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THE INVENTION of the compressed air brake by George Westinghouse in 1869 was followed rather promptly by investigations of wheel slip and wheel slide. This is reported in the famous Galton Westinghouse Tests of 1878 and 1879.1 Prior to that time, wheel slip or slide was unknown on cars because the hand brakes used did not supply sufficient braking effort. Wheel slip frequently occurred during acceleration on locomotives, but this was very easy to detect due to the fact that the number of driving wheels was relatively few and they were usually connected in such a manner that all would slip in unison. Wheel slide in braking the locomotive was also not unknown since emergency stops were usually made by reversing the drivers of the locomotive and applying steam for compression braking, sometimes to the extent of spinning the drivers backwards.