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Webster 1913 Edition

adv.

In proportion; in due degree; adapted relatively;

as, all parts of the building are

proportionally

large

Sir I. Newton.

adv.

In proportion; in due degree; with suitable comparative relation; as all parts of a building being proportionally large.

proportionally ‎(comparative more proportionally, superlative most proportionally)

in proportion

proportionally in The Century Dictionary, The Century Co., New York, 1911
proportionally in Webster’s Revised Unabridged Dictionary, G. & C. Merriam, 1913