1. On March 15, 1820, New Hampshire became the ninth state to ratify the United States Constitution, which had been amended to include the Missouri Compromise. This compromise addressed the issue of slavery in newly admitted states by prohibiting slavery north of the 36°30' parallel.

2. On March 15, 1860, the first American intercollegiate baseball game was played between Amherst College and Williams College at Pittsfield, Massachusetts. Both institutions were founded by New Hampshire residents, and the game's popularity quickly spread to colleges and universities throughout the country.

3. On March 15, 1875, the first telephone exchange in New Hampshire was established in Manchester. A few months later, the first long-distance telephone line in the state was installed, connecting Manchester with Boston.

4. On March 15, 1934, Merrimack River flooded its banks, causing widespread damage throughout the state. This event led to the construction of flood control dams, levees, and dikes throughout New Hampshire, and the development of the state's first floodplain mapping program.

5. On March 15, 1979, the Seabrook Nuclear Power Plant, located on the coast of New Hampshire, was the site of a major anti-nuclear protest. More than 1,400 protestors were arrested for civil disobedience, and the event drew national attention to the issue of nuclear energy in the United States.