For the purpose of estimating the duties on importations of grain, the number of bushels shall be ascertained by weight, instead of by measuring; and sixty pounds of wheat, fifty-six pounds of corn, fifty-six pounds of rye, forty-eight pounds of barley, thirty-two pounds of oats, sixty pounds of peas, and forty-two pounds of buckwheat, avoirdupois weight, shall respectively be estimated as a bushel.