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This library is built in the open.
If you spot an error, have a suggestion, or just want to say hello — we’d love to hear from you.

The book is structured in such a way that each part forms a complete whole, which can, if necessary, be studied separately.
I have made every effort to be as clear as possible; the public, who have so kindly given a warm welcome to my previous works, will surely forgive the imperfections inherent in a task of this nature.
Paris, March 2, 1889