The pressure of the atmosphere decreases with increasing height above the earth’s surface, because the thickness, and therefore the weight, of the belt of air above the observer decreases. The rate of fall in pressure is almost uniform over fairly small heights-about 85mm mercury per km. But at great altitude the rate of fall diminishes. At the height of Everest, 9000m, the atmospheric pressure is about 280mm of mercury. On account of the fall in atmospheric pressure, the boiling point of water falls with increasing height. Cooking-pots for use in high mountainous districts, such as the Ades, are therefore fitted with clamped lids. As the water boils, the steam accumulates in the pot, and its pressure rises above atmospheric. At about 760mm mercury a safety values opens, so that the pressure does not rise above that values, and the cooking is done at 100C.
The fall in the boiling-point with atmospheric pressure gives a simple way of determining one’s height above sea-level. One observes the steam point with a thermometer and hypsometer. Knowing how the steam point falls with pressure, and how atmospheric pressure falls with increasing height, one can then find one’s altitude. The hypsometer was, in fact, devised for this purpose, and takes its name from it; hypsos is Greek for height. Hypsometers have been his companions in Amundsen’s abandoned tent at the south Pole.
Variation of Latent Heat with Temperature
When we speak of the latent heat of evaporation of a liquid, we usually mean the heat required to vaporize unit mass of it at its normal boiling-point, that is to say, under normal atmospheric pressure. But since evaporation takes place at all temperatures, the latent heat has a value for every temperature. Regnault measured the latent heat of steam over a range of temperatures, by boiling water at controlled pressures, as in measuring its saturated vapor pressure. His apparatus was in principle similar to Berthelot’s; but the connected the outlet tube to an air reservoir, manometer, and pump, as in modern measurements give, approximately,
l = 2520 – 2.50
Where is the specific latent heat in KJ kg-1 at 0C.