Attack by a spar-torpedo boat on the United States frigate Minnesota, April 9th, 1864. In this attack the Minnesota was unprotected; the torpedo-boat was discovered about fifty yards away and fire was opened on her. Her torpedo was properly planted and exploded, damaging the frigate considerably, but the charge was not heavy enough to sink her. Attack by a spar-torpedo boat on the United States corvette Housatonic, February 17th, 1864. The boat used in this attack was in reality a submarine boat, but there are good reasons for believing that on this occasion she was used as a spar-torpedo boat. She was discovered about fifty yards away, but there was no time to open fire on her. She planted her torpedo fairly and exploded it, sinking the vessel and herself at the same time. Besides these attacks there were several during the American civil war, in which the boats were discovered at a distance and driven off. PART IV. ARCHITECTURE AND CONSTRUCTION. [Illustration: American Revolving Turret (Ericsson’s System).] [Illustration: English Revolving Turret (Coles’ System).] [Illustration: French Revolving Turret.] [Illustration: French Barbette Fixed Turret and Revolving Gun Carriage.] UNARMORED FLEETS. ARCHITECTURAL DEVELOPMENTS. Ever since the introduction of steam propulsion in ships of war, England, France, and the United States have taken the lead in the development of types of naval vessels, and in examining the various methods followed it is necessary to at first fully understand the conditions under which each nation acted and the obstacles against which each had to contend.