252 DOC. 42 SPECIAL AND GENERAL RELATIVITY
TWO
The
System
of
Co-ordinates
On
the
basis
of the
physical interpretation
of distance
which
has
been
indicated,
we
are
also in
a
position to
establish the distance between
two points
on a
rigid
body by
means
of
measurements.
For this
purpose
we
require
a
"distance"
(rod S)
which
is
to
be used
once
and
for
all,
and
which
we
employ
as a
standard
measure.
If, now,
A
and B
are
two points
on a
rigid
body,
we
can
construct
the line
joining
them
according
to
the
rules of
geometry;
then,
starting
from
A,
we can
mark off the distance S time after time until
we
reach
B.
The
number of these
operations required is
the numerical
measure
of the distance
A
B.
This
is
the
basis
of
all
measure-
ment
of
length.1
Every description
of
the
scene
of
an
event
or
of
the
position
of
an
object
in
space is
based
on
the
specification
of the
point
on
a
rigid
body (body
of
reference)
with which
that
event
or
object
coincides.
This
applies
not only to
scientific
descrip-
1
Here
we
have assumed that there
is nothing
left
over, i.e.
that the
measurement gives a
whole
number. This
difficulty is got
over by
the
use
of divided
measuring-rods,
the introduction of
which does
not
demand
any fundamentally
new
method.
6