Try counting. When you get to one hundred you will have reached and completed your first century, and will have to start on your second, That will take you through all the one hundreds till you get to 200, and are ready to start on your third century (consisting of numbers beginning in 2).
Incidentally, not all the 1800s are in the 19th century: the year 1800 itself is still in the 18th century (which it makes complete). .
Primary school arithmetic.