Complex sentence: The Army of the Potomac was accompanied by the Union Army Balloon Corps commanded by Prof. Thaddeus S. C. Lowe, who had established two balloon camps on the north side of the river, one at Gaines's Farm and one at Mechanicsville.
Split into simpler sentences: The Army of the Potomac was accompanied by the Union Army Balloon Corps commanded by Prof. Thaddeus S. C. Lowe. Lowe had established two balloon camps on the north side of the river, one at Gaines' Farm and one at Mechanicsville.