The Cossacks (Don, Kuban Terek, Astrakhan, Orenburg, Ural, Siberia, Semiryetchensk, Transbaikalia, Amur and Usuri) are divided in three classes; the first in active service, the second on furlough with their arms and horses; the third with arms and without horses. Some of the Cossack cavalry serves with the regular cavalry. Military service is also obligatory in Finland. The Russian army consists of 31 corps. The lowest estimate of its peace strength is about 1,100,000 with 42,000 officers; the war strength about 75,000 officers, 4,500,000 men and 562,000 horses. Owing to its widely separated seas, the Russian navy maintains four squadrons: the Baltic, the Black Sea, the Pacific and the Caspian. Cronstadt is the chief base of the Baltic Fleet; Sebastopol of the Black Sea; and Vladivostok and Port Arthur of the Pacific. The Caspian fleet is comparatively insignificant. In 1903, the navy consisted of 26 battleships, 14 coast defence ships, 24 first-class cruisers, 15 second-class cruisers, 161 gunboats and torpedo craft. The ocean shipping of the Russian Empire is not relatively large, but its lake and river shipping is very extensive. In 1900, the sea-going marine consisted of 2,293 sailing vessels and 745 steamers. The total length of railway open for traffic and travel on January 1, 1903, was 35,336 miles (not including 1,753 miles in Finland). Of this 4,965 miles were in Asiatic Russia. The legal unit of money is the silver rouble of 100 kopecks of the value of 2s. 1.6d., or about fifty cents of American money. The coins called imperial and half-imperial contain 15 and 7-1/2 roubles respectively. There are also credit notes of 100, 25, 10, 5, 3 and 1 rouble. Russia's chief source of revenue is the liquor traffic. Her chief exports are spirits, tallow, wool, tow, bristles, timber, hides and skins, grain, raw and dressed flax, linseed and hemp. Her principal imports are tea, cotton and other colonial produce, iron, machinery, wool, wine, fruits, vegetables and oil. Russia is the second largest European grower of wheat. Hemp, flax, potatoes and tobacco are also raised in large quantities. Barley, buckwheat, oats, millet and rye form the staple food of the inhabitants. Mines of great value exist in the Ural, Obdorsk and Altai mountains, which produce gold, copper, iron, silver, platinum, rock-salt, marble and kaolin or china clay. Rich naphtha springs exist on the Caspian and an immense bed of coal has been discovered between the Donetz and Dnieper rivers.