With a faint hum and rattle the moving racks crawled imperceptibly through the weeks and the recapitulated aeons to where, in the Decanting Room, the newly-unbottled babes uttered their first yell of horror and amazement.
Faint almost to imperceptibility, but appalling, she suddenly heard the grinding of his teeth.
But inexorably, every thirty seconds, the minute hand of the electric clock above his bed jumped forward with an almost imperceptible click.
There are no more uses of "imperceptible" in the book.
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...and he let the line slip down, down, down, unrolling off the first of the two reserve coils. As it went down, slipping lightly through the old man’s fingers, he still could feel the great weight, though the pressure of his thumb and finger were almost imperceptible.