Owing to the ability of submarines to deliver stealthy attacks by day, naval tacticians have designated this type of craft “Daylight torpedo-boats,” but they are rapidly passing beyond the purely torpedo and coastal defensive stage and are taking on to themselves the _rôle_ of the ocean cruiser. The size of these vessels has increased from 50 to 1,000 tons displacement in ten years. They now carry not only a considerable number of the largest size torpedo but also quick-firing guns for repelling attacks by small surface vessels, and are capable of accompanying fleets to sea. The Australian naval submarines A.E.1 and A.E.2 both made the voyage from Barrow to Sydney under their own power and without convoy. The radius of action of the latest vessels both of the British and French Navies amounts to several thousand miles. In the case of the British “F” class the displacement has risen to 1,500 tons, the speed to 20 knots and the armament to six torpedo tubes and four 12-pounder quick-firing guns, thus making them in every sense ocean cruisers capable of keeping the sea in almost any weather and possessing wide range of action and considerable offensive power. Hitherto British submarines, like the ordinary surface torpedo-boats, have been known by numbers only, but the latest vessels are to receive names which indicates the increase in size and importance of these craft. They may be termed the advance guards of the submarine battleships of the future. The torpedo, which is the principal arm of the submarine boat, is itself a submarine projectile. After being discharged from the firing tube it sinks a short distance below the surface and is propelled by its own engines at a high rate of speed in a straight line towards its object of attack. Great improvements have been made during the past ten years in the construction of these delicate weapons. The extreme effective range and speed has risen from 4,000 yards at 18 knots to 7,000 yards at 45 knots or 11,000 yards at 30 knots. The “war-head” or front section of the torpedo contains a charge of about 200 pounds of damp gun-cotton which is fired by a detonator on the torpedo striking an object. This very powerful explosive charge is capable of tearing open an enormous hole in the unprotected under-water skin of the surface warship. The type of weapon used in the British, French, Russian and Japanese Navies is the _Whitehead_ torpedo (18-inch and 21-inch). The German Navy uses the _Schwartzkopf_ torpedo (18-inch and 21-inch), which is very similar to the Whitehead and is a very powerful weapon. In the first phase of the naval war no less than eight warships have been sunk by submarine torpedoes. The activity of the British submarines _in the theatre of war_, from the beginning of hostilities, is admirably set forth in the following dispatch from Commodore Roger B. Keyes, C.B., which is the first dispatch in the history of naval warfare to describe in detail submarine attack and reconnaissance:—