What is agenda setting theory? what initial research was done to show agenda setting in the media?

Read the opening lines of sojourner truth's speech "ain't i a woman? " well, children, where there is so much racket there must be something out of kilter. i think that 'twixt the negroes of the south and the women at the north, all talking about rights, the white men will be in a fix pretty soon. but what's all this here talking about? what tone do these lines convey?