Approximating

ppl. a. [f. APPROXIMATE v. + -ING².] That approximates or brings near.

1774.  Goldsm., Nat. Hist., I. 12. Attraction: a sort of approximating influence, which all bodies … are found to possess.

1783.  Cowper, Corr. (1824), I. 293. The approximating powers of the telescope.