Progress was slow at first, as Carlyle harboured doubts about his subject and his ability to complete the task he had set for himself, as well as finding difficulty in accessing source materials. Nonetheless, he persisted, writing copious notes, a large collection of which are now in the Beinecke Library at Yale University. Various assistants helped with his research, making trips to the British Library and State Paper Office among other collections, also copying extracts from German histories.