Rope Brainteaser

In antiquity, a King wanted to sleep for exactly one and half hours, and so he asked his servant to wake him up after this time period had elapsed. The servant had no clock to check the time. However, he did have two ropes of different lengths. The servant had a box of matches as well. Each rope was uneven (of varying thickness), and the servant knew that each rope took one hour to burn if it was set on fire at either end. It was a tradition in those days that you would be killed if you did not obey the orders of the King, even it was beyond your powers to obey. Therefore, the servant would also be killed by the King if he did not wake him up after one and half hours. What strategy could the servant use to measure the time of one and half hours? Note that there was no way to cut the ropes into parts.