An extense must impull, an elapse must elopes,

Space and time...

Extense: to stretch out (extend) in space (and 'tense' relating to forms of time, ie past / future tense). Ex-tense (past tense?).

Impull: impel (to urge, move, propel, push). Pull: opposite of extend / extense.

Elapse: relates to the movement time; an elapse of many hours. The word also contains 'lap' which is a spatial (and circular - elipse?) reference.

Elopes: To run away with a lover, especially with the intention of getting married.

This sentence seems to be stating that if one extends in space, one cannot do it without time. Space and time are inherently interconnected and are therefore continuously eloping (Einstein's 'spacetime'). Relativity is invoked by the unification of push and pull in 'impull', where an extension is both a push and pull. Joyce is stating a Law of Nature ('must') - one that resonates clearly with the idea that if one rises one must fall. Giordano Bruno, who appears two lines previous ("Till Nolans go volants and Bruneyes come blue."), believed that every unity was made up of opposites.