"As far as was known there was nothing to lower his mental mercury, for his rent-roll was a large one..."
See in text (The Striding Place)
These phrases introduce us to Wyatt Gifford, who has recently disappeared from the castle. His “mental mercury” refers to his mental acuity. “Mercury” here indicates both the reading of a mercury thermometer—a handy metaphor for intelligence, scaled vertically—as well as wit. Those who are said to be mercurial are lively and quick-witted. A “rent-roll” refers to the list of renters who live on the property of a landowner. Gifford is a landowner and, it appears, a wealthy one at that.