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For this reason non-rigid reference-bodies are used, which are as

a whole not only moving in any way whatsoever, but which also

suffer alterations in form ad lib. during their motion. Clocks,

for which the law of motion is of any kind, however irregular,

serve for the definition of time. We have to imagine each of

these clocks fixed at a point on the non-rigid reference-body.

These clocks satisfy only the one condition, that the "readings"

which are observed simultaneously on adjacent clocks (in space)

differ from each other by an indefinitely small amount. This

non-rigid reference-body, which might appropriately be termed a

"reference-mollusc", is in the main equivalent to a Gaussian

four-dimensional co-ordinate system chosen arbitrarily. That

which gives the "mollusc" a certain comprehensibility as compared

with the Gauss co-ordinate system is the (really unjustified)

formal retention of the separate existence of the