DOC. 42 SPECIAL AND
GENERAL RELATIVITY 253
The
System
of
Co-ordinates
7
tion,
but
also
to
everyday
life.
If
I analyse
the
place
specifi-
cation
"Trafalgar Square, London,"1 I arrive at
the
following
result.
The
earth
is
the
rigid
body to
which the
specification
of
place
refers;
"Trafalgar Square,
London,"
is
a
well-defined
point,
to
which
a name
has
been
assigned,
and with which the
event
coincides in
space.2
This
primitive
method of
place
specification
deals
only
with
places
on
the surface of
rigid
bodies,
and
is
dependent
on
the
existence of
points
on
this surface which
are
distinguishable
from each other.
But
we can
free
ourselves
from
both
of
these
limitations without
altering
the
nature
of
our
specification
of
position. If,
for instance,
a
cloud
is hovering
over
Trafalgar
Square,
then
we can
determine
its
position
relative
to
the
surface of the earth
by
erecting
a
pole perpendicularly
on
the
Square,
so
that it
reaches
the
cloud.
The
length
of the
pole
measured with
the
standard
measuring-rod,
combined
with
the
specification
of the
position
of the foot of the
pole, sup-
plies
us
with
a
complete place specification.
On the
basis
of
this
illustration,
we
are
able
to
see
the
manner
in
which
a
refinement of the
conception
of
position
has
been
developed.
(a)
We
imagine
the
rigid
body,
to
which the
place specifi-
cation
is referred, supplemented
in
such
a
manner
that the
object
whose
position
we
require
is
reached
by
the
completed
rigid body.
1
I
have
chosen this
as being more
familiar
to
the
English
reader than the "Potsdamer
Platz,
Berlin,"
which
is referred
to
in
the
original.
(R. W. L.)
2
It
is
not necessary
here
to investigate
further
the
significance
of
the
expression
"coincidence
in
space."
This
conception
is
sufficiently
obvious
to
ensure
that differences of
opinion are scarcely
likely to
arise
as
to
its applicability in practice.