SIDELIGHTS ON RELATIVITY

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is infinite in relation to practically-rigid bodies, assuming that
the laws of disposition for these bodies are given by Euclidean
geometry.

Another example of an infinite continuum is the plane. On a plane
surface we may lay squares of cardboard so that each side of any
square has the side of another square adjacent to it. The construction
is never finished; we can always go on laying squares--if their laws
of disposition correspond to those of plane figures of Euclidean
geometry. The plane is therefore infinite in relation to the
cardboard squares. Accordingly we say that the plane is an infinite
continuum of two dimensions, and space an infinite continuum of
three dimensions. What is here meant by the number of dimensions,
I think I may assume to be known.

Now we take an example of a two-dimensional continuum which is
finite, but unbounded. We imagine the surface of a large globe and