=The National Taxing Power.=--The lack of the power of Congress to levy taxes was, as we have seen, one of the chief weaknesses of the Articles of Confederation, voluntary contributions by the states being the chief source of revenue for the national government. When, therefore, the framers of the Constitution came to deal with this subject, they wisely provided that Congress should have power to levy and collect its own revenues. The power conferred is almost absolute, the only limitations being that no duties shall be levied upon exports; that excises and duties levied on imports shall be uniform throughout the United States, that is, they shall be the same in amount on a given article everywhere; and that where direct taxes are levied, they shall be apportioned among the states on the basis of population. =Forms of Federal Taxes.=--The two general forms of taxes recognized by the Constitution are direct and indirect taxes. The only direct taxes, in the sense of the Constitution of the United States, are poll taxes, and taxes on real or personal property, all of which are required to be apportioned among the states on the basis of population, whenever they are levied. On account of the obvious injustice of levying a tax on the states on the basis of population, inasmuch as there might easily be twice as much property in one state as in another having the same population, this method has fallen into general disuse. Indeed, it has been resorted to by Congress only five times in our history, and then only for very short periods in each case. It does not seem likely that this method of federal taxation will ever again be resorted to. Until recently the two principal sources of federal revenue have been duties on imports, and internal revenue, or excise taxes, on certain articles produced in this country. =Customs Duties.=--_Specific and Ad Valorem._--Customs duties are taxes levied on articles imported into the United States from abroad. They are of two kinds, specific and _ad valorem_. Specific duties are those levied on the articles according to their weight or measurement without regard to their value. Thus a duty of one and a half cents a pound on imported tin plate, or five cents a pound on dyestuffs, or ten cents a yard on silk would be specific. An _ad valorem_ tax is one levied with reference to the value of the article. Thus a duty of 50 per cent on the value of imported woolen goods is an example of an _ad valorem_ duty. Sometimes both forms of duty are levied on the same article.