 Complete the following phrase:  During the Franco-Prussian War, Paris was surrounded by the Prussians and the French attempted to use balloons to smuggle messages and supplies in and out of the besieged city. In response, Krupp developed a 37 mm cannon which was mounted on a pedestal which sat on a four-wheeled horse cart and was designated  or BaK and this was the first practical anti-aircraft gun. Even though armed aircraft were used as early as the 