SCIENTIFIC BALLOON ASCENSIONS— VON BEZOLD 293 



of condition and curves of changes of condition to a definite and 

 more general question to the elucidation of which they are pecul- 

 iarly appropriate, that is, the so-called convective equilibrium. 



The mechanics of elastic fluids offers disproportionately far 

 greater difficulties than that of liquids. A drop of oil that is placed 

 at the bottom of a vessel full of water rises in the water without 

 experiencing any change in its volume or its temperature. On the 

 other hand, a particle of air enlarges its volume in proportion as it 

 approaches the surface, it also cools and consequently its volume 

 does not change to the same extent as when the temperature 

 remains unchanged. 



Still more complicated is the case when a mass of air that has been 

 locally warmed rises in an atmosphere whose temperature and density 

 themselves change with altitude and when, moreover, aqueous vapor 

 is mixed with the air which condenses at a definite temperature. 



In order to deal with this question one must make the simplifying 

 assumption that the mass of air after being once warmed over the 

 region receives or loses no heat thereafter, that is to say, the expan- 

 sion during ascent is adiabatic. Under this assumption the tem- 

 peratures through which the ascending air will pass can be com- 

 puted and corresponding to these the curve of the change of con- 

 dition or the so-called "adiabat" can be drawn. 



This adiabat is a straight line inclined 45 to the axes of the 

 coordinates so long as the saturation temperature is not attained. 

 At this point the line experiences a sharp bend and thence rises 

 more or less steadily as a flat curve convex to the right and above. 

 It rises steeply so long as the temperatures are high, that is to say, 

 so long as large quantities of water are present whose latent heat 

 of condensation is able to perform much work; it approaches more 

 and more the adiabat of the dry stage the lower its temperature, 

 that is, the less the water contained in the saturated air. 



Fig. 34 shows three adiabats that I take from the memoir 

 of O. NeuhofP now in press and to be published in the "Abhandlun- 

 gen" or Memoirs of the Prussian Meteorological Institute. These 

 curves correspond to masses of air that ascend from the earth's sur- 

 face with temperatures of — io°C, + io° C. and +30 C. respectively 

 and a barometric pressure of 76o mm . and relative humidity of 62 

 per cent. 



If in this way we have attained an idea of the course of such curves 



2 O. Neuhoff : Adiabatische, etc. Adiabatic changes of condition of moist 

 air and their determination, numerically and graphically. Memoirs Preuss. 

 Meteorolog. Institute, vol. I. No. 6, Berlin, 1900. [See No. xxi, p. 436 et seq.] 



