Book: # 2

     A rocket tube is mounted on gymbal bearings making it free to turn in any directions. When the gun is hauled along the snow-covered surface of a frozen lake by a ski-doo, the gymbals allow the tube to remain vertically pointing upward in spite of some roughness of path. Equally spaced lamps along the path allow on to judge whether the ski-doo has constant velocity or whether it is accelerating. A preliminary run shows the entire scene; the setting is in the Laurentian mountains in Quebec at dusk. 

Four scenes are photographed. In each case the flare is fired vertically upward. With care you can determine the paths experimentally. 

Scene 1: The ski-doo is stationary relative to the earth. Scene 2: The ski-doo moves at uniform velocity relative to the earth. Scene 3 and 4: The ski-doo's speed changes after the shot is fired.