what does "the second possible antecedent" and "hollowed by precedent"mean?
But the Madison papers appeared too late. The second possible antecedent for "the Same"—"the said Office"—had become hallowed by precedent. In 1841 William Henry Harrison caught cold during his inauguration and became the first American Chief Executive to die in office. His Vice Presi¬dent, John Tyler, learned the news while playing marbles with his children in Williamsburg, Virginia.

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The original intent of the Constitution was that the Vice President would only perform the duties of the President until the citizens chose a new President.  The wording was changed by a committee of the Constitutional Convention and the original intent became less clear.  (When the committee changed the wording, it was no longer clear if the Vice-President would assume the Presidency or just assume his duties until a new President could be chosen)  The following events of President Harrison's death would set a precedent for the Vice-President to assume the Presidency.  
 When President Harrison died in office, Vice President John Tyler interpreted the constitution to mean that the Vice President assumed the office of the Presidency (in the death of the President) and he acted under that assumption.  Creating a precedent for the Vice-President to assume the office of the Presidency in the event of the death of the President.
The first issue was the vagueness of the Constitution when it came to Presidential succession and the second was the precedent set by Vice-President Tyler when he assumed the office of the Presidency after the death of the President--Harrison was the first President to die while in office.