Heat and its Importance in Meteorology. 367 



grows ; when the inflow is prevented, or when it brings dry 

 air instead of moist air, then the condensation of moisture stops 

 and the storm subsides. 



(b.) Ferrel worked out, from the point of view of hydro- 

 mechanics, the theory of the circulation of air in cyclones 

 and in the surrounding peri cyclonic region ; he also developed 

 the mechanics of cyclones with cold centers ; he then showed 

 how ordinary areas of high pressnre and cold air (which Gal ton 

 in 1863 first called anti cyclones) may result from the inter- 

 action of two or more pericyclones ; he mentions without 

 further development the radiation of heat from the air but 

 gives greater weight to the conduction to the ground and thus 

 explains the inversion of temperature in the clear still air of 

 winter. Much of this was done before the publication of 

 the weather maps of the Signal Service or indeed of any 

 European weather bureau. Ferrel subsequently developed the 

 thermodynamics of the condensation theory of Espy and 

 showed its excellent agreement with all the known facts of 

 local storms, hence it was assumed to also apply to the large 

 storms or areas of low pressure. 



(c.) Hann, having shown that the fohn wind is a rapidly 

 descending mass of air whose warmth is due to an initial rain 

 or snowfall with the evolution of latent heat and to the subse- 

 quent rapid compression of the air, follows this up by the 

 study of the slowly descending movements as exemplified in 

 the European area of high pressure of 1889, Nov. 11-23. He 

 finds, as Ferrel had done before him, that in the high areas 

 the temperature at a few thousand feet altitude is higher than 

 that at the surface of the ground ; he concludes that the 

 average temperature for a depth of 3000 meters is somewhat 

 higher in an anti-cyclone than in a cyclone. Hence Hann 

 throws doubt upon the idea that an area of high pressure with 

 cold air near the surface of the ground only, is really a mass 

 of descending cold air ; like most others he also explains that 

 the coldness of the lowest stratum of air under a clear sky is 

 due to radiation from the surface of the ground. Hann's 

 array of data requires that we now consider more closely the 

 mechanical theories of atmospheric motion ; these have 

 usually assumed an incompressible atmosphere of about 8000 

 meters in depth. 



(8.) Some features of recent researches in the mechanics of 

 the atmosphere may be presented as follows : A comprehensive 

 study of fluid motions shows that air and water alike may be 

 forced to ascend without being warmer and lighter or to de- 

 scend without being colder and denser than the surrounding 

 fluid. The currents and whirls behind any obstacle in streams 

 of air or water are almost wholly independent of differences 



