Extract of comments: ... In case this is not a joke ... It happens only at daytime ... so it's probably dew to the vibrations caused by the public ... because the friction between stone and glass is small.
Note: An important detail is that it's not a "perfect circle", it's really only a half turn, so gravity is a possible explanation.
Extract of comments: ... In case this is not a joke ... It happens only at daytime ... so it's probably dew to the vibrations caused by the public ... because the friction between stone and glass is small.
Note: An important detail is that it's not a "perfect circle", it's really only a half turn, so gravity is a possible explanation.
Another thread: https://news.ycombinator.com/item?id=5927620 (3 points, 4 ours ago, 4 comments)