Vessels, for purposes of taxation, have, generally speaking, a situs at their home ports, that is, where registered, irrespective of where they are doing business. Where, however, it appears that a boat is permanently located in another State and doing business there, it may be taxed there.10

In determining for purposes of taxation the amount of rolling stock of an interstate carrier, it has been held that a State may ascertain the average number of cars continuously employed in the State, though no particular car may in fact be kept permanently employed in the State.11

"When valuing the property of carrier companies whose property extends over several States, each State is permitted to tax the amount of property within its own limits and to give to that amount a value bearing the same proportion to the value of the entire property of the company as the length of railway or telegraph or telephone line bears to the total length of the carrier system which is assessed. This, the court declared proper in W. U. Telegraph Co. v. Mass.12 and again in Pullman Palace Car Co. v. Pennsylvania,13 in the latter case saying that the method "was a just and equitable method of assessment, and, if it were adopted by all the States through which these cars ran, the company would be assessed upon the whole of its capital stock and no more."

The court have, however, at times pointed out that this method of assessment is after all but a convenient one applicable in some cases, and that it is not to be erected into an absolute principle; for it might not be acceptable in those cases where it would work obvious injustice. An example of this would be where a railroad company has a large mileage in one State, but over land where construction expenses had been very inexpensive, and where terminal facilities were few and not costly, while in another State its mileage is small, but of expensive construction, and its terminal facilities elaborate and costly.14

10 Cf. Judson, Taxation, § 139.

11 Pullman Palace Car Co. v. Penn., 141 V. S. 18: 11 Sup. Ct. Rep. 876: 35 L. ed. 613; Union Refrigerator Transit Co. v. Ky.. 199 V. S. 194; 26 Sup. Ct. Rep. 36: 50 L. ed. 150; American Refrigerator Transit Co. v. Hall, 174 70; 19 Sup. Ct. Pep. 599; 43 L. ed. 899.

12 125 U. S. 530; 8 Sup. Ct. Rep. 961 : 31 L. ed. 790.

13 141 U. S. 18; 11 Sup. Ct. Rep. 876; 35 L. ed. 613.

The chief constitutional objection to this method of valuation has been that the value of the property is based in very great degree upon its use as an instrument of interstate commerce, and that, therefore, a tax assessed upon this value is, in effect, a tax upon that commerce. This contention was urged with especial force, but without success, in the case of Adams Express Co. v. Ohio.15 In this case the state statute required the board of assessors" to proceed to ascertain and assess the value of the property of express, telegraph and telephone companies in Ohio, and in determining the value of the property of said companies in this State to be taxed within the State and assessed as herein provided, said board shall be guided by the value of said property as determined by the value of the entire capital stock of said companies, and such other evidence and rules as will enable said board to arrive at the true value in money of the entire property of said companies within the State of Ohio, in the proportion which the same bears to the entire property of said companies, as determined by the value of the capital stock thereof, and the other evidence and rules as aforesaid." 16

14 Cf. Judson, Taxation, § 261.

15 165 U. S. 194; 17 Sup. Ct. Rep. 305; 41 L. ed. 683.

16 In behalf of the express companies it was contended that the law sought to tax property beyond the territorial jurisdiction of the State, and that it imposed a burden on interstate commerce. The court, however, speaking through Chief Justice Fuller, said: "Although the transportation of the subjects of interstate commerce, or the receipts received therefrom, or the occupation or business of carrying it on, cannot be directly subjected to state taxation, yet property belonging to corporations or companies engaged in such commerce may be; and, whatever the particular form of the exaction, if it is essentially only property taxation, it will not be considered as falling within the inhibition of the Constitution. Corporations and companies engaged in interstate commerce should bear their proper proportion of the burdens of the governments under whose protection they conduct their operations, and taxation on property, collectible by the ordinary means, does not affect interstate commerce, otherwise than incidentally, as all business is the theory that the whole property of the company might be regarded as a unit plant, with a unit value, a proportionate part of which value might be reached by the state authorities on the basis indicated."

In the Express Company case was thus established what is known as the "unit of use" rule, according to which the property of a company may be determined as a unity, if used as a single system, and that its value may be assessed for purposes of taxation at the value which, as such a unity, it has in use, namely, the net profits which it produces, and irrespective of what may be the value of the tangible property which is owned or employed; and affected by the necessity of contributing to the support of government. As to railroad, telegraph, and sleeping-car companies engaged in interstate commerce, it has been often held by this court that their property in the several States through which their lines or business extended might be valued as a unit for the purposes of taxation taking into consideration the uses to which it was put, and all the elements making up aggregate value, and that a proportion of the whole fairly and properly ascertained might be taxed by the particular State without violating any federal restriction. The valuation was thus not confined to the wires, poles and instruments of the telegraph company, or the roadbed, ties, rails, and spikes of the railroad company, or the cars of the sleeping company, but included the proportionate part of the value resulting from the combination of the means by which the business was carried on,- value existing to an appreciable extent throughout the entire main of operation. And it has been decided that a proper mode of ascertaining the assessable value of so much of the whole property as is situated in a particular State is, in the case of railroads, to take that part of the value of the entire road which is measured by the proportion of its length therein to the length of the whole (Railway Co. v. Backus, 154 U. S. 439; 14 Sup. Ct. Rep. 1122; 38 L. ed. 1041), or taking as the basis of assessment such proportion of the capital stock of a sleeping-car company as the number of miles of railroad over which its cars are run in a par-titular State bears to the whole number of miles traversed by them in that and other States (Pullman's Palace Car Co. v. Pennsylvania, 141 U. S. 18; 11 Sup. Ct. Rep. 876; 35 L. ed. 613), or such a proportion of the whole value of the capital stock of a telegraph company as the length of its lines within a State bears to the length of its lines everywhere, deducting a sum equal to the value of its real estate and machinery subject to local taxation within the State (W. U. Tel. Co. v. Taggart, 163 U. S. 1; 16 Sup. Ct. Rep. 1064; 41 L. ed. 49). Doubtless there is a distinction between the property of railroad and telegraph companies and that of express companies. The physical unity existing in the former is lacking in the latter; but there i- the same unity in the use of the entire property for the specific purpose, and there are the same elements of value arising from such use. The cars of the Pullman Company did not constitute a physical unity, and their value as separate ears did not bear a direct relation to the valuation which was sustained in that case. The cars were moved by railway carriers under contract, and the taxation of the corporation in Pennsylvania was sustained on that where this system extends into two or more States each State may, for purposes of taxation, consider as within its borders, an amount of property proportioned to the whole, as the amount of business done within the State is proportioned to total amount of business done.17