Source Data Reference for:

City = Leiden
Time = 1850

City	Year	Size	Size Comment	Year Comment	Author	Title	Location
Leiden	1840	37,500			Jan Lahmeyer	http://www.populstat.info/	n/a
Leiden	1850	37,000			Paul Bairoch	La population des villes europeennes de 800 a 1850
Leiden	1850	35,000			Tertius Chandler	Four thousand years of urban growth	p26