At the outset the Court appears to have thought that it could solve all difficulties by the simple device of falling back on Marshall's opinion in Brown _v._ Maryland;[653] and on the same day that it set aside Pennsylvania's freight tax by appeal to that transcendent precedent, it sustained, by reference to the same authority, a Pennsylvania tax on the gross receipts of all railroads chartered by it, the theory being that such receipts had, by tax time, become "part of the mass of property of the State."[654] This precedent stood fourteen years, being at last superseded by a ruling in which substantially the same tax was held void as to a Pennsylvania chartered steamship company.[655] A year later the Court sustained Massachusetts in levying a tax on Western Union, a New York corporation, on account of property owned and used by it in the State, taking as the basis of the assessment such proportion of the value of its capital stock as the length of its lines within the State bore to their entire length throughout the country.[656] The tax was characterized by the Court as an attempt by Massachusetts "to ascertain the just amount which any corporation engaged in business within its limits shall pay as a contribution to the support of its government upon the amount and value of the capital so employed by it therein."[657] And drawing on certain decisions in which it had sought to limit the principle of tax exemption as applied in the case of railroads chartered by the United States, it expressed concern that "the necessary powers of the States" should not be destroyed or "their efficient exercise" be prevented.[658] Three years later Pennsylvania, still in quest of revenue, was sustained in applying the Massachusetts idea to Pullman's Palace Car Company, a "foreign" corporation.[659] Pointing to the fact that the company had at all times substantially the same number of cars within the State and continuously and constantly used there a portion of its property, the Court commended the State for taking "as a basis of assessment such proportion of the capital stock of the company as the number of miles over which it ran cars within the State bore to the whole number of miles, in that and other States, * * *" This, said the Court, was "a just and equitable method of assessment;" one which, "if it were adopted by all the States through which these cars ran, the company would be assessed upon the whole value of its capital stock, and no more."[660] THE UNIT RULE