How does Dr. Mortimer know that Charles Baskerville stayed by the moor-gate for quite a while?

According to Dr. Moritmer, he waited because the ground was wet. Holmes did not agree.

“You think that he was waiting for someone?”

“The man was elderly and infirm. We can understand his taking an evening stroll, but the ground was damp and the night inclement. Is it natural that he should stand for five or ten minutes, as Dr. Mortimer, with more practical sense than I should have given him credit for, deduced from the cigar ash?”