Leonhard Euler 
Letters of Euler on Different Subjects in Physics and Philosophy [PDF ebook] 
Addressed to a German Princess

Whilst the greatest effort has been made to ensure the quality of this text, due to the historical nature of this content, in some rare cases there may be minor issues with legibility. The fubjeets of thefe Letters, and the Author’s method of treating them, fcem to me much adapted to this purpofe. With the aflifiance of a very moderate apparatus, they might conduet youth of both fexes, with equal delight and emog lument, to avery competent knowledge of natural philofophy: very little pre vions elementary knowledge is necef fary to a profitable perufal of them, and that little may be very eafily acquired.