18.01: ha-ha: "A sunk fence; a fence, wall, or ditch, not visible till one is close upon it" (W2); "A boundary to a garden, pleasure-ground, or park, of such a kind as not to interrupt the view from within, and not to be seen till closely approached" (OED). A ha-ha features prominently in the description of Mansfield Park's Sotherton Court, whose topography Nabokov liked to impress on his students' minds with the help of a map that shows the ha-ha (LL 31). A pun, of course, on "ha-ha" as laughter, stressing the absurdity of transferring Russia across the ocean. The “ha” lost in the transformation of “sleight of hand” into “sleight of land” has been doubly repaid.

As I pointed out in several articles (Russian and English), "the ha-ha of a doubled ocean" hints at the absence of the Bering Strait on a globe which drove mad the geography teacher in Ilf and Petrov's "The Golden Calf" (chapter XVI: "Jahrbuch fuer Psychoanalytik").