256 SMITHSONIAN MISCELLANEOUS COLLECTIONS' VOL. 51 



quickly that I prefer to recommend it in all cases such as this where 

 a general orientation is all that is needed. 7 



The altitude is the only datum not taken from Hertz's diagram 

 of adiabats, but has been directly computed since this quantity 

 as given by the diagram cannot be absolutely relied on if we wish 

 to avoid large error. But when the object is to attain a general 

 idea of the magnitude of the quantities in question I think it justi- 

 fiable to be content with approximations. 



From this example we perceive that an ascending current of air 

 needs to pass only a very little beyond the saturation point in order 

 to develop a supersaturation whose dissipation completely suffices 

 to produce a rise of pressure of such magnitude as is observed in 

 thunderstorms. 



For instance, under the above assumed conditions the rising air 

 has only to pass 120 meters above the altitude for saturation in 

 order to produce a rise of pressure of 1.6 mm. when the change occurs 

 and in fact 75 meters will produce a sudden rise of 1 mm. in pressure. 

 Hence we utilize no risky hypotheses in explaining by means of 

 supersaturation the sudden rise in barometric pressure observed 

 during thunderstorms. 



In this explanation we must not attribute too much importance 

 to the fact that hitherto we have not yet been able to prove the 

 existence of such supersaturation in the free atmosphere. For 

 independent of the fact that our ordinary apparatus for measuring 

 humidity, such as the psychrometer and the hair hygrometer, are 

 not available as indicators of supersaturation, we note that the 

 greater frequency of fog in the neighborhood of large manufacturing 

 cities indicates that the point of saturation is not infrequently 

 attained, and even exceeded, in the free atmosphere without the 

 attendant condensation because of the absence of the necessary fog 

 nuclei. Moreover, the fact recently demonstrated by Hellmann 8 

 that the rainfall from thunderstorms west of Berlin are heavier than 

 from those over the city or east of it, argues for the occurrence of 

 supersaturation at places where there is a dearth of nuclei and espe- 

 cially during thunderstorms. 



Furthermore, in the powerful movements and in the peculiar 



7 The new adiabatic tables and diagrams by O. Neuhoff as published in 

 the Abhandlungen of the Preuss. Met. Institute, Vol. I, No. 6, Berlin. 1900, 

 can now be more conveniently used for such computations. See No. XXI 

 of this collection of translations. 



* Jahresbericht des Berliner Zweigvereins der Deutsche Met. Gesell. fur 

 1 89 1, p. 21, Berlin, 1892. 



