He meant that the act was made to increase the power of slave states.
Explanation:
He made that statement when he's commenting about the Fugitive slave act of 1850. This act was made to made sure slaves that escaped the free states to be returned to the owners.
Douglass believed that this act was a conspiracy that made by legislators from Sothern states to expand the power of the slave states.
Mason & Dixon lines were the lines that separated the slaves states and free states region. Douglass believed that the fugitive slave act of 1850 made this line basically obsolete. The free states wouldn't be able to provide protection to African American like they intended to.
