A characteristic feature of gun cotton is that it may be detonated even when completely saturated with and immersed in water, if only some dry gun cotton be detonated in contact with it. Thus in one experiment a disk of dry gun cotton was covered with a water-proof coating and the detonator inserted in the detonator hole of this disk. This dry disk was laid upon four uncoated disks, the five lashed tightly together, and sunk in Newport Harbor, where the column remained until the uncoated disks were saturated with salt water, when the mine was fired and the saturated disks were found by measurement of the work done to have been completely exploded. I have found that three ounces of dry compressed gun cotton will cause the detonation of wet compressed gun cotton in contact with it, but forty ounces of dry gun cotton are used as the primer in our naval mines and torpedoes, so as to give a large coefficient of assurance. [Illustration: GUN-COTTON SPAR TORPEDO.] [Illustration: BLOWING UP THE SCHOONER JOSEPH HENRY.] In the mining and other industries the fulminate is used in smaller quantities and it is generally mixed with potassium chlorate, the mixture being compressed in small copper cases and sold as blasting caps. They are fired by means of a piece of Bickford or running fuse, consisting of a woven cotton or hemp tube containing a core of gunpowder, which is inserted in the mouth of the copper cap and made fast within it by crimping. The capped fuse is then inserted in a dynamite cartridge so that the cap is firmly in contact with the dynamite, the mouth of the cartridge is fastened securely, and the charge inserted in the bore-hole in the rock and tamped. The protruding end of the fuse is lighted, and the fire travels at the rate of three feet per minute down the train of gunpowder to the fulminate, which then detonates and causes the detonation of the dynamite.