Black Cow: A German spy signalling device, according to the newspapers, during the German advance into France and Belgium in 1914. Peasants in German pay drew in charcoal, it was said, black cows on walls, gates, etc., in varying sizes and attitudes, each of which had a special significance in regard to the presence and force of the Allied troops. The size and shape of the horns had special meanings, as had the direction in which the cow was looking, and so on. In addition to the Black Cow there were, of course, many other German spy devices at that time; the moving of the sails of windmills spelled out certain information; the method of driving flocks of sheep similarly had a significance; all in accordance with a pre-arranged code. 
References / notes
- Edward Fraser and John Gibbons (1925). Soldier and Sailor Words and Phrases. Routledge, London p.24.
Glossary of words and phrases
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