witness-post
n.— «It is extremely important that the points S and L of each spiral should be secured by permanent monuments in the centre of the track, and by witness-posts at the side of the road. The posts should be painted and lettered so that they may serve as guides to the trackmen in their subsequent efforts to grade and “line up” the track.» —by William H. Searles The Railroad Spiral (New York) , 1882. (source: Double-Tongued Dictionary)
Further reading
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Amid court-ordered busing in the 1970s, a middle-school teacher tried to distract her nervous students on the first day of class with this strange assignment: find a monarch caterpillar. The result? A memorable lesson in the miracle of metamorphosis...