[89] So developed in sea and land maneuvers undertaken for the purpose of establishing the very points here mentioned.
[90] It is estimated, from careful calculations, that to put out of action a searchlight at night with shipfire at a range of 6,000 yards, more than a thousand shots from 3-inch guns should be required. The fact is mentioned here to illustrate the great strength of harbor defenses against fire from the sea, if there be enough mobile troops on the land to prohibit destruction by landing parties.
[91] That the American harbor defense system and construction are of the very highest type, has been acknowledged many times by the technical experts of the world. More than once the author has heard foreign officers express the belief that they were practically impregnable to naval fire, providing they were fully supplied and equipped with the material necessary for continuous defense.
[92] A generous system of reliefs is imperative in harbor defenses during war. Peace time maneuvers have developed the fact that the mere strain of incessant watchfulness while waiting for an enemy who may appear at unexpected points suddenly, is so great that unless the men have frequent relief, they cannot exert that concentrated energy which is needed instantly in the crisis.
[93] This system of night attack has been developed and tested by actual trial, and is such as is now laid down for battle practice in the tactics of most navies. “The ... squadron will enter ..., and will maneuver at range of about 9,000 yards from Fort ..., firing heavily, to induce the defense to expend as much ammunition as possible.”—Extract from actual orders in author’s possession, given to a squadron of battleships and cruisers for night attack. It will be noted that this distance is less than one-half the range of the 12-inch rifled mortars in a harbor defense battery.
[94] The search-light system, recognized as a vital part of harbor defense by the Endicott Board on harbor defense (appointed in 1885) has grown steadily in importance with the steady increase in ship armament and ship speed. A thoroughly efficient installation of search-lights for modern harbors demands as much scientific calculation and interlocation as do the gun-systems. If the search-lights cannot infallibly find any vessel that may approach within range, the guns of the fortification are useless.
[95] The inadequacy of the installation has been made the subject of continuous reports. It is a fact that a few years ago, when a mock attack on one of the most important Atlantic defenses was ordered by the War Department, the commander had to requisition search-lights from other coast defenses, and that during the maneuvers the search-light defense, because of its inadequacy and temporary character, failed at several critical points, permitting attacking ships to come within less than 4,000 yards of one important battery.
[96] Usually the firing zones are: first, 6,000 yards to the extreme range of biggest guns; second or intermediate, 3,000 yards to 6,000 yards; third (mine field zone), 3,000 yards. The order of fire is worked out absolutely for every condition that is possible. The movements of attacking ships, and their combinations, although very numerous, can be predicated with some accuracy beforehand.
[97] Estimated number of shots required at night from ships afloat at 6,000 yards: to destroy position-finding tower which is visible, 22 12-inch shells, 250 4-inch shells or 2,500 3-inch shells; to destroy invisible station without tower, 400 12-inch shells, 5,000 4-inch shells; to destroy search-light, 24 12-inch shells, 300 4-inch shells or 3,000 3-inch shells. This fact makes it feasible to protect outlying and secondary range stations perfectly if sufficient troops can guard each station to fight off landing parties. An enemy will surely land men to destroy them unless he knows they are well defended.
[98] Actual records of American harbor batteries: three 6-inch guns on disappearing carriages, 15 shots in 1 minute, 27 seconds.