Complex sentence: Welby's replies did not conceal that she found Peirce hard to follow, but by duplicating and circulating some of Peirce's letters to her she did much to introduce Peirce to British thinkers.
Split into simpler sentences: Welby's replies did not conceal that she found Peirce hard to follow. But by duplicating and circulating some of Peirce's letters to her, she did much to introduce Peirce to British thinkers.