Percussion-caps instead of flintlocks were now used to explode contact mines. A new type of anchored torpedo, set off by an electric spark through a wire running to an operator on shore, was also a favorite with the Confederates. Because they are exploded not by contact with the ship’s hull but by the closing of the circuit by the operator when he observes an enemy’s vessel to be above one of them, these are called “observation mines.” In the Civil War, many effective mines of this sort were made out of whisky demijohns. One of these blew up the gunboat _Cairo_, in the Yazoo River, in the autumn of 1862. The double-ended, river gunboat _Commodore Jones_ was blown to pieces by an observation mine, whose operator was subsequently captured and tied to the cut-water of another Federal gunboat as a warning and a hostage. During the bombardment of Fort Sumter by the United States fleet in 1863, the _New Ironsides_ lay for an hour directly above an observation mine made of boiler iron and containing a ton of gunpowder but which failed to explode despite all the efforts of the operator. He was naturally accused of treachery and it would have gone hard with him had it not been discovered, soon after the _New Ironsides_ ceased firing and stood out to sea, that the shore end of the wire had been severed by the wheel of an ammunition wagon. [Illustration: U. S. IRON-CLAD “CAIRO” (BLOWN UP BY CONFEDERATE TORPEDO). From Scharf’s History of the Confederate States Navy. First Warship Destroyed by a Mine.] During the Franco-Prussian War, the powerful French fleet blockaded the German coast but did not attack the shore batteries, which were well protected by mines. After peace was declared the foreign consuls at one of the North German seaports congratulated the burgomaster on having planted and taken up so many mines without a single accident. Unknown to any one, the prudent burgomaster had unloaded them first, and they kept the French away just as well.