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 space co-ordinates as opposed to the time co-ordinate. Every point on the mollusc is treated as a space-point, and every material point which is at rest relatively to it as at rest, so long as the mollusc is considered as
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The reference body.
