Their sole intention is to point out that, if a meagre naval force could produce so great an effect against a country _mainly agricultural_, a very powerful naval force, blockading effectively the principal ports of a _manufacturing country_, would have an effect so great that it can hardly be estimated.
It is plainly to be seen that the effect of a blockade against a purely commercial country by a modern navy would be incomparably greater now than it was fifty years ago, for two very important reasons.

One reason is that the progress of modern engineering has made navies very much more powerful than they were fifty years ago; and the other reason is that the same cause has made countries very much more vulnerable to blockade, because it has made so many millions of people dependent upon manufacturing industries and the export of manufactured things, and forced them to live an artificial life.

While the United States, for instance, does not depend for its daily bread on the regular coming of wheat from over the sea, yet millions of its people do depend, though indirectly, upon the money from the export of manufactured things; for with countries, as with people, habits are formed both of system and of mode of life, which it is dangerous suddenly to break; so that a country soon becomes as dependent upon outside commerce as a man does upon outside air, and a people suddenly deprived of a vigorous outside commerce would seem to be smothered almost like a man deprived of outside air.
A rough idea of the possible effect of a blockade of our coast may be gathered from the fact that our exports last year were valued at more than $2,000,000,000; which means that goods to this amount were sold, for which a return was received, either in money or its equivalent, most of it, ultimately, as wages for labor.

Of course no blockade could stop all of this; but it does not seem impossible that it could stop half of it, if our fleet were destroyed by the enemy.

Supposing that this half were divided equally among all the people in the United States, it would mean that each man, woman, and child would lose about $10 in one year.