244 THE HUMAN MOTOR 



179. The Influence of Altitude Atmospheric pressure is 

 measured by the barometer (Technics, 245). This pressure is a 

 function of the altitude decreasing as the altitude increases, and 

 increasing consequently in proportion as one descends below the 

 surface of the earth. 



The temperature of the air does the same. It becomes colder 

 as we ascend, and inversely it Rets hotter as we descend. The 

 temperature is 1 C. lower at 180 metres above sea level, and it 

 increases by 1 when 33 metres below the ground. 



More exactly, the barometric pressure and the temperature 

 of a place are functions of its altitude and latitude. Knowing 

 the pressures of the barometer at the foot and at the summit of a 

 mountain, as well as the temperatures, H, H' and t, t', respect- 

 ively, and knowing the latitude X, Laplace's formula allows the 

 altitude A to be deduced : 



A = 18,405 m (1 + 0-002552 cos 2X (l + ^p) log jj', 



Conversely, the value of H' can be deduced from A and the 

 other factors. The height A thus calculated must have added 

 to it the height of the foot of the mountain above sea-level ; if 

 this is B metres, the true height will be : A -f B. This is called 

 the reduced altitude (reduced to sea-level). 



The variation of temperature with altitude is affected by 

 numerous factors, which it is difficult to determine exactly. The 

 winds and the position of the place itself in regard to its surround- 

 ings are variables not easily determined. 



The variation of temperature with depth is quite as irregular. 

 Experience has shown that at about 30 metres below the surface 

 in our climates, there is an " invariable stratum " ; for example, 

 a cave at that depth would have a fairly constant temperature 

 both summer and winter. 



Below this invariable stratum, the temperature of the earth 

 increases with the depth about 1 C. per 33 metres. The differ- 

 ences are remarkable. Thus we find 1 per 25 metres in the 

 Sperenberg mines, near Potsdam, up to a depth of 628 metres ; 

 below that the rise is 1 per 31 metres and 1 per 64 metres. In 

 the Anzin pits it is 1 per 15 to 26 metres ; in the Neuffen mines, 

 in Wurtemburg, 1 per 11 metres. More exact results can be 

 obtained by measuring the tempeiature in Artesian wells, that 

 of Crenelle, 548 metres deep, shows an increase of 1 C. per 30 

 metres. A man who climbs a mountain, goes up in a balloon, or 

 descends the shaft of a mine, experiences a change of atmospheric 

 pressure and of temperature. This change will be slow or rapid, 

 according to the speed with which he moves. It is useful to re- 

 member this factor of speed, to which one attributes, not without 

 reason, certain consequences of high altitudes. 



