Chemical smoke puffs represent exploding shells…
Surveyed through field glasses that make it appear miles away, a novel war map at Princeton University makes artillery practice realistic to students of the Princeton unit of the Reserve Officers Training Corps.
…Each student takes his turn at directing the miniature “barrage.” The ingenious map is operated by the instructor, who follows the student’s data and commands to fire. A small adjoining map is criss-crossed with lines showing where shells with various ranges would strike. Over this key chart moves a lever which, placed at the spot where the student’s shot would fall, swings a glass nozzle to a corresponding position on the large map; at the student’s word “fire” a puff of artificial smoke is released.
Popular Mechanics, May 1927
Making Maps: DIY Cartography 