Passage: The National Monument of Scotland, on Calton Hill in Edinburgh, is Scotland's national memorial to the Scottish soldiers and sailors who died fighting in the Napoleonic Wars. It was intended, according to the inscription, to be "A Memorial of the Past and Incentive to the Future Heroism of the Men of Scotland".The monument dominates the top of Calton Hill, just to the east of Princes Street. It was designed during 1823-6 by Charles Robert Cockerell and William Henry Playfair and is modelled upon the Parthenon in Athens. Construction started in 1826 and, due to the lack of funds, was left unfinished in 1829. This circumstance gave rise to various nicknames such as "Scotland's Folly", "Edinburgh's Disgrace", "the Pride and Poverty of Scotland" and "Edinburgh's Folly".

Question: When did construction start?
1826