Charles Darwin's Voyage of the Beagle is a journal of his travels and discoveries as a young man in South America. The entry for March 21st, 1835, is interesting; 165 years ago today the naturalist was on an excursion high in the Andes, and he tried to make sense of what he saw.
After describing the remarkable transparency of the air at high altitudes, he goes on: "Travellers having observed the difficulty of judging heights and distances amidst lofty mountains, have generally attributed it to the absence of objects of comparison. It appears to me, however, that it is fully as much owing to the transparency of the air confounding objects at different distances. I am sure that this extreme clearness of the air gives a peculiar character to the landscape, all objects appearing to be brought nearly into one plane, as in a drawing or panorama."
The "singular facility with which electricity is excited" he correctly attributed to the dryness of the atmosphere. "My flannel waistcoat, when rubbed in the dark, appeared as if it had been washed with phosphorus; every hair on a dog's back crackled; and even the linen sheets, and leathern straps of the saddle, when handled, emitted sparks."
Later that evening the party had trouble cooking dinner. "Our potatoes," wrote Darwin, "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. My two companions discussed the cause and came to the simple conclusion that the cursed pot (which was a new one) did not choose to boil the potatoes."
Darwin himself, however, was well aware of the real reason for their inconvenience. "At the place where we slept," he explained, "water necessarily boiled, from the diminished pressure of the atmosphere, at a lower temperature than it does in a less lofty country."
Atmospheric pressure decreases with increasing height in the lower atmosphere, at a rate of about one hectopascal for every 30 ft. If the party had camped, for example, at a height of 10,000 feet above sea level, one would expect the boiling point of water at that altitude to be in the region of 80 Celsius - as opposed to the normal 100 Celsius we assume at sea level. Obviously, 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 a great deal longer at this lower temperature.