Last year it was reported that an en i giueer of Boston had perfeeted a - vanee by which a gun of any size could be fired under water in such a manner that the shot froui ifc sbould tako effect ajeo under water. We have heard. notljing recently of this dísoovery or contri- vanee, but we find, frora a report roade lately of soiue experimenta condueted last year in England by the offiaers of the British gunnery ship Excellent, that therc a full trial has been made, and it is now certain that submtrged eaunon can be effectually fired. The interesting experimenta at Portsmouth are thua described : 'A stage was erected ín the harbor within the tide-mark ; on this an Armstrong 110 pounder was mountcd, loaded and aimed, at low water, at a target placed also within the rise of the tide. - When both gun and target were oovered by the water to a depth of .six feet the gun was fired by means of a tube. The targets were placed at from 20 to 25 feet from the muzzle of the gun. One was oomposed of piles and oak planking, of a thiokness of 21 inohes ; another consisted of the h.ull of an old vessel, tbs Griper, laid on a mud bank ; a third was made up of three iuches in thiekness of iron boi'er-plutes, bolted together and backcd with timber. On all these the effaot of shot and shell trom the sublucied guu as very sia.tiing The woodeu target was pierced through and through, tho iron targut was broken into pieees and driveu into the backing, the solid shot passed right thrcugh both sides of the ve8sel, niakingva huge hole through which the water poured in torrents ; shell with percussion fuse burst in entering, opening up a chas'm of 4 faet by 3 in the planking, shattoi'iog the ribs and bursting up the deck beams above." Our ordnancu bureau ought to look to this matter. If it is practicable to load and fire oannon under water, then the defense of harbors will ieceive a new help ; and iron-claas no matter tíow heavily they are armorod above water wil! bo as weak and defenseless against sueh a submerged battery as the merest shell of a woodeu frigate. It is impossible to arm a floating vessel ngaiust suoh an assault. Huge as the British Warrior and Prench Gloire are, they can barely support the iron plating whioh defends their upper works - their hulls below the wator-line are no more impervious to shot than auy wooden ship's. Suppose now that we had a battery of submerged cannon so arrM)ged in this harbor that they could be loaded, aimed and fired at will, no. iron-olad would dare encounter suoh a foe. Harbor defense vvould be complete.