. . . "This last service was exacted in England, in consequence of what was called the Royal prerogative of purveyance and pre-emption,???a prerogative which was understood ???to give the Crown a right of buying up provisions and other necessaries, by the intervention of the King???s purveyors, for the use of his royal household, at an appraised valuation, in preference to all others, and even without co" .