user: Question: What is the Population n1891 of the County with Sleaford Headquarters?
Columns: ['Administrative County', 'Area (statute acres) 1891', 'Population 1891', 'Area (statute acres) 1961', 'Population 1961', 'Headquarters']
Types: ['text', 'real', 'real', 'real', 'real', 'text']
Sample row: ['Bedfordshire', 298494.0, 160704.0, 302940.0, 380837.0, 'Bedford']
SQL:

assistant: SELECT "Population 1891" FROM "Table" WHERE "Headquarters" = 'Sleaford';