About a century ago there was no switch we could flip to cool things down, except perhaps for a simple fan... but now, in sweltering and humid summers we have much to thank Mr Willis Carrier for.
Willis Haviland Carrier's family came from New England. His male ancestor emigrated from Wales in 1663, and married an early settler from Massachusetts, a woman who was later accused of being a witch because she defied the Andover town fathers in a boundary dispute. Refusing to admit to being a witch she had been hung from the gallows in Salem.
Her much later descendant, Willis Carrier, also had a mind of his own, and after earning an engineering degree from Cornell spent his life controlling humidity and temperature -- which has benefitted all of us.
Now, how would we cool ourselves 100 years ago? Using a newspaper we could gently fan a stream of air -- not too energetically, for that would make us hotter -- across our faces and necks. The perspiration would be carried off, leaving us slightly cooler. But why is this?
A pivotal experiment is due to Ben Franklin, who suspected the key was evaporation. In 1758, together with a Cambridge chemistry professor friend, he wetted the bulb of a regular thermometer with ether and a feather. Blowing across it by means of bellows the evaporation was amplified, and on a hot summer day in England -- if there is such a thing -- he and his friend managed to cool the thermometer bulb down to about 7F (= -14C.) Franklin is recorded as saying "... one may see the possibility of freezing a man to death on a warm summer's day ..." [Ref.1.]
A cup of coffee may also be cooled by evaporation. The fluid has a whole range of molecular speeds, some fast, some slow. The fastest ones, or the hottest ones, are continually jumping out above the surface -- one can see the vapor -- thus leaving the average speeds of the coffee itself somewhat slower than before. So, when one blows across the coffee one is sweeping those fastest hottest molecules away.
A different way of cooling is due to sudden expansion. Try this: hold your hand in front of your open mouth and gently blow. The air feels warm on your hand. Now close your mouth and let out a tiny stream of air; your hand feels cooled. This effect (actually called the Joule-Thomson effect) is used in present-day air-conditioners, refrigerators and dehumidifiers the world over.
In 1915 Willis Carrier pooled resources with half a dozen engineering friends, and formed the Carrier Engineering Corporation. It still exists, based in Syracuse NY, but has recently gone the way of banks, airlines, pharmacies and so on, being absorbed into United Technologies in 1979.
If one researches Carriers' various early machines, one also finds his interesting "psychrometric graph" relating parameters such as humidity and temperature. The prefix "psychro", with an "r" -- not to be confused with "psycho" -- comes from the Greek word "psychros" for cold. Very loosely, a thermometer could be called a kind of psychrometer.
However, not all was well with the wonders of air conditioners and dehumidifiers. Long after Mr Carrier died (1950) it was found that the working fluids -- which contained CFCs -- were seriously damaging the protective ozone layer in our upper atmosphere, and they have now been replaced with less ozone-depleting refrigerants. Interestingly, the most harmless, though not the most efficient refrigerant would be air -- as in our 'mouth experiment' described above.
Willis Carrier died at age 73 in 1950. 15 years later he was inducted into the National Inventors Hall of Fame, and in 1998 was listed as 'the inventor of modern air conditioning' in Time Magazine's "100 Most Influential People of the Century."
References:
1. "Benjamin Franklin's Science", by I.B.Cohen, 1990. Cohen also has other books on Franklin's experiments. Franklin wrote a detailed letter about this 1758 experiment with his friend in Cambridge, which can be found in Wikipedia. In the letter he mentions that others, particularly in hot desert climes, knew of this property of cooling when water evaporated.
