On the other hand, where mines are unprotected by the fire of shore batteries it is possible for an enemy to remove or disable them. Therefore guns and mines, the two elements of the fixed defenses of a harbor, are mutually dependent, and when the location of one has been decided upon that of the other must conform thereto. Within the zone between 4,000 and 8,000 yards of the main defense the fire of heavy guns is destructive for warships, yet the latter are at such a distance that their rapid-fire guns will be of little effect against the batteries. Moreover, at 4,000 yards vessels are just beyond the inner limit of mortar fire. If possible, therefore, hostile vessels should be held in this zone by some obstacle. Such obstacle is afforded by a mine field. On the other hand, attacks upon a mine field are most liable to be made by small boats at night. If the mine field be at too great a distance from the defenses, these boats will not be revealed by the mine searchlights. Furthermore, for protection against such attacks, the defense relies upon rapid-fire guns of relatively limited range. Due to the above considerations the outermost mines are usually placed between 3,000 and 4,500 yards from the main defense. In general, there should be in each main channel at least three lines of mines. ELEMENTS OF A MINE SYSTEM. The elements of a mine system are: 1. =The mining casemate=, consisting typically of four rooms: (1) The operating room, containing the power panel and the operating boards; (2) the engine room, containing the engine and the generator; (3) the battery room, containing the storage battery; and (4) the sleeping room for the personnel. 2. =The multiple cables=, 7 and 19 conductor, leading from the casemate out to the distribution boxes, one of which is in the center and rear of each group of mines. 3. =The single-conductor cables=, radiating to the front from the distribution boxes, one leading to each mine. 4. =The mines=, in groups of 19 or less, extending across the waterway to be defended, planted approximately 100 feet apart and anchored so as to have a submergence of about 10 feet at low water. The groups are numbered 1, 2, 3, etc., from left to right of the observer stationed in rear of the line, and the mines in each group are numbered similarly, No. 1 being on the left, No. 10 in the center, and No. 19 on the right. The groups composing a line of buoyant mines are not usually planted in prolongation of each other, but with a space for the passage of friendly vessels, and also for the movement of the planter when at work upon adjacent groups. Groups of ground mines may be placed in prolongation of each other or between the groups of buoyant mines, as they will always be below the hulls of passing vessels. 5. =The mine planters= and other boats with the necessary equipment for planting and maintaining the planted mines.