November 29, 1895.] 



SCIENCE. 



711 



Very similar results were published by 

 Oberbeck in 1882 and 1888, employing 

 more elegant mathematical methods and 

 advancing a step beyond Ferrel's first pub- 

 lications. It may, however, be stated that 

 the general solution of the hydro-dynamic 

 equations presupposes a definite knowledge 

 of the distribution of temperature or of 

 density in the atmosphere; and, of course, 

 the solutions given by Oberbeck and Ferrel 

 are intended to apply only to the atmos- 

 phere as we observe it. 



The thermo-dynamic phenomena attend- 

 ing the ascent and descent of the air have 

 been treated analytically by many authors, 

 such as Sir William Thomson, Reye, Cham- 

 bers, Hann, Guldberg and Mohn; within 

 the last few years this subject has been 

 worked out in a very elegant, graphic way 

 by Herz and von Bezold. 



The memoir of Herz considers only adi- 

 abatic changes, while the memoir of Bezold 

 considers the changes that are not strictly 

 adiabatic. It is evident on a slight consid- 

 eration that the quantity of heat within a 

 given mass of air is continually changing 

 by reason of several processes: First, the 

 direct absorption from the sun; second, radi- 

 ation to colder objects; third, the loss by 

 convection of heat attending the precipita- 

 tion of rain or snow; fourth, the gain by con- 

 vection attending evaporation from the 

 earth into the air; fifth, the process of 

 mixture that is constantly going on. There- 

 fore atmospheric processes are by no means 

 always adiabatic, and Bezold's graphic 

 methods enable us quickly to solve any 

 problem that may be presented. Bezold 

 and Helmholtz have agreed in adopting 

 and recommending the term ' potential 

 temperature ' as defining the temperature 

 that a mass of gas would have if brought 

 to a normal pressure, without loss or gain 

 of heat. 



Helmholtz added to our knowledge of at- 

 mospheric movements by his studies on the 



conditions of stability among masses of air 

 that have a discontinuous motion, such as 

 two vortex rings encircling the earth in 

 different latitudes and having different 

 temperatures. In general, stable equili- 

 brium is possible only when the warm ring 

 is on the polar side of the cold ring. 



A pupil of Helmholtz, Professor Diro 

 Kitao, of the Imperial Academy of Agricul- 

 ture of the University of Tokio, has made 

 an elaborate study of the forms of motion 

 that attend the meeting of two horizontal 

 currents, which then pile up and roll back 

 on themselves. 



Finally, Helmholtz has given us very 

 remarkable memoirs on waves in the at- 

 mosphere and the distribution of energy in 

 the winds and the ocean waves. Moeller, 

 Sprung, Hann, Wien and others have elabo- 

 rated the ideas thus contributed. 



The so-called ' convection theory of 

 storms ' that we call Espy's assumes that 

 the latent heat of vapor is the maintaining 

 power and that the original ascent of the 

 moist, warm air is due to its buoyancy. 

 Therefore we could have no continued 

 cyclonic motion without ascending moisture 

 and clouds and rain. But the other studies 

 have, I think, put it beyond doubt that 

 there is another equally important cause 

 at work, which undoubtedly is the fact that 

 the upper air flowing northward from the 

 equator as a return trade is slowly cooling 

 by radiation and descending. It eventually 

 reaches the earth here and there in spots 

 which are small areas of clear sky in the 

 tropical regions, but are large areas of cold 

 dry air and high pressure in northern lati- 

 tudes. If the air is cooled by radiation 

 faster than it is warmed up by the compres- 

 sion attending its slow descent, then it 

 descends as clear, cold and dry air, and only 

 after reaching the earth's surface does it 

 begin to warm up again in the daytime faster 

 than it can cool at night. As this dry cold 

 air under-runs the moist, warm air at the 



