Letters to a German princess on various topics in physics and philosophy
The Euler Archive also has a PDF of Brewster's 1823 English translation.
Euler expresses his views on a wide range of scientific topics, as he also does in E343 and E344.
Original Source Citation
St. Petersburg: Imperial Academy of Sciences, Volume 3, pp. 1-404.
Opera Omnia Citation
Series 3, Volume 11, pp.1-312.