Sentence:  It would be also of great Use to us to form our deliberate Judgments of Persons and Things in the calmest and serenest Hours of Life, when the "Passions of Nature" are all silent, and the Mind enjoys its most perfect <WSD>Composure</WSD> ... 
Meaning of the word marked with <WSD> tags:  Calmness of mind or matter, self-possession.