rationably

From rationable +‎ -ly.^[1] Piecewise doublet of reasonably.

rationably (comparative more rationably, superlative most rationably)

1. (now rare) Rationally, reasonably. [from 1540]^[1]
   • 1665, R[obert] Hooke, “Observ[ation] X. Of Metalline, and Other Real Colours.”, in Micrographia: Or Some Physiological Descriptions of Minute Bodies Made by Magnifying Glasses. […], London: […] Jo[hn] Martyn, and Ja[mes] Allestry, printers to the Royal Society, […], →OCLC, page 69:

     By this Hypotheſis there is no one experiment of colour that I have yet met with, but may be, I conceive, very rationably ſolv'd, and perhaps, had I time to examine ſeveral particulars requiſite to the demonſtration of it, I might prove it more than probable, […]