Letter from Admiral Sir John Fisher [Commander-in- Chief, Portsmouth] (Admiralty House, Portsmouth [Hampshire]) to Arthur Balfour [Prime Minister] on submarine warfare, stating that the submarine would displace the gun and revolutionise naval tactics, suggesting that the output of submarines should be doubled at any cost. [Hand-written copy, with annotations by Fisher, written in 1911; typescript copy at CHAR 13/2/132-134].