How is a witness permitted to refer to a ship’s log?

In Burrough v. Martin (1809) 2 Camp. 112, 170 E.R. 1098, the witness was permitted to refer to a ship’s log when it was established that the witness had examined the log from time to time when the occurrences mentioned were fresh in his memory. But the questions put to him were: (1) Whether he saw that entry at a time when he remembered the facts; and (2) whether looking at the entry, he could now state positively upon his oath when the ship arrived at her port of destination. It is quite obvious in this case that the witness could not, looking at this document, have said that he could now swear positively to that date.