Lights were already shining through its windows, and Archer, as the carriage stopped, caught a glimpse of his father-in-law, exactly as he had pictured him, pacing the drawing-room, watch in hand and wearing the pained expression that he had long since found to be much more efficacious than anger.
There are no more uses of "efficacious" in the book.
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The drug is safe and efficacious.
He would weep for hours together, and I verily believe that to the very end this spoiled child of life thought his weak tears in some way efficacious.