We are kindly permitted to make an extract from a private letter, from a gentleman in the South to his near relative in this State, the lesson of which we commend to that class of our fellow-citizens who feel a deep concern lest the action of the government, in vigorously prosecuting the war, should prejudice the Union men of the South, and lead them to make common cause with the rebels. Though we do not choose to give the name or exact locality of the writer, or to give any due to his present whereabouts, we will state that he is a native of Maine, of Democratic antecedents, a graduate of one of our New England colleges, that be studied a profession, went South where he engaged in teaching, finally married into the family of a wealthy cotton planter, and at the time the rebellion broke out, he was engaged in a lucrative legal practice.