famous patients of the Beverly
HillsHollywood scene.
Given a triangle, put
three circles in it, so that each circle is tangent to the other two and to two
sides of the triangle. These
circles are the Malfatti circles.
These were for many years considered to provide a solutions for the socalled
Malfatti´s problem, later it appeared they are not.
Giovanni Francesco Malfatti (17311807) was an Italian
mathematician in Ferrara.
Four circles
Archimedes constructed the salinon as a
combination of four semicircles. The centers of the circles are lying on a line,
the two small circles are of equal size. Archimedes described the curve,
that is also called the saltcellar ^{4)},
in his Book of Lemmas.
More concentric circles can be found at a target for shooting or darting.
In the 16th century scientists as Giordano Bruno and John Dee looked at
the spiritual meaning of socalled hermetic forms, which often contained
circles. By
placing those circles in the right place a resonance with his celestial (pure) counterpart
could be reached. An example of these forms is  with four circle arcs  John Dee's magic hieroglyphic
^{5)}.
The circle
skating rink curve is formed by four circle arcs, one four each side, so
that the curve is tangent to two concentric circles (of different size).
The four arcs are from circles with the same size, but different center.
The curve is a simplification of the SaintHilaire
skating rink curve. The physicist
SaintHilaire was the first to calculate the profile of this curve.
An application of the profile has been found in engine
design. The Quasiturbine (or
Qurbine) is a rotating engine with a
rotor,
trapped inside an internal housing contour, which does not require a central shaft
or support. This concept brings the engine dead time to zero.
The method of operation allows the engine to be extremely flexible. The
developer claims that it is possible to use it for pneumatics, hydraulics,
steam, petrol, natural gas and hydrogen.
The precise formula is not yet available to me, it can be found in the patent
for this engine.
Five circles
A combination of five
circles we can see in the symbol of the car Audi  circles placed on one line  and in the
symbol of the Olympic games  the five continents, the circles now positioned in two
lines. Is this similarity the explanation for the sponsorship of Audi of the Dutch Olympic
team?
A circle placed around a cross is the symbol of the Kuklux klan ^{6)}
movement.
Infinite circles
An infinite sequence of tangent
circles for which the radius converges to zero, is called a Hawaiian
earring.
The radius can have forms like 1/n (see picture) or 1/n^{2} (n=1,2,
...).
The topology of this set and its generalizations has been studied by Eda
and Kawamura.
notes
1) Bi (Gr.) = two.
2) He wrote in the journal Leonardo, The Tiling Patterns of Sebastien Truchet
and the Topology of Structural Hierarchy, Leonardo 20, pp 373385 1987.
3) Memoir sur les Combinasions
4) Two explations of the word 'salinon' can be found:
 the word 'salinon' resembles the Greek word for a vessel for water,
what is the form of the curve, turned upside down
 the word salinon is a graecised form of the Latin word salinum, what is a saltcellar.
The lower curve resembles an ordinary type of saltcellar.
5) Davis 1981 p. 100.
6) Kuklux is derived from the Greek word for circle: kyklos.
