SCIENTIFIC BALLOON ASCENSIONS— VON BEZOLD 



295 



of motion in either direction suffices to cause the motion to continue 

 with increasing acceleration. If the particle receives a push upward 

 or in the direction from a toward b t along the adiabat, then in con- 

 sequence of the smaller cooling, the temperature difference between 

 the particle and^the surrounding air when it arrives at 6, will 

 steadily increase] in proportion as the motion continues. Con- 

 versely for a descending particle the temperature, as shown by the 

 line a b 2 representing the change of condition, will continually depart 

 more and more from that of the surrounding air, so that this move- 

 ment also progresses with increasing acceleration. 



fig. 36 



fig. 37 



The condition corresponding to this state of affairs is unstable and 

 can exist only temporarily. The condition of indifferent or neutral 

 equilibrium occurs when the change of temperature with altitude 

 exactly follows the adiabatic line, but since this condition changes 

 over into the unstable condition with the slightest change of tem- 

 perature, therefore it rarely occurs in nature but immediately 

 becomes unstable. 



In the neighborhood of the earth's surface, where the atmosphere 

 is generally in the dry stage, a temperature gradient of i° per 100 

 meters constitutes the limit, that is never or very rarely exceeded. 

 But the problem changes entirely when saturation occurs, since then 

 a comparatively much smaller gradient of temperature suffices to 

 produce unstable equilibrium. 



For in this case, as above remarked, the adiabat has a form such as 

 is shown in fig. 37 by the complex line A t 5y4 2 brokenatthepointS'. 



If then we compute the temperature gradient from observations 

 at two points, one of which lies somewhat above, but the other 

 below the limit of condensation, e.g., from the temperatures observed 

 at A t and A 2 , fig. 37, then, as is evident from the figure, we obtain a 

 value of the gradient that is less than i°C. per 100 meters no matter 

 whether neutral or even unstable equilibrium prevails. If, for 



