However, not for long did British commerce sail unmolested.  A new and terrible menace was to appear.  This was the submarine boat, the invention of Mr. John Holland, an American, but improved and enlarged by the Germans.  In one of the early months of the war three British warships, the Hogue, the Cressy, and the Aboukir, were cruising about, guarding the waters of the North Sea.  There was the explosion of a torpedo, and the Hogue began to sink.  One of her sister ships rushed in to pick up the crew as they struggled in the water.  A second torpedo struck and a second ship was sinking.  Nothing daunted by the fate of the other two, the last survivor steamed to the scene of the disaster—­the German submarine once more shot its deadly weapon, and three gallant ships with a thousand men had gone down.

This startled the world.  It was plain that battleships and cruisers were not enough.  While England controlled the surface of the sea, there was no way to prevent the coming and going of the German submarine beneath the waters.  All naval warfare was changed in a moment; new methods and new weapons had to be employed.

At the outset of the war the English and French fleets had set up a strict blockade of Germany.  There were certain substances which were called “contraband of war” and which, according to the law of nations, might be seized by one country if they were the property of her enemy.  On the list of contraband were all kinds of ammunition and guns, as well as materials for making these.  England and France, however, added to the list which all nations before the war had admitted to be contraband substances like cotton, which was very necessary in the manufacture of gun-cotton and other high explosives, gasoline—­fuel for the thousands of automobiles needed to transport army supplies, and rubber for their tires.  Soon other substances were added to the list.

An attempt was made to starve Germany into making peace.  The central empires, in ordinary years, raise only about three-fourths of the food that they eat.  With the great supply of Russian wheat shut off and vessels from North America and South America not allowed to pass the British blockade, Germany’s imports had to come by way of Holland, Switzerland, and the Scandinavian countries.  When Holland in 1915 began to buy about four times as much wheat as she had eaten in 1913, it did not take a detective to discover that she was secretly selling to Germany the great bulk of what she was buying apparently for herself.  In a like manner Switzerland and the Scandinavian countries suddenly developed a much greater appetite than before the war!  The British blockade grew stricter.  It was agreed to allow these countries to import just enough food for their own purposes.  The British trusted that they would rather eat the food themselves than sell it to Germany even at very high prices.  The Germans soon began to feel the pinch of hunger.  They had slaughtered many of their cows for beef and as a result grew short of milk and butter.