How did the crew of the HMS Challenger measure the depth of the deepest part of the ocean?

I was just reading about the Challenger expedition, which was the first to measure the depth of Challenger Deep, the deepest place in the oceans of the world. They did this in 1875, apparently with some apparatus involving a rope made of Italian hemp. They found Challenger Deep to be 8,184m deep, which is a bit off from the 2009 sonar measurement of 10,971m. How did they make this measurement? How did they know that the weight on the end of the rope had reached the bottom? Did it really reach the bottom?