delivered the opinion of the court.

The principal question presented by the record of' this, case is,, whether a municipal corporation of a State, having by the law of its organization an exclusive right to make wharves, collect wharfage, and regulate wharfage rates, can, consistently with the Constitution of the. United States* charge and collect wharfage proportioned to the tonnage.of the vessels from the' owners of enrolled and licensed steamboats mooring and landing at' the wharves constructed cm- the banks of a navigable river.

The city of Keokuk is such a corporation, existing by virtue of a special charter granted by the legislature of Iowa. To determine whether the charge prescribed by the ordinance in question is a duty of tonnage, within the meaning of the Constitution, it is necessary to observe carefully its object and essence. If the charge is clearly a duty, a tax, or burden, which in its essence is a contribution claimed for the privilege of entering the port of Keokuk, or remaining in it, or departing from it, imposed, as .it is, by authority of .the State, and measured by the capacity of the vessel, it is doubtless embraced by the constitutional prohibition of such a duty. But a charge for services rendered or for conveniences provided is in no sense a tax pr a duty. ' It is not a hindrance or impediment to free navigation 'The prohibition to the State against the iinposi *85 tion of a duty of- tonnage was designed to. guard against local hindrances* to trade and carriage by vessels, not to relieve them from liability to claims for assistance rendered and facilities furnished for trade and commerce. It is a tax ór a duty, that is prohibited: something.imposed by virtue of sovereignty, not claimed in right of proprietorship. Wharfage' is of the latter character. Providing a wharf to which vessels may make fast, or at which they may conveniently load or unload, is rendering them a service. The character of the service is the same whether the wharf is built and offered for use by a State, a municipal corporation, or a private individual; and, when compensation is demanded for the, use of the wharf, the demand is an assertion, not of sovereignty, but of a right of property. A passing vessel may use the wharf or not, at its election, and thus may incur liability for wharfage or not, at the choice of the master or owner. No one would claim that a demand of compensation for the use of a dry-dock for repairing a vessel, or a demand for towage in a harbor, would be a demand of a tonnage tax, no matter whether the dock was the property of a private individual or of a State, and no matter whether proportioned or not to the size or tonnage of the vessel. There is no essential difference between such a demanu and one for the use of a wharf. It has always been held that wharfage dues may be exacted; and it is believed that they have been collected in ports where the wharves have belonged to the State or a municipal corporation ever since the adoption of the Constitution. In Cannon v. New Orleans, 20 Wall. 577, this court, while holding an ordinance void that fixed dues upon steamboats which should moor or land in any part of the port of New Orleans, measured by the number of tons of the boats, because substantially a tax for the privilege of stopping in the port, and, therefore, a. duty or tonnage, carefully guarded the right to exact wharfage. The language of the court rvas: “ In saying this (namely, denying the validity of the ordinance then before it), -we do not understand that this principle interposes any hindrance to the recovery from any vessel landing at a wharf or pier owned by an individual, or by a municipal or other corporation, a just compensation for the use of such property. It is a doctrine too well settled, and a practice too *86 common and too essential to the interests of commerce and navigation, to admit of a doubt, that for the use of such structures, erected by individual enterprise and recognized everywhere as private property, a reasonable compensation can be exacted.. And it may be safely admitted, also, that it is within the power of the State to regulate this compensation, so as to prevent extortion, a power which is often very properly delegated to the local municipal authority. Nor do we see any reason why, when a city ,or other municipality is the owner of such structures, built by its own money, to assist vessels landing within its limits in the pursuit of their business, the city should not be allowed to exact and receive this reasonable compensation, as'well as individuals.” .

No doubt, neither a State nor a municipal corporation can be permitted to impose a tax upon tonnage under cover of laws or ordinances ostensibly passed to collect wharfage. This has sometimes bqen attempted, but the ordinances will always be-carefully scrutinized. In Cannon v. New Orleans, the ordinance was held invalid, not because the charge was for wharf-age, nor éven because it was proportioned to the tonnage of the vessels, but because the charge was not for wharfage or any service rendered. It was for stopping in the harbor, though no wharf was used. Such, also, was North-western Packet Co. v. St. Paul, 3 Dill. 454. So, in Steamship Company v. Port Wardens, 6 Wall. 31, the statute held void imposed a tax upon every ship entering the port. This was held to be alike a regulation of commerce and a duty of tonnage. It was a sovereign exaction, not a charge for compensation. Of the same character was the tax held prohibited in Peete v. Morgan, 19 id. 581.

