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signal mirror: 1927 Dutch Heliograph Diagram - Figure 13
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signal mirror: 1927 Dutch Heliograph Diagram - Figure 14
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signal mirror: 1927 Dutch Heliograph Diagram - Figure 15
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signal mirror: 1927 Dutch Heliograph Diagram - Figure 16
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signal mirror: 1927 Dutch Heliograph Diagram - Figure 17