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In practice, the rigid surfaces which constitute the system of

co-ordinates are generally not available; furthermore, the

magnitudes of the co-ordinates are not actually determined by

constructions with rigid rods, but by indirect means. If the

results of physics and astronomy are to maintain their clearness,

the physical meaning of specifications of position must always be

sought in accordance with the above considerations.[5]

[5] A refinement and modification of these views does not become

necessary until we come to deal with the general theory of

relativity, treated in the second part of this book.

We thus obtain the following result: Every description of events

in space involves the use of a rigid body to which such events

have to be referred. The resulting relationship takes for granted

that the laws of Euclidean geometry hold for "distances;" the

"distance" being represented physically by means of the

convention of two marks on a rigid body.

III.

SPACE AND TIME IN CLASSICAL MECHANICS