What aspect of United States history does the speaker mainly talk about?

Last week, we discussed the early development of railroads in the United States. Today I want to talk about the wooden sailing ship. [11] From colonial times sailing ships were vital to the economy. Many coastal towns depended on fishing or whaling for employment and income. This was especially true in the northeastern states. And there the wood from nearby forests also formed the basis of an important shipbuilding industry. But the big profits were to be made on trade with faraway places. [12lAnd since sea captains often became part owners of their ships, they had a strong interest in the commercial success of their voyages. So these US sailors carried on a very profitable trade with other parts of the world. The high point of this trade came in the mid- 19th century with the introduction of the clipper ship, [13] the enormous clippers with huge sails reaching nearly two hundred feet into sky. She’d carry passengers and cargo from New York around South America to San Francisco in less than three months. [13] At that time this seemed unbelievably fast and efficient. But in the 1860s, more reliable steam-powered ships began to take over. [11]And soon the important role of sailing ships in the US economy would come to an end.