More recently, in his _Harmony of Interests_, Mr. Carey has published a full examination of the great question of commercial policy, with a view to show that protection, as it exists in this country, is the true and _only_ road to free trade. He has brought to the illustration of this important doctrine a mass of facts, greater, probably, than was ever before displayed in support of any position in political economy. It commences with an examination of our whole commercial policy for the last thirty years, and shows the effect of protection in increasing the sum of production and consumption, the means of transportation, internal and external, and the influx of population from abroad, always an evidence of the increased productiveness of labor. In this work it is shown conclusively, that shipping grows with protection, because protection tends to promote immigration, or the import of men, the most valuable of commodities, and thus to diminish the cost of _sending_ to market the less valuable ones, grain, tobacco, and cotton. The question is examined in every point of view--material, moral, intellectual, and political; and the result arrived at is, "that between the interests of the treasury and the people, the farmer, planter, manufacturer and merchant, the great and little trader and the ship-owner, the slave and his master, the land-owners and laborers of the Union and the world, the free-trader and the advocate of protection, there is perfect harmony of interests, and that the way to the establishment of universal peace and universal free trade, is to be found in the adoption of measures tending to the destruction of the _monopoly of machinery_, and the location of the loom and the anvil in the vicinity of the plough and the harrow."