Letters to a German princess on various topics in physics and philosophy
The Euler Archive also has a PDF of the 1823 English version, Brewster translation.
Euler expresses his views on a wide range of scientific topics, as he also does in E344 and E417.
Original Source Citation
St. Petersburg: Imperial Academy of Sciences, Volume 1, pp. 1-314.
Opera Omnia Citation
Series 3, Volume 11.