Sentence:  He that has to do with young scholars, especially in mathematics, may perceive how their minds open by degrees, and how it is exercise alone that opens them. Sometimes they will <WSD>stick</WSD> a long time at a part of a demonstration, not for want of perceiving the connection of two ideas; that, to one whose understanding is more exercised, is as visible as any thing can be. 
Meaning of the word marked with <WSD> tags:  To be puzzled ("at" something), have difficulty understanding.