If many simultaneously and variously directed forces act on a given body, the direction of its motion cannot coincide with any one of those forces, but will always be a mean—what in mechanics is represented by the diagonal of a parallelogram of forces.
There are no more uses of "diagonal" in the book.
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The artist used diagonal lines to create tension.
A pink scar started under his right eye, hit the corner of his nose, and ran diagonally across his upper lip.