Krupp is a well-established German weapons manufacturer, commissioned to create an anti-aircraft gun during the Franco-Prussian War of 1870, in response to the French use of balloons.
Naturally these sorties were not at all to the taste of the Germans, and Krupp was ordered to make a cannon suitable for bringing the balloons to earth. It was to be capable of being tilted almost into a vertical position, and to have a special gun-carriage fitted to it. But it was not a success and was soon relegated to the Zeughaus in Berlin. The outposts, however, were constantly on the look-out, and the result of their firing was to drive the French to start their balloons by night.
The German artillery knew the diameter of these balloons to be 50ft., and were therefore able to tell the distance approximately.