Sentence:  [I]t requires a Critical Nicety both of Wit, and of Judgment, to find out the "Genius", or the Propenſions, of a Child, ... The Juſt Seaſon of Doing Things must be <WSD>Nick'd</WSD>, and All Accidents Obſerv'd and Improv'd; for Weak Minds are to be as Narrowly Attended, as Sickly Bodies: ... 
Meaning of the word marked with <WSD> tags:  To hit at, or in, the nick; to touch rightly; to strike at the precise point or time.