The Union is in danger again. The Herald has discovered that the National Theatre is in league with Senator SEWARD and other incendiaries for the destruction of our beloved Confederacy. The first step in this grand conspiracy is to win over a score or so of pea-nut boys, for which purpose the play of "Uncle Tom's Cabin" has been put upon the stage. And to make the plot still more horrible, a commendatory notice of the dramatic effect of the play has been discovered among the theatrical notices of the TIMES, and is paraded by the Herald as proof positive of the impending ruin. We are glad to see that the Herald is properly alive to the necessity of doing something desperate in this extremity of our national affairs. We shall watch its future movements with the profoundest interest.