It is insisted, however, on behalf of the plaintiffs in error, that the charge prescribed by the ordinance must be considered as an imposition of a duty of tonnage, because it, is regulated by and proportioned to the number of tons of the vessels using the wharf; and the argument is attempted to be supported by the ruling of this court in State Tonnage Tax Cases, 12 Wall. 204. But this is a misconception of those cases. The statute of Alabama declared invalid was not a provision to secure or regulate compensation for wharfage, or for any services rendered *87 to the vessels taxed. It imposed a tax “upon all steamboats, vessels, and other water-crafts plying in the navigable waters of the State,” to be levied “ at the rate of- one dollar per ton of the registered tonnage thereof.” It did not tax the boats as property in proportion to their value, but according to their capacity, or, as was said, “ solely and exclusively on the basis of their cubical contents, as' ascertained by the rules of admeasurement -and computation prescribed by Congress.” It was the nature of the tax or duty, coupled with the mode of assessing it, which made the law a violation of the Constitution. As stated, the vessels taxed were such as were plying in the navigable waters- of the State. If not plying.'in those waters, they were not taxed. The tax was, therefore, an impediment to navigation in those waters, which led the court to say that it was as instruments of commerce and not as property the vessels were required to contribute to the revenues of the State. The fact that the-tax was proportioned to the tonnage of the vessels taxed was relied upon only as supporting the conclusion that they were not taxed as property, but as instruments of commerce; and the court, in view of all these considerations, remarked, “ Beyond all question, the act is an act to raise revenue without any corresponding or- equivalent benefit- or advantage to the vessel's taxed or to the ship-owners, and consequently it is not to be upheld by virtue of the rules applied in the construction of -laws regulating pilot dues and-port charges.” Nothing in these eases justifies the -assertion that either wharfage or port charges are duties of tonnage, merely because they are proportioned to the actual tonnage or- cubical capacity of vessels: It would be a strange -misconception of the purpose of the framers of the Constitution were its provisions thus understood. What was ’ intended by" the provisions of the second clause of the tenth section of the first article was to protect the freedom of commerce, and nothing more. The prohibition of a duty of tonnage should, therefore, be-construed so as to carry out that intent. A-mere adherence to the letter, without reference to the. spirit and purpose, may in this case, mislead, as itr has'' misled in other cases.' It cannot be thought the framers of the Constitution, when they drafted the prohibition, had in mind charges for services rendered or for conveniences Furnished to *88 vessels in port, which are facilities to commerce rather than hindrances to its freedom; and, if such charges were not in mind, the mode of ascertaining their reasonable amount could not have been. In Cooley v. The Board of Port Wardens, 12 How. 299, this court recognized a clear distinction between wharfage and duties on imports or exports, or duties on tonnage. Referring to the second paragraph of sect. 10, art. 1, of the Constitution, Curtis, J., speaking for the court, said: “ This provision of the Constitution was intended to operate upon subjects actually existing and' well understood when the Constitution was formed. . Imposts, and duties on imports, exports, and tonnage, were.then known to the commerce of the civilized world to be as distinct from fees and charges for pilotage, and from the penalties by which commercial States enforced their laws, as they were from charges for wharfage or towage, or any other local port charges for services rendered to vessels or cargoes, and to declare that such pilot fees or penalties are embraced within the words imposts, or duties on imports, exports, or tonnage, would be to confound things essentially different, and which must have been known to be actually different by those who used this language. ... It is. the thing and not the name that is to be considered.”

For these reasons, we hold that the ordinance' cannot be considered as imposing a duty of tonnage, and what we have said is sufficient to show that most of the other objections of the plaintiffs in error to its validity have no substantial foundation. It is iii no sense a regulation of commerce between the States,, nor does, it impose- duties upon vessels bound to or from one State to another, nor compel entry or clearance in the port of Keokuk; n.or is it contrary to the compact contained in the ordinance of 1787, since it levies no tax for the navigatien of the river.;, nor is it in conflict with the act of Congress respecting the enrolment and license of vessels for the coasting trader All these objections rest on the mistaken assumption that port charges, and especially wharfage, are taxes, duties,.and restraints of commerce.

In nothing that we .have said do we mean to be understood as affirming that a city can, by ordinance or otherwise, charge or collect wharfage for merely entering its port, or stopping *89 therein, or for the use of that which is not a'tv barf, but merely the natural and unimproved shore of a navigable river. Such a question does not arise in this case. The record shows that the wharfage charged to these plaintiffs in error was for the use of a wharf, built, paved, and improved by the city at large expense. So far as the ordinance- imposes and regulates such a charge, it is not obnoxious to the accusation that it is-in conflict with the Constitution. A different question-would be presented had the steamboats landed at the bank of the river where no wharf had been constructed or' improvement made to afford facilities for receiving or discharging cargoes. We adhere to all that was decided in Cannon v. New Orleans. In that case, the city ordinance imposed what were called “ levee dues ” on all steamboats that should mooi; or land in any part of the harbor of New Orleans. It was subsequently-amended by the substitution of the words “ levee and wharf-age dues” for “levee- dues;” but, even as amended, it did not profess to demand wharfage. The plaintiff filed a petition for.an injunction' against the collection of the dues prescribed by it, and for the. recovery of those he had been compelled to pay. It did hot appear that he had -éver made use of any wharf or improved levee; and what we decided was, that the city could not impose a.charge for merely stopping in the harbor. The case in hand is different. The ordinance •of Keokuk has imposed no charge upon these plaintiffs'which it was beyond the power of the city to impose. To the extent to which they are affected Ty it there is no valid objection to it. Statutes that are constitutional in part only, will be upheld ■ so far as they are not in conflict with the Constitution, provided the allowed and prohibited parts are severable; We think a. severance is .possible in this case.. It may be conceded the ordinance is too broad, and that some of its provisions are unwarranted. When those provisions are attempted to be enforced, a different question, may be presented.

Judgment affirmed