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By making use of the following modification of this abstract

experiment, we recognise that there must also be cases in which

the experiment would be unsuccessful. We shall suppose that the

rods "expand" by in amount proportional to the increase of

temperature. We heat the central part of the marble slab, but not

the periphery, in which case two of our little rods can still be

brought into coincidence at every position on the table. But our

construction of squares must necessarily come into disorder

during the heating, because the little rods on the central region

of the table expand, whereas those on the outer part do not.