In this fragment from The Boy at Mugby within paragraph 60:
The swelling tumult was arrested in its rise. Sniff, bore away by his servile disposition, had drored up his leg with a higher and a higher relish, and was now discovered to be waving his corkscrew over his head. It was at this moment that Mrs. Sniff, who had kep’ her eye upon him like the fabled obelisk, descended on her victim.
What is "the fabled obelisk" alluding to?