When I was a kid, BBC2 would play Harold Lloyd shorts at 6pm every weekday. I was late for Scouts on more than one occasion because of these, brilliant, films.
And, yes, Safety Last ... the clock scene was filmed using forced perspective. Here's the scene:
As for how it's done, the set was built on a rooftop with the camera on a rig giving just the right angle:
I always preferred Harold Lloyd to Buster Keaton or Charlie Chaplin ... I think because he just looked so unlikely to do these sorts of stunts.
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