This is evidently very elastic.

Many can remember that during the Civil War, after a night attack on the United States fleet off Charleston, the Confederates next morning sent out a steamer with some foreign consuls on board, who so far satisfied themselves that no blockading vessel was in sight that they issued a declaration to that effect.

On the strength of this declaration some Southern authorities claimed that the blockade was technically broken, and could not be technically re-established without a new notification.

Is it necessary, to constitute a real danger to blockade-runners, that the blockading fleet should be in sight?
Half a dozen fast steamers, cruising twenty miles off-shore between the New Jersey and Long Island coast, would be a very real danger to ships seeking to go in or out by the principal entrance to New York; and similar positions might effectively blockade Boston, the Delaware, and the Chesapeake.

The main body of the blockading fleet, prepared not only to capture merchant-ships but to resist military attempts to break the blockade, need not be within sight, nor in a position known to the shore.