In English law. A duty Imposed by parliament upon merchandise exported and imported, according to a certain rate upon every ton. Brown. In American law. A tax laid upon vessels according to their tonnage or cubical capacity. A tonnage duty is a duty imposed on vessels in proportion to their capacity. The vital principle of a tonnage duty is that it is imposed, whatever the subject, solely according to the rule of weight either as to the capacity to carry or the actual weight of the thing itself. In man S. S. Co. v. Tinker, 94 U. 8. 238, 24 W Ed. 118. The term “tonnage duty,” as used in the constitutional prohibition upon state laws imposing tonnage duties, describes a duty proportioned to the tonnage of the vessel a certain rate on each ton. But it is not to be taken in this restricted sense in the constitutional provision. The general prohibition upon the states against levying duties on imports or exports would have been ineffectual if it had not been extended to duties on the ships which serve as the .vehicles of conveyance. The prohibition extends to any duty on the ship, whether a fixed sum upon its whole tonnage or a sum to be ascertained by comparing the amount of tonnage with the rate of duty. Southern S. S. Co. v. New Orleans,. 0 Wall. 31. 18 L. Ed. 749. A tonnage tax is denned to be a duty levied on a vessel according to the tonnage or capacity. It is a tax upon the boat as an instrument of navigation, and not a .tax upon the property of a citizen of the state. The North, Cape, 0 Biss. 505. Fed. Cas. No. 10,316.