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The observer performs experiments on his circular disc with

clocks and measuring-rods. In doing so, it is his intention to

arrive at exact definitions for the signification of time- and

space-data with reference to the circular disc K′, these

definitions being based on his observations. What will be his

experience in this enterprise?

To start with, he places one of two identically constructed

clocks at the centre of the circular disc, and the other on the

edge of the disc, so that they are at rest relative to it. We now

ask ourselves whether both clocks go at the same rate from the

standpoint of the non-rotating Galileian reference-body K. As

judged from this body, the clock at the centre of the disc has no

velocity, whereas the clock at the edge of the disc is in motion

relative to K in consequence of the rotation. According to a

result obtained in Section XII, it follows that the latter clock

goes at a rate permanently slower than that of the clock at the

centre of the circular disc, i.e. as observed from K. It is

obvious that the same effect would be noted by an observer whom

we will imagine sitting alongside his clock at the centre of the

circular disc.