In like manner when a spirit is said to be 11 u. p., or under-proof, it
means that 100 gal. of such spirit contains 11 gal. of water, and 89 gal.
of 'proof spirit.' By deducting the per-centage under-proof from 100, we
not only obtain the number of proof gal. contained in 100 gal. of such
spirit, but, as in the last case, a factor which multiplied by any number
of gal., and divided by 100, gives the exact number of 'proof gallons'
contained in any quantity of the given strength. Thus:--An ullage brandy
piece containing 45 gal. of spirit at 10 u. p., would have the proof value
of--
Per cent. u. p. of sample 10, }
subtracted from 100 } 90
No. of gall 45
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4050