Final answer:
Northern cities like Lowell, Massachusetts experienced a significant increase in population due to industrialization and the availability of jobs near urban centers.
Step-by-step explanation:
The population of northern cities like Lowell, Massachusetts saw such dramatic increases due to rapid changes in industrialization during the mid-19th century. The development of the steam engine allowed factories to be located near urban centers, attracting more people to move to these areas for job opportunities. The availability of jobs in these urban centers, although often low-paying and monotonous, contributed to the population growth.