The surprising certainty with whicb. shots from modern rifled cannon reach :argets several miles distant depenfls upon au accurate knowledge of the range. In a combat with hostile ships :he range must be continually changng and one of tbe rneans for quick ascertainment is the range-flnder invented by Lieut. Fiske, of our navy. Two teleacopes are fixed on the deck of a s'hip at a known diBtance apart. Au observer at each telescope keeps it directed upon the enemy's ship. The teleacopes are electrieally connected, and a Wheatstone bridge and galvanometera are included in the circuit. As long as the telescopes are perfectly parallel in direction, the Wheatstone bridge remains in equilibrium, but when they are converged upon a single object, the equilibrium is destroyed, and the needie of the galvanometer swings over a distance corresponding to the angular displacement of the telescopes. A properly graduated disk behind the needie shows at a glance the, distance of the object at which the telescopes are aimed.