four changes that took place in america between 1840 and 1860?

Question

political, technology and population changes

Answer 1

1. Death of the whigs and emergence of the Republican party in the 1850s

2. Rapid expansion of railroads, at least as far west as Kansas

3. Growing economic division between the north and south, with north becoming industrial as south remained agrarian

4. Huge growth in population, especially in major cities; westward growth of population
Here's a good map on how the US Center of Population has shifted through history:
http://en.wikipedia.org/wiki/Mean_center_of_U.S._population