19: Sudden Changes in Channels of Trade
Less advanced economies generally enjoy greater stability: when the sole product is the necessities of life, people are generally predictable in their consumption - a man will not consume significantly more food in one month or year than the last, nor will he consume significantly less, but requires a certain amount that does not change individually, nor in aggregate except as the population grows or diminishes, which is generally a gradual process in time of peace.
A more advanced economy, in which much productive activity is diverted from necessities to luxuries, is more subject to fluctuations in consumptions with the prosperity and tastes of its people. It is also the manufactured goods, of a high value per unit of bulk, that are more frequently imported or exported to meet the demands of consumers, placing domestic producers in more direct competition with imported goods.
As such, taxes on manufactures can destroy the competitive advantage of domestic manufacturers over foreign competition, and a change in taxation can cause market upheavals as producers must shift their capital from production that is no longer competitive to other pursuits.
Such upheavals will not be detrimental merely to the market in which a tax was imposed, but to others whose commodities were imported or exported; and they are disruptive to trade in general, given that nothing can be brought into a country without sending out something else in trade.
As such, the chain reaction is that domestic goods become more expensive than foreign due to taxation, consumers in the local market buy more imports, foreign exporters must produce more of the good in question, domestic producers must shift to producing something else, and more or different domestic goods (or wealth) is sent overseas to pay for imported goods.
It's also noted that the commencement of war in time of peace, and restoration of peace after a time of war, are also disruptive to an economy. Production shifts to goods to support the military effort, or back to fill civilian needs; channels of trade are interrupted or restored. Aside of the destruction of population, the facilities and skills to produce goods are wholly lost during time of war, and may be difficult to restore when peace resumes.
And it is generally the pattern that domestic economy is degraded by war, since so much of the domestic production is expended in warfare than consumed or traded for other goods; whereas the return to peace brings a rapid shift back to consumption and trade.
There is a stray remark about a "revulsion of trade," which is not entirely clear, but my sense is Ricardo is referring to trade sanctions that, even in time of peace, harm both sides.
Economic disruption is also more distressing to a wealthy and advanced economy, as greater capitals are expended on improvements that are not easily replaced. Raze every field in a farming community and crops can be replanted the following season. Raze bridges, roads, factories, and such, and the economy will take years to recover.
It is likewise more difficult for an advanced economy to react nimbly to changes of lesser drama: a field sown with one crop may be sown with a different one in the next season; but a factory with machines to produce one good cannot as quickly be retooled to produce something entirely different. Likewise, retraining a wheat farmer to grow barley is easier than retraining a weaver to become a blacksmith.
Returning to war, its detrimental effects extend beyond the nations involved in a conflict, but to all who trade with them, who are denied imported goods for domestic markets and have lost foreign markets for domestic goods.
Ricardo suggest that "the best policy of a state would be to lay a tax ... on the importation of foreign corn for a limited number of years." This would favor domestic producers for a time, ensuring restoration of basic necessities and a gradual return of capital to more advanced goods. He goes on in some detail how this might benefit the economy of the nation in general until stability can be restored.
(EN: I find the argument unconvincing, the examples contrived, and the practicality questionable. Much of the worst and most destructive laws in times of stability are vestiges of those passed in times of upheaval that were not repealed after the emergency situation abated. I expect he means well, from an economic perspective, but likely forgets the tendencies of politics to accumulate and defend the power of state.)