LVII THE DEVELOPMENT OF MATERIAL KNOWLEDGE

adapted to work in heavy seas. The paddles smash too easily, and the boat is then disabled. The screw steamship followed rather slowly. Many difficulties had to be surmounted before the screw was a practicable thing. Not until the middle of the century did the tonnage of steamships upon the sea begin to overhaul that of sailing ships. After that the evolution in sea transport was rapid. For the first time men began to cross the seas and oceans with some certainty as to the date of their arrival. The transatlantic crossing, which had been an uncertain adventure of several weeks—which might stretch