EFFECT ON LIFE OF BAROMETRIC PRESSURE 211 



tension of water vapour at t must be deducted from p. The per- 

 centage composition of the air measured by volume is the same on 

 hot or cold days, at high or low altitudes, but in given volume 

 contains less air by weight on hot days or at high altitudes. 



The presence of water vapour in the atmosphere lowers the 

 tension of oxygen. For example, the barometric pressure was 756 '6 

 and the water- vapour tension 49, and the 2 tension on this occasion 



equalled (756'6 - 49) x ^j^ = 148-1 mm. 



The intake of 2 is not directly affected by variations in tempera- 

 ture and dryness of the air, for as the lung air is always saturated at 

 37 it does not matter whether the air is hot or cold, moist or dry. 

 The 2 intake is, however, indirectly affected by the mechanisms which 

 regulate the body temperature. 



In 1640 Torricelli invented the barometer, and eight years later 

 Perier, on the suggestion of Pascal, showed by an ascent of the 

 Puy de Dome that the barometric pressure fell with the increase 

 in altitude. The mean pressure at sea-level and latitude 45 is 

 760 mm. Hg. It varies slightly with, the latitude in proportion to 

 the variation of the acceleration due to gravity. 



The calculations of the height of the atmosphere vary from 

 42,000 to 320,000 kilometres, and have been made from the fall of 

 barometric pressure in balloon ascents, from the height of meteors 

 which become incandescent from friction on- entering the atmosphere, 

 and from the duration of twilight, which is caused by the scatter- 

 ing of light by the dust and vapour particles in the atmosphere. 

 As the atmosphere is not homogeneous in composition, and the 

 centrifugal force and the acceleration due to gravity which act 

 upon it vary with the distance from the earth, a complicated 

 formula (Laplace) is required to deduce an altitude from the baro- 

 metric pressure. 



At 5000 m. the pressure is roughly 400 mm. Hg, and the 

 oxygen by weight 11 per cent., or almost half that at sea-level. 

 For each 300 ft. of ascent the temperature falls about 1 F., and 

 thus the refugee from the hot plains of India finds at a height of 

 6000 ft. on the Himalayan slopes the mean temperature of the 

 Riviera. 



The higher strata of the air are very cold ( 52 C. at 10,500 m.) 

 owing to the diathermancy of pure air, the absorption of the heat 

 radiated from the earth by the dust and water vapour with which 



