From a recent repairclinic.com newsletter:

Air conditioning was not invented for comfort. In 1902, Willis Haviland Carrier, a recent engineering graduate of Cornell University, was hired by the Buffalo Forge Printing Company. Fluctuations in heat and humidity in the printing plant were causing the paper to change dimensions, which led to misalignment of the colored inks. Carrier created an air conditioning machine to add moisture to the air in order to create stable temperature and humidity levels in the plant. His success at Buffalo Forge made aligned four-color printing possible. Carrier came up with the concept of air conditioning while waiting for a train. It was a foggy night and he was going over in his mind the problem of temperature and humidity control. By the time the train arrived, Carrier had an understanding of the relationship between temperature, humidity and dew point. In 1928, Carrier developed an air conditioner designed for human comfort and private home use called the Weathermaker.