Quantity of proof spirit = 4050 / 100 = 40·50

Or exactly 40-1/2 gallons.

The strength of absolute alcohol (sp. gr. ·7938) is estimated at 75-1/4%

over-proof. It therefore contains 175-1/4% of 'proof spirit,' whilst

proof spirit (sp. gr. ·91984) contains 57·06% of 'absolute alcohol,' both

being by measure or volume. Thus--

(meas. of alc. × 175-1/4) / 100 = equiv. meas. of pf. spt.