There would be no advantage, therefore, in imposing duties of this kind, with a view to gain by them, in the manner which has been pointed out. But so long as any other kind of taxes on commodities are retained, as a source of revenue, these may often be as unobjectionable as the rest. It is evident, moreover, that considerations of reciprocity, which are quite unessential when the matter in debate is a protecting duty, are of material importance when the repeal of duties of this other description is discussed. A country cannot be expected to renounce the power of taxing foreigners, unless foreigners will in return practise towards itself the same forbearance. The only mode in which a country can save itself from being a loser by the duties imposed by other countries on its commodities, is to impose corresponding duties on theirs. Only it must take care that these duties be not so high as to exceed all that remains of the advantage of the trade, and put an end to importation altogether; causing the article to be either produced at home, or imported from another and a dearer market. It is not necessary to apply the principles which we have stated to the case of bounties on exportation or importation. The application is easy, and the conclusions present nothing of particular interest or importance. 6. Any cause which alters the exports or imports from one country into another, alters the division of the advantage of interchange between those two countries. Suppose the discovery of a new process, by which some article of export, or some article not previously exported, can be produced so cheap as to occasion a great demand for it in other countries. This of course produces a great influx of money from other countries, and lowers the prices of all articles imported from them, until the increase of importation produced by this cause has restored the equilibrium. Thus, the country which acquires a new article of export gets its imports cheaper. This is not a case of mere alteration in the division of the advantage; it is a new advantage created by the discovery. But suppose that the invention, to which the nation is indebted for this increase of the return to its industry, comes into use also in the other country, and that the process is one which can be as perfectly and as cheaply performed in the one country as in the other. The new exportation will cease; trade will revert to its old channels, the money which flowed in will again flow out, and the country which invented the process will lose that increase of its gain by trade, which it had derived from the discovery. Now the exportation of machinery comes within the case which we have just described.