The film runs ninety minutes, and the (…) print of it was on two 16mm reels of exactly the same length. (…) I asked the projectionist to help me. (…) [He] set up two projectors side by side but sufficiently apart. On the left-hand projector he threaded up reel one of ‘The Clock’, to play forward, with sound. On the right-hand projector, he loaded the second reel, with all the film on the lower, or uptake, reel, to play in reverse and without sound.
— We started the two projectors at the same time. (…)
The effect was ravishing: it may be the most exciting film show I have ever seen. (…) You may be able to try it for yourself, though those projectors and prints are harder to find nowadays.
David Thomson, ‘The Big Screen – The Story of The Movies and What They did to Us’, Pinguin Books, 2012: 522-523.