Meteorologists, by nature, are very much preoccupied with pressure. They watch its variations closely, and chart its evolution on their weather maps with isobars. Such is their fixation with these ethereal vicissitudes that A.P. Herbert once surmised in Punch that:
In some high mansion, I suppose,
The weather-men con- front the stars,
Giving "the glass" tremendous blows,
And drinking deep at isobars.
It is possible that the feasibility of drinking isobars may have occurred to the 17th-century meteorologist Blaise Pascal, since with Gallic elan, panache and savoir faire he used Burgundy to fill his French barometer. He found it worked quite well, except that it needed a tube some 30ft in length to achieve the desired effect. The reason, of course, is that wine has only one-thirteenth the weight of mercury, the fluid that we normally use, so a greater "head" was created before the system came in balance.
Be that as it may, Pascal - after whom, incidentally, we named the hectopascal - would have been aware that high mansions are not necessarily the most suitable locations for meteorological experiments. It was he who first discovered that atmospheric pressure decreases with increasing height, at a rate of about one hectopascal for every 30 feet in the layers of the atmosphere most adjacent to the ground.
The phenomenon caused some culinary difficulties for Charles Darwin and his companions in the high Andes in South America in 1835. "Our potatoes," Darwin wrote, "after remaining for some considerable time in the boiling water, were nearly as hard as ever.
"The pot was left on the fire all night and the next morning it was boiled again, but yet the potatoes were not cooked. I found this out by overhearing my two companions discussing the cause: they had come to the simple conclusion that the cursed pot did not choose to boil the potatoes."
Darwin himself, however, was well aware of the real reason for this inconvenience: "At the place where we slept, water necessarily boiled, from the diminished pressure of the atmosphere, at a lower temperature than it does in a less lofty country."
The boiling point was 80C as opposed to the normal 100C we assume at sea level. Moreover, since the process known as "cooking" involves transferring heat energy from the boiling water to the food immersed in it, it follows that it must take much longer at this lower temperature.