Served on board the U.S.S. "Lackawanna" during successful attacks against Fort Morgan, rebel gunboats and the ram "Tennessee", in Mobile Bay, 5 August 1864. Displaying great coolness and exemplary behavior as first sponger of a gun, Cassidy, by his coolness under fire, received the applause of his officers and the guncrew throughout the action which resulted in the capture of the prize ram "Tennessee" and in the destruction of batteries at Fort Morgan.