Berlin, 31 July 1916

An explanation of how to locate 'the field of a rotating ring in the vicinity of the axis'.

'As a first approximation, the field is easily provided by direction integration of the field equations. The second approximation is provided by the field equations in a vacuum ... The first approximation gives the Coriolis forces, the second the centrifugal forces. It is evident from the general covariance of the equations that the latter can be calculated precisely, so that an actual calculation has no further interest. This interest is only present if one does not know whether rotation-transformations are "permitted", i.e. if one is not sure of the properties of transformation of the equations, a stage which has, thank god, been decisively surmounted'. Einstein is pleased to hear that Mileva [Einstein's first wife, from whom he is separated] is feeling better – does Besso know what she was suffering from? He goes on to explain why he called Besso 'Uncle Toby' in a previous letter – the reference was to Laurence Sterne's novel Tristram Shandy, as they used to compare themselves to the characters of the novel when they were together in Zurich.