So what nationality are you simplex - French ?
Did the launching rail have a 'hold back' latch or similar to lock the aircraft in place while the engine rpm was run up and the pilot was satisfied that the engine was operating normally ?
I doubt the Wrights were dumb enough to launch the aircraft without checking engine rpm etc.
So the witnesses may have seen the aircraft pushed forward to the 'latch' prior to engine run up !
I am asking you because you know everything about the Wright brothers
Please remember that the witnesses were not technically trained and might not get technical details absolutely correct -
however they would be able to estimate time/distance fairly well
