39 6 THE POPULAR SCIENCE MONTHLY. 



another. The velocity of rotation of the atmosphere in tropical 

 latitudes must therefore lag behind the rotation of the earth, and 

 it must outstrip it in higher latitudes, mathematical calculation 

 proving that the thirty-fifth parallel is, in both hemispheres, the 

 line of division between the two. The general system of air cir- 

 culation deduced from these principles is very similar in its re- 

 sults to the system of Ferrel ; but the interest and importance of 

 Siemens^ views lie elsewhere. His memoirs were an appeal and 

 an attempt to apply the principles of thermodynamics to the 

 aerial currents, and they have opened the way for a series of im- 

 portant researches, which, however, are not yet sufficiently ad- 

 vanced to be discussed in these pages. 



And, finally, a third new point of view has been introduced 

 into the same discussions by Helmholtz. Sitting one day by the 

 seaside, and observing how wind blows on the surface of the sea 

 by sudden gushes, how it originates waves, and how they grow 

 when wind blows with an increasing force, Helmholtz came to 

 consider what would happen with two air currents blowing one 

 above the other in different directions. A system of air waves, 

 he concluded, must arise in this case, in the same way as they are 

 formed on the sea. The upper current, if it is inclined toward 

 the earth's surface (as is often the case), must originate in the 

 lower current immense aerial waves rolling at a great speed. 

 We do not generally see them, but when the lower current is so 

 much saturated with moisture that clouds are formed in it, we do 

 see a system of wavelike parallel clouds, which often extend over 

 wide parts of the sky. To calculate the sizes of the waves in 

 different cases is extremely difficult, if not impossible; but by 

 taking some simpler cases Helmholtz and Oberbeck showed that 

 when the waves on the sea attain lengths of from sixteen to thirty- 

 three feet, the air waves must attain lengths of from ten to twenty 

 miles, and a proportional depth. Such waves would make the 

 wind blow on the earth's surface in rhythmical gushes, which we 

 all know, and they also would more thoroughly mix together the 

 superposed strata, dissipating the energy stored in strong cur- 

 rents. These views are so correct that they undoubtedly will 

 throw some new light, as they already begin to do, upon the 

 theory of cyclones.* 



At the same time, Bezold is now endeavoring to reconstruct 

 meteorology from the point of view of thermodynamics ; f and 

 the well-known Austrian meteorologist, J. Hann, whose work is 



* H. Helmholtz, Zur Theorie von Wind und Wetter, and Die Energie der Wogen und 

 des Windes, in the Sitzungsberichte of the Berlin Academy, 1889, ii, and 1890, ii. Ober- 

 beck's calculations of the waves are given in the Meteorologische Zeitung, 1890, p. 81. 



f Zur Thermodynamik der Atmosphare, in Sitzungsberichte of the Berlin Academy of 

 Sciences, 1888, p. 485; same year, p. 1189; 1890, p. 355; and 1892, p. 279. 



