PRESSURE 



79 



Effects. 



The effects on mankind of the slighter variations of atmospheric 

 pressure are quite unknown and unstudied, scientifically, as far as 

 we know, though the subject merits careful research. It is possible 

 • that they are without effect, because they decrease with altitude in 

 the same ratio in which the pressure diminishes, but without produc- 

 ing any obvious effect from a climatic point of view. When atmo- 

 spheric pressure is increased considerably, as in the use of special 

 apparatus for deep diving and in the caissons filled with compressed 

 air which engineers use to lay the underwater foundations of 

 bridges, then a series of symptoms are produced which are called 

 caisson disease, and to this we shall refer in a later chapter, as the 

 conditions are pathological and not physiological. 



There is also the condition of diminished atmospheric pressure which is 

 found in mountain climates. Thus, if the pressure at sea-level is 762 milli- 

 metres of mercury at 25° C, then at an altitude of 500 metres, and at the same 

 temperature, this becomes 720; at 1,000 metres 679; at 2,000 metres 604; 

 at 3,000 metres 536; at 4,000 metres 475; at 5,000 metres 420; and at 6,000 

 metres 370. But if the temperature at 5,000 metres were 0° C, instead of 

 25° C, then the pressure would be 394 millimetres, and at 6,000 metres 343 

 millimetres; while at 3,000 metres it is 517 at 0° C, and 536 at 25° C. 



It must therefore be noted that the mean pressure is not the same at similar 

 altitudes in the tropics and in temperate climates, but somewhat higher. 



Mankind can live and has formed permanent habitations in Tibet at places 

 over 4,900 metres above sea-level, and Hahn states that in the Bolivian 

 province of Chichas people live at an altitude of 5,000 metres. 



Hahn, quoting Poppig and Reck, states that natives living on the high 

 Andean plateaus suffer from certain disagreeable effects, but we have been 

 unable to find what they mean exactly by this expression. 



Mountain sickness belongs to the pathological portion, and will therefore 

 be considered later; and all we can write with regard to the physiological 

 effects of pressure is to invite the attention of observers, who live at or visit 

 high altitudes, to the great lack of accurate information with regard to these 

 matters. 



WINDS. 



The motion of the air in passing from regions of high to those of 

 low pressure constitute the winds, which may be classified into per- 

 manent, periodical, variable, and local. 



Permanent winds are caused by the expansion of the hot equa- 

 torial air which rises to high altitudes and passes polewards. 



If Fig. 7, which demonstrated Ferrell's latest ideas as to the 

 circulation of the air, be studied, in general it will be noticed that 

 the zones of equatorial calms and rains are indicated, as well as 

 the zones of subtropical calms and drought. 



The unbroken arrows indicate the surface winds, and the broken 

 arrows the upper currents. The margin of the circle shows a vertical 

 section of the atmosphere in which the arrows indicate the move- 

 ments. The equatorial area is marked in the centre. 



If this figure be carefully examined, it will be seen rom the sectior} 

 of the atmosphere on the sides of the figure that the heated aii 

 rises over the Equator to high altitudes, and flows to the poles; 



