Before long there came the sound of hoofs, at first hardly more than a tremor of the ground perceptible only to Aragorn as he lay upon the grass, then growing steadily louder and clearer to a quick beat.
Hardly perceptibly his head moved to the left and the right, and first one eye and then the other opened a slit.
At the same moment the stirring of the air became perceptible to them; and it grew very cold.
There are no more uses of "perceptible" in the book.
Show samples from other sources
The car makes a barely perceptible clicking sound at high speeds.