Definition of "changeful"

When we will tempt the frailty of our powers, presuming on their changeful potency.

c. 1602, William Shakespeare, “The Tragedie of Troylus and Cressida”, in Mr. William Shakespeares Comedies, Histories, & Tragedies […] (First Folio), London: […] Isaac Iaggard, and Ed[ward] Blount, published 1623, [Act IV, scene iv], line 99

So, deprived of one leg, and the strange ship of course being altogether unsupplied with the kindly invention, Ahab now found himself abjectly reduced to a clumsy landsman again; hopelessly eyeing the uncertain changeful height he could hardly hope to attain.

1851 November 14, Herman Melville, “Leg and Arm. The Pequod, of Nantucket, Meets the Samuel Enderby, of London.”, in Moby-Dick; or, The Whale, 1st American edition, New York, N.Y.: Harper & Brothers; London: Richard Bentley, page 486