of San Fracisco, and are quite strong from 9 o'clock in the morning until 11 at night; with the rapid current they would give fromten to twelve knots speed to sailing vessels. With such speed ships will be under fire thirty minutes and under effectual range twenty minutes. The South Pass, between Alcatraz and San Francisco, is one and a third miles wide; the North Pass is one mile and three- quarters. The distrance froM Alcatraz to Point Stewart is tow miles and a half, as laid down on Ringgold's map. In view of the prominence of Alcatraz Island in the system of defense for San Francisco Bay, on account of the width of the passes commanded by its batteries, and the rapidity with which sailing vessels, under favorable circumstances, may pass by the works, the board now recommend a few additional guns on each pass, as represented in the accomopanying drawing in blue. The number indicated is the minimum- eight 42- pounders on the South Pass, and one 10-inch and five 8- inch columbiads on the North Pass. These last guns sweep the waters of the bay as far as the line of direction to Yerba Bueana. one gun additonal can be placed on the flank of the three - gun battery by prolonging the breast- geight wall. The cost of thiese additons will not exceed $2,500 per gun. By the arrangement oflt he additonal battery on the south of the island the 42- pounders for hot shot are taken away from the vicinity of the large magazine under the caponiere. The objectionable feature of this battery is its high reference- seventy feet above high tide. The geenal character of the lines of defense betweent he north batterya and wharf is not changed in the plan submitted. The guard house is enlarged, made stronger, and is provided with a postern draw bridge and ditch. The room below the gun room will serve as a prison. THe outer gun room will be occupied by the guard, the inner bythe officer of the guard. The terre- plein is designed to rest upon arches supported by railroad iron. The ditch between guard house and north battery and the escarpment to the wharf are represented as revetted. This revetment of brick is two feet thick against the rocky bank from the wharf to the prolongation of scarp of the left flank of south battery, and at same point betweent he south and north batteries, w illsecure the island against surprise with proper vigilance at the dead angles. The barracks should be enlargedt o accommodate a war garrison corresponding to the increased defenses of the island without changing the design of the building. It may be made 120 feet long, the portion marked officers' quarters separated from the soldiers and store- rooms by closing the doorwayw ith a brick wal and appropriated as a ospital. For officers' quarters three plain one- and- a- half story cottages are recommended to be placed about the middle of the island on the north slope.
Z. B. TOWER,
Brevet Major of Engineers.
R. E. DE RUSSY,
Lieutenatn- Colonel of Engineers and President of Board.
In addition to the proposed number of guns tot he north battery on Alcatraz Island, the senior member of the Board would respectfully recommend ten guns instead of six, and would add a hot- shot furnance at the temination of that battery.
R. E. DE RUSSY,
Lieutenant- Colonel of Engineers,
37 R R- VOL L, PT II