"At 30 minutes past midnight this morning of Palm-Sunday the 4th of April 1841, died William Henry Harrison, precisely one calendar month as President of the United States after his inauguration. The first impression of this event, here where it occurred is of the frailty of all human enjoyments, and the awful vicissitudes woven into the lot of mortal man. He had reached but one short month since the pinnacle of honour and power in his own country. He lies a lifeless corpse in the Palace provided by his Country for his abode. He was amiable and benevolent. Sympathy for his sufferings and his fate, is the prevailing sentiment of his fellow-citizens. The bereavement and distress of his family, is felt intensely, albeit they are strangers here, and known to scarcely any one. His wife had not yet even left his residence at North Bend, Ohio, to join him here. An express was sent for her two or three days since, but the tidings of death must meet her before she can reach this city.
The influence of this event upon the condition and history of the Country, can scarcely be foreseen. It makes the Vice-President of the United States, John Tyler, acting President of the Union, for four years, less one month. Tyler is a political sectarian of the Slave-driving, Virginian Jeffersonian school. Principled against all improvement. With all the interests and passions, and vices of Slavery rooted in his moral and political constitution -- with talents not above mediocrity, and a spirit incapable of expansion to the dimensions of the station upon which he has been cast by the hand of Providence unseen through the apparent agency of chance. I trust in humble hope of the good, which it always brings forth out of evil. In upwards of half a century, this is the first instance of a Vice-President's being called to act as President of the United States, and brings to the test that provision of the Constitution which places in the Executive Chair a man never thought of for it by any body. This day was in every sense gloomy."
-- Former President John Quincy Adams, who spent almost his entire post-Presidential life serving in the U.S. House of Representatives after leaving the White House, on the death of President William Henry Harrison and succession of John Tyler, in an entry to JQA's personal diary, April 4, 1841.
Since President Harrison was the first President to die in office and Tyler the first Vice President to succeed to office, it was not entirely clear whether that meant Tyler was merely "acting President" or had fully assumed the Presidency. Adams was one of many leaders who believed that Tyler's role was meant to be that of a caretaker, but Tyler claimed all the powers and trappings of an incumbent President, setting the "Tyler Precedent", which established Presidential succession for everyone who followed.
[April 4, 1841 diary entry by John Quincy Adams, courtesy the Adams Family Papers at the Massachusetts Historical Society.]