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This is the original sense of the word airline. Now obsolete, it survived in works on surveying and military history into the 1960s. The OED says:
airline, n. 1. a. Chiefly U.S. A direct line through the air; a straight line between two points on the earth's surface.
with citations dating back to the early 19th century:
1829 J. F. Cooper Wept of Wish-...
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Possibly the American Civil Rights Movement.
Living as he did in Tennessee, in the American South, Foote would have seen a lot of changes during the decades surrounding his writing of The Civil War: A Narrative. The fact that he was writing about the US Civil War, as a man from the South, would only have complicated this further for him. From the article "...
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