Unpublished letter, 'Concerning the invention of sempiternal motion upon Earth' from William Herschel de Griesbach to Sir John Lubbock De Griesbach explains that in the summer of 1820 he first began to entertain the idea that the invention of sempiternal motion upon Earth was possible. He describes sempiternal motion as 'having a beginning, but no end', therefore preferring this term to 'perpetual motion'. He writes on the importance of the centripetal and centrifugal force on the Earth. Subject: Astronomy Written by de Griesbach in Vegesack, Bremen [Germany]. 30 January 1843