What is the content of the manuscript presented by dr. mortimer

The tale of The Hound of the Baskervilles by Arthur Conan Doyle begins with a visitor, Dr. Mortimer, approaching Holmes to solve one murder and potentially prevent a second.    

Dr. Mortimer's visit is occasioned by two things, the mysterious death of Sir Charles Baskerville and the impending arrival of his heir, Sir Henry. Although the doctor is not by nature superstitious, he does consider it important to apprise Holmes of the legend surrounding the "hound of the Baskervilles", a legend that begins with Hugo Baskerville. The story of the legend is told in the 1742 manuscript.

Hugo Baskerville had abducted a peasant girl. She escaped from the room in which he had imprisoned her and fled. He followed on horseback using hounds to hunt her down. His companions followed him, and saw his horse standing riderless and a great black hound tearing out his throat. Since then, several members of the family have died mysterious deaths. The document warns the Baskervilles avoid the moors at night. 

The legend is made relevant by Dr. Mortimer's description of the scene of Sir Charles' death:

Dr. Mortimer looked strangely at us for an instant, and his voice sank almost to a whisper as he answered: "Mr Holmes, they were the footprints of a gigantic hound!” This is the answer! I hope it helped...

dramatic irony occurs when the reader/audience knows more about a character's situation than the character does. the one main effect is that it gives the reader a sense of superiority, or detached superiority. the reader not only seemingly discovers what is revealed, but feels superior for doing so.