Why are old movies sped up?

Why are old movies sped up?
The hand-cranked cameras ran at a speed slower than what was neededto make the motion seem normal. Paradoxically, the slower the filmruns through a camera--given that the projector speed isconstant--the faster the motion appears on screen. Contrariwise,when movies are shot in "slow motion," the film runs through thecamera at faster than normal speeds.
Some old movies--silent comedies with chase scenes, forexample--were shot deliberately at slower speeds so that the motionwould seem fast and jerky: directors found that the comedic effecton the audience increased.