How does Sacks help Mr. MacGregor in Chapter Seven of The Man Who Mistook His Wife for a Hat and Other Clinical Tales?

When we first meet Mr. MacGregor he tilts to one side when he walks but is unaware of it. Together Sacks and Mr. MacGregor work to construct a pair of glasses from which a pendulum is suspended. Mr. MacGregor watches the pendulum and is then able to determine if he is tilting or straight up